docs: move many specification-like documents into specifications/
authorBrian Warner <warner@lothar.com>
Sun, 22 Feb 2009 06:40:54 +0000 (23:40 -0700)
committerBrian Warner <warner@lothar.com>
Sun, 22 Feb 2009 06:40:54 +0000 (23:40 -0700)
28 files changed:
docs/CHK-hashes.svg [deleted file]
docs/Makefile
docs/URI-extension.txt [deleted file]
docs/dirnodes.txt [deleted file]
docs/file-encoding.txt [deleted file]
docs/file-encoding1.svg [deleted file]
docs/file-encoding2.svg [deleted file]
docs/file-encoding3.svg [deleted file]
docs/file-encoding4.svg [deleted file]
docs/file-encoding5.svg [deleted file]
docs/file-encoding6.svg [deleted file]
docs/mut.svg [deleted file]
docs/mutable.txt [deleted file]
docs/specifications/CHK-hashes.svg [new file with mode: 0644]
docs/specifications/Makefile [new file with mode: 0644]
docs/specifications/URI-extension.txt [new file with mode: 0644]
docs/specifications/dirnodes.txt [new file with mode: 0644]
docs/specifications/file-encoding.txt [new file with mode: 0644]
docs/specifications/file-encoding1.svg [new file with mode: 0644]
docs/specifications/file-encoding2.svg [new file with mode: 0644]
docs/specifications/file-encoding3.svg [new file with mode: 0644]
docs/specifications/file-encoding4.svg [new file with mode: 0644]
docs/specifications/file-encoding5.svg [new file with mode: 0644]
docs/specifications/file-encoding6.svg [new file with mode: 0644]
docs/specifications/mut.svg [new file with mode: 0644]
docs/specifications/mutable.txt [new file with mode: 0644]
docs/specifications/uri.txt [new file with mode: 0644]
docs/uri.txt [deleted file]

diff --git a/docs/CHK-hashes.svg b/docs/CHK-hashes.svg
deleted file mode 100644 (file)
index 22bd524..0000000
+++ /dev/null
@@ -1,723 +0,0 @@
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-       x="217.67101"
-       style="font-size:20px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#000000;fill-opacity:1;stroke:none;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
-       xml:space="preserve"><tspan
-         y="978.66254"
-         x="217.67101"
-         id="tspan3481"
-         sodipodi:role="line">B is derived from A by encryption or erasure coding</tspan></text>
-    <path
-       id="path3483"
-       d="M 99.408806,997.06966 L 147.39926,997.06966"
-       style="fill:none;fill-opacity:1;fill-rule:evenodd;stroke:#1bcc03;stroke-width:1.5;stroke-linecap:butt;stroke-linejoin:miter;marker-end:url(#Arrow1Lend);stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1"
-       sodipodi:nodetypes="cc" />
-    <text
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-       style="font-size:28px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#000000;fill-opacity:1;stroke:none;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
-       x="65.129906"
-       y="1006.5616"
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-         sodipodi:role="line"
-         id="tspan3487"
-         x="65.129906"
-         y="1006.5616">A</tspan></text>
-    <text
-       xml:space="preserve"
-       style="font-size:28px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#000000;fill-opacity:1;stroke:none;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
-       x="164.53871"
-       y="1006.0856"
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-       sodipodi:linespacing="100%"><tspan
-         sodipodi:role="line"
-         x="164.53871"
-         y="1006.0856"
-         id="tspan3491">B :</tspan></text>
-    <text
-       xml:space="preserve"
-       style="font-size:20px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#000000;fill-opacity:1;stroke:none;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
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-         sodipodi:role="line"
-         id="tspan3495"
-         x="217.67101"
-         y="1006.0856">A is used as an index to retrieve data B</tspan></text>
-    <text
-       xml:space="preserve"
-       style="font-size:32px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#1bcc03;fill-opacity:1;stroke:#1bcc03;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
-       x="625.5899"
-       y="740.42419"
-       id="text3497"
-       sodipodi:linespacing="100%"><tspan
-         sodipodi:role="line"
-         id="tspan3499"
-         x="625.5899"
-         y="740.42419">SHARE</tspan></text>
-    <text
-       xml:space="preserve"
-       style="font-size:32px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#000000;fill-opacity:1;stroke:none;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
-       x="249.20799"
-       y="30.851007"
-       id="text3501"
-       sodipodi:linespacing="100%"><tspan
-         sodipodi:role="line"
-         id="tspan3503"
-         x="249.20799"
-         y="30.851007">CHK File Hashes</tspan></text>
-  </g>
-</svg>
index 04db86d8621d56e7aa2be476bf27dd95db6f1bcc..490072177f6c62baa47a3a82726cc8e26e55a32e 100644 (file)
@@ -1,7 +1,5 @@
 
-SOURCES = CHK-hashes.svg file-encoding1.svg file-encoding2.svg \
-       file-encoding3.svg file-encoding4.svg file-encoding5.svg \
-       file-encoding6.svg subtree1.svg lease-tradeoffs.svg
+SOURCES = subtree1.svg lease-tradeoffs.svg
 
 PNGS = $(patsubst %.svg,%.png,$(SOURCES))
 EPSS = $(patsubst %.svg,%.eps,$(SOURCES))
diff --git a/docs/URI-extension.txt b/docs/URI-extension.txt
deleted file mode 100644 (file)
index 8ec383e..0000000
+++ /dev/null
@@ -1,61 +0,0 @@
-
-"URI Extension Block"
-
-This block is a serialized dictionary with string keys and string values
-(some of which represent numbers, some of which are SHA-256 hashes). All
-buckets hold an identical copy. The hash of the serialized data is kept in
-the URI.
-
-The download process must obtain a valid copy of this data before any
-decoding can take place. The download process must also obtain other data
-before incremental validation can be performed. Full-file validation (for
-clients who do not wish to do incremental validation) can be performed solely
-with the data from this block.
-
-At the moment, this data block contains the following keys (and an estimate
-on their sizes):
-
- size                5
- segment_size        7
- num_segments        2
- needed_shares       2
- total_shares        3
-
- codec_name          3
- codec_params        5+1+2+1+3=12
- tail_codec_params   12
-
- share_root_hash     32 (binary) or 52 (base32-encoded) each
- plaintext_hash
- plaintext_root_hash
- crypttext_hash
- crypttext_root_hash
-
-Some pieces are needed elsewhere (size should be visible without pulling the
-block, the Tahoe3 algorithm needs total_shares to find the right peers, all
-peer selection algorithms need needed_shares to ask a minimal set of peers).
-Some pieces are arguably redundant but are convenient to have present
-(test_encode.py makes use of num_segments).
-
-The rule for this data block is that it should be a constant size for all
-files, regardless of file size. Therefore hash trees (which have a size that
-depends linearly upon the number of segments) are stored elsewhere in the
-bucket, with only the hash tree root stored in this data block.
-
-This block will be serialized as follows:
-
- assert that all keys match ^[a-zA-z_\-]+$
- sort all the keys lexicographically
- for k in keys:
-  write("%s:" % k)
-  write(netstring(data[k]))
-
-
-Serialized size:
-
- dense binary (but decimal) packing: 160+46=206
- including 'key:' (185) and netstring (6*3+7*4=46) on values: 231
- including 'key:%d\n' (185+13=198) and printable values (46+5*52=306)=504
-
-We'll go with the 231-sized block, and provide a tool to dump it as text if
-we really want one.
diff --git a/docs/dirnodes.txt b/docs/dirnodes.txt
deleted file mode 100644 (file)
index adc8fca..0000000
+++ /dev/null
@@ -1,433 +0,0 @@
-
-= Tahoe Directory Nodes =
-
-As explained in the architecture docs, Tahoe can be roughly viewed as a
-collection of three layers. The lowest layer is the distributed filestore, or
-DHT: it provides operations that accept files and upload them to the mesh,
-creating a URI in the process which securely references the file's contents.
-The middle layer is the filesystem, creating a structure of directories and
-filenames resembling the traditional unix/windows filesystems. The top layer
-is the application layer, which uses the lower layers to provide useful
-services to users, like a backup application, or a way to share files with
-friends.
-
-This document examines the middle layer, the "filesystem".
-
-== DHT Primitives ==
-
-In the lowest layer (DHT), there are two operations that reference immutable
-data (which we refer to as "CHK URIs" or "CHK read-capabilities" or "CHK
-read-caps"). One puts data into the grid (but only if it doesn't exist
-already), the other retrieves it:
-
- chk_uri = put(data)
- data = get(chk_uri)
-
-We also have three operations which reference mutable data (which we refer to
-as "mutable slots", or "mutable write-caps and read-caps", or sometimes "SSK
-slots"). One creates a slot with some initial contents, a second replaces the
-contents of a pre-existing slot, and the third retrieves the contents:
-
- mutable_uri = create(initial_data)
- replace(mutable_uri, new_data)
- data = get(mutable_uri)
-
-== Filesystem Goals ==
-
-The main goal for the middle (filesystem) layer is to give users a way to
-organize the data that they have uploaded into the mesh. The traditional way
-to do this in computer filesystems is to put this data into files, give those
-files names, and collect these names into directories.
-
-Each directory is a series of name-value pairs, which maps "child name" to an
-object of some kind. Those child objects might be files, or they might be
-other directories.
-
-The directory structure is therefore a directed graph of nodes, in which each
-node might be a directory node or a file node. All file nodes are terminal
-nodes.
-
-== Dirnode Goals ==
-
-What properties might be desirable for these directory nodes? In no
-particular order:
-
- 1: functional. Code which does not work doesn't count.
- 2: easy to document, explain, and understand
- 3: confidential: it should not be possible for others to see the contents of
-                  a directory
- 4: integrity: it should not be possible for others to modify the contents
-               of a directory
- 5: available: directories should survive host failure, just like files do
- 6: efficient: in storage, communication bandwidth, number of round-trips
- 7: easy to delegate individual directories in a flexible way
- 8: updateness: everybody looking at a directory should see the same contents
- 9: monotonicity: everybody looking at a directory should see the same
-                  sequence of updates
-
-Some of these goals are mutually exclusive. For example, availability and
-consistency are opposing, so it is not possible to achieve #5 and #8 at the
-same time. Moreover, it takes a more complex architecture to get close to the
-available-and-consistent ideal, so #2/#6 is in opposition to #5/#8.
-
-Tahoe-0.7.0 introduced distributed mutable files, which use public key
-cryptography for integrity, and erasure coding for availability. These
-achieve roughly the same properties as immutable CHK files, but their
-contents can be replaced without changing their identity. Dirnodes are then
-just a special way of interpreting the contents of a specific mutable file.
-Earlier releases used a "vdrive server": this server was abolished in the
-0.7.0 release.
-
-For details of how mutable files work, please see "mutable.txt" in this
-directory.
-
-For the current 0.7.0 release, we achieve most of our desired properties. The
-integrity and availability of dirnodes is equivalent to that of regular
-(immutable) files, with the exception that there are more simultaneous-update
-failure modes for mutable slots. Delegation is quite strong: you can give
-read-write or read-only access to any subtree, and the data format used for
-dirnodes is such that read-only access is transitive: i.e. if you grant Bob
-read-only access to a parent directory, then Bob will get read-only access
-(and *not* read-write access) to its children.
-
-Relative to the previous "vdrive-server" based scheme, the current
-distributed dirnode approach gives better availability, but cannot guarantee
-updateness quite as well, and requires far more network traffic for each
-retrieval and update. Mutable files are somewhat less available than
-immutable files, simply because of the increased number of combinations
-(shares of an immutable file are either present or not, whereas there are
-multiple versions of each mutable file, and you might have some shares of
-version 1 and other shares of version 2). In extreme cases of simultaneous
-update, mutable files might suffer from non-monotonicity.
-
-
-== Dirnode secret values ==
-
-As mentioned before, dirnodes are simply a special way to interpret the
-contents of a mutable file, so the secret keys and capability strings
-described in "mutable.txt" are all the same. Each dirnode contains an RSA
-public/private keypair, and the holder of the "write capability" will be able
-to retrieve the private key (as well as the AES encryption key used for the
-data itself). The holder of the "read capability" will be able to obtain the
-public key and the AES data key, but not the RSA private key needed to modify
-the data.
-
-The "write capability" for a dirnode grants read-write access to its
-contents. This is expressed on concrete form as the "dirnode write cap": a
-printable string which contains the necessary secrets to grant this access.
-Likewise, the "read capability" grants read-only access to a dirnode, and can
-be represented by a "dirnode read cap" string.
-
-For example,
-URI:DIR2:swdi8ge1s7qko45d3ckkyw1aac%3Aar8r5j99a4mezdojejmsfp4fj1zeky9gjigyrid4urxdimego68o
-is a write-capability URI, while
-URI:DIR2-RO:buxjqykt637u61nnmjg7s8zkny:ar8r5j99a4mezdojejmsfp4fj1zeky9gjigyrid4urxdimego68o
-is a read-capability URI, both for the same dirnode.
-
-
-== Dirnode storage format ==
-
-Each dirnode is stored in a single mutable file, distributed in the Tahoe
-grid. The contents of this file are a serialized list of netstrings, one per
-child. Each child is a list of four netstrings: (name, rocap, rwcap,
-metadata). (remember that the contents of the mutable file are encrypted by
-the read-cap, so this section describes the plaintext contents of the mutable
-file, *after* it has been decrypted by the read-cap).
-
-The name is simple a UTF-8 -encoded child name. The 'rocap' is a read-only
-capability URI to that child, either an immutable (CHK) file, a mutable file,
-or a directory. The 'rwcap' is a read-write capability URI for that child,
-encrypted with the dirnode's write-cap: this enables the "transitive
-readonlyness" property, described further below. The 'metadata' is a
-JSON-encoded dictionary of type,value metadata pairs. Some metadata keys are
-pre-defined, the rest are left up to the application.
-
-Each rwcap is stored as IV + ciphertext + MAC. The IV is a 16-byte random
-value. The ciphertext is obtained by using AES in CTR mode on the rwcap URI
-string, using a key that is formed from a tagged hash of the IV and the
-dirnode's writekey. The MAC is a 32-byte SHA-256 -based HMAC (using that same
-AES key) over the (IV+ciphertext) pair.
-
-If Bob has read-only access to the 'bar' directory, and he adds it as a child
-to the 'foo' directory, then he will put the read-only cap for 'bar' in both
-the rwcap and rocap slots (encrypting the rwcap contents as described above).
-If he has full read-write access to 'bar', then he will put the read-write
-cap in the 'rwcap' slot, and the read-only cap in the 'rocap' slot. Since
-other users who have read-only access to 'foo' will be unable to decrypt its
-rwcap slot, this limits those users to read-only access to 'bar' as well,
-thus providing the transitive readonlyness that we desire.
-
-=== Dirnode sizes, mutable-file initial read sizes ===
-
-How big are dirnodes? When reading dirnode data out of mutable files, how
-large should our initial read be? If we guess exactly, we can read a dirnode
-in a single round-trip, and update one in two RTT. If we guess too high,
-we'll waste some amount of bandwidth. If we guess low, we need to make a
-second pass to get the data (or the encrypted privkey, for writes), which
-will cost us at least another RTT.
-
-Assuming child names are between 10 and 99 characters long, how long are the
-various pieces of a dirnode?
-
- netstring(name) ~= 4+len(name)
- chk-cap = 97 (for 4-char filesizes)
- dir-rw-cap = 88
- dir-ro-cap = 91
- netstring(cap) = 4+len(cap)
- encrypted(cap) = 16+cap+32
- JSON({}) = 2
- JSON({ctime=float,mtime=float}): 57
- netstring(metadata) = 4+57 = 61
-
-so a CHK entry is:
- 5+ 4+len(name) + 4+97 + 5+16+97+32 + 4+57
-And a 15-byte filename gives a 336-byte entry. When the entry points at a
-subdirectory instead of a file, the entry is a little bit smaller. So an
-empty directory uses 0 bytes, a directory with one child uses about 336
-bytes, a directory with two children uses about 672, etc.
-
-When the dirnode data is encoding using our default 3-of-10, that means we
-get 112ish bytes of data in each share per child.
-
-The pubkey, signature, and hashes form the first 935ish bytes of the
-container, then comes our data, then about 1216 bytes of encprivkey. So if we
-read the first:
-
- 1kB: we get 65bytes of dirnode data : only empty directories
- 1kiB: 89bytes of dirnode data : maybe one short-named subdir
- 2kB: 1065bytes: about 9 entries
- 3kB: 2065bytes: about 18 entries, or 7.5 entries plus the encprivkey
- 4kB: 3065bytes: about 27 entries, or about 16.5 plus the encprivkey
-
-So we've written the code to do an initial read of 2kB from each share when
-we read the mutable file, which should give good performance (one RTT) for
-small directories.
-
-
-== Design Goals, redux ==
-
-How well does this design meet the goals?
-
- #1 functional: YES: the code works and has extensive unit tests
- #2 documentable: YES: this document is the existence proof
- #3 confidential: YES: see below
- #4 integrity: MOSTLY: a coalition of storage servers can rollback individual
-                       mutable files, but not a single one. No server can
-                       substitute fake data as genuine.
- #5 availability: YES: as long as 'k' storage servers are present and have
-                       the same version of the mutable file, the dirnode will
-                       be available.
- #6 efficient: MOSTLY:
-      network: single dirnode lookup is very efficient, since clients can
-               fetch specific keys rather than being required to get or set
-               the entire dirnode each time. Traversing many directories
-               takes a lot of roundtrips, and these can't be collapsed with
-               promise-pipelining because the intermediate values must only
-               be visible to the client. Modifying many dirnodes at once
-               (e.g. importing a large pre-existing directory tree) is pretty
-               slow, since each graph edge must be created independently.
-      storage: each child has a separate IV, which makes them larger than
-               if all children were aggregated into a single encrypted string
- #7 delegation: VERY: each dirnode is a completely independent object,
-                to which clients can be granted separate read-write or
-                read-only access
- #8 updateness: VERY: with only a single point of access, and no caching,
-                each client operation starts by fetching the current
-                value, so there are no opportunities for staleness
- #9 monotonicity: VERY: the single point of access also protects against
-                  retrograde motion
-     
-
-
-=== Confidentiality leaks in the vdrive server ===
-
-Dirnode (and the mutable files upon which they are based) are very private
-against other clients: traffic between the client and the storage servers is
-protected by the Foolscap SSL connection, so they can observe very little.
-Storage index values are hashes of secrets and thus unguessable, and they are
-not made public, so other clients cannot snoop through encrypted dirnodes
-that they have not been told about.
-
-Storage servers can observe access patterns and see ciphertext, but they
-cannot see the plaintext (of child names, metadata, or URIs). If an attacker
-operates a significant number of storage servers, they can infer the shape of
-the directory structure by assuming that directories are usually accessed
-from root to leaf in rapid succession. Since filenames are usually much
-shorter than read-caps and write-caps, the attacker can use the length of the
-ciphertext to guess the number of children of each node, and might be able to
-guess the length of the child names (or at least their sum). From this, the
-attacker may be able to build up a graph with the same shape as the plaintext
-filesystem, but with unlabeled edges and unknown file contents.
-
-
-=== Integrity failures in the vdrive server ===
-
-The mutable file's integrity mechanism (RSA signature on the hash of the file
-contents) prevents the storage server from modifying the dirnode's contents
-without detection. Therefore the storage servers can make the dirnode
-unavailable, but not corrupt it.
-
-A sufficient number of colluding storage servers can perform a rollback
-attack: replace all shares of the whole mutable file with an earlier version.
-TODO: To prevent this, when retrieving the contents of a mutable file, the
-client should query more servers than necessary and use the highest available
-version number. This insures that one or two misbehaving storage servers
-cannot cause this rollback on their own.
-
-
-=== Improving the efficiency of dirnodes ===
-
-The current mutable-file -based dirnode scheme suffers from certain
-inefficiencies. A very large directory (with thousands or millions of
-children) will take a significant time to extract any single entry, because
-the whole file must be downloaded first, then parsed and searched to find the
-desired child entry. Likewise, modifying a single child will require the
-whole file to be re-uploaded.
-
-The current design assumes (and in some cases, requires) that dirnodes remain
-small. The mutable files on which dirnodes are based are currently using
-"SDMF" ("Small Distributed Mutable File") design rules, which state that the
-size of the data shall remain below one megabyte. More advanced forms of
-mutable files (MDMF and LDMF) are in the design phase to allow efficient
-manipulation of larger mutable files. This would reduce the work needed to
-modify a single entry in a large directory.
-
-Judicious caching may help improve the reading-large-directory case. Some
-form of mutable index at the beginning of the dirnode might help as well. The
-MDMF design rules allow for efficient random-access reads from the middle of
-the file, which would give the index something useful to point at.
-
-The current SDMF design generates a new RSA public/private keypair for each
-directory. This takes considerable time and CPU effort, generally one or two
-seconds per directory. We have designed (but not yet built) a DSA-based
-mutable file scheme which will use shared parameters to reduce the
-directory-creation effort to a bare minimum (picking a random number instead
-of generating two random primes).
-
-
-When a backup program is run for the first time, it needs to copy a large
-amount of data from a pre-existing filesystem into reliable storage. This
-means that a large and complex directory structure needs to be duplicated in
-the dirnode layer. With the one-object-per-dirnode approach described here,
-this requires as many operations as there are edges in the imported
-filesystem graph.
-
-Another approach would be to aggregate multiple directories into a single
-storage object. This object would contain a serialized graph rather than a
-single name-to-child dictionary. Most directory operations would fetch the
-whole block of data (and presumeably cache it for a while to avoid lots of
-re-fetches), and modification operations would need to replace the whole
-thing at once. This "realm" approach would have the added benefit of
-combining more data into a single encrypted bundle (perhaps hiding the shape
-of the graph from a determined attacker), and would reduce round-trips when
-performing deep directory traversals (assuming the realm was already cached).
-It would also prevent fine-grained rollback attacks from working: a coalition
-of storage servers could change the entire realm to look like an earlier
-state, but it could not independently roll back individual directories.
-
-The drawbacks of this aggregation would be that small accesses (adding a
-single child, looking up a single child) would require pulling or pushing a
-lot of unrelated data, increasing network overhead (and necessitating
-test-and-set semantics for the modification side, which increases the chances
-that a user operation will fail, making it more challenging to provide
-promises of atomicity to the user). 
-
-It would also make it much more difficult to enable the delegation
-("sharing") of specific directories. Since each aggregate "realm" provides
-all-or-nothing access control, the act of delegating any directory from the
-middle of the realm would require the realm first be split into the upper
-piece that isn't being shared and the lower piece that is. This splitting
-would have to be done in response to what is essentially a read operation,
-which is not traditionally supposed to be a high-effort action. On the other
-hand, it may be possible to aggregate the ciphertext, but use distinct
-encryption keys for each component directory, to get the benefits of both
-schemes at once.
-
-
-=== Dirnode expiration and leases ===
-
-Dirnodes are created any time a client wishes to add a new directory. How
-long do they live? What's to keep them from sticking around forever, taking
-up space that nobody can reach any longer?
-
-Mutable files are created with limited-time "leases", which keep the shares
-alive until the last lease has expired or been cancelled. Clients which know
-and care about specific dirnodes can ask to keep them alive for a while, by
-renewing a lease on them (with a typical period of one month). Clients are
-expected to assist in the deletion of dirnodes by canceling their leases as
-soon as they are done with them. This means that when a client deletes a
-directory, it should also cancel its lease on that directory. When the lease
-count on a given share goes to zero, the storage server can delete the
-related storage. Multiple clients may all have leases on the same dirnode:
-the server may delete the shares only after all of the leases have gone away.
-
-We expect that clients will periodically create a "manifest": a list of
-so-called "refresh capabilities" for all of the dirnodes and files that they
-can reach. They will give this manifest to the "repairer", which is a service
-that keeps files (and dirnodes) alive on behalf of clients who cannot take on
-this responsibility for themselves. These refresh capabilities include the
-storage index, but do *not* include the readkeys or writekeys, so the
-repairer does not get to read the files or directories that it is helping to
-keep alive.
-
-After each change to the user's vdrive, the client creates a manifest and
-looks for differences from their previous version. Anything which was removed
-prompts the client to send out lease-cancellation messages, allowing the data
-to be deleted.
-
-
-== Starting Points: root dirnodes ==
-
-Any client can record the URI of a directory node in some external form (say,
-in a local file) and use it as the starting point of later traversal. Each
-Tahoe user is expected to create a new (unattached) dirnode when they first
-start using the grid, and record its URI for later use.
-
-== Mounting and Sharing Directories ==
-
-The biggest benefit of this dirnode approach is that sharing individual
-directories is almost trivial. Alice creates a subdirectory that she wants to
-use to share files with Bob. This subdirectory is attached to Alice's
-filesystem at "~alice/share-with-bob". She asks her filesystem for the
-read-write directory URI for that new directory, and emails it to Bob. When
-Bob receives the URI, he asks his own local vdrive to attach the given URI,
-perhaps at a place named "~bob/shared-with-alice". Every time either party
-writes a file into this directory, the other will be able to read it. If
-Alice prefers, she can give a read-only URI to Bob instead, and then Bob will
-be able to read files but not change the contents of the directory. Neither
-Alice nor Bob will get access to any files above the mounted directory: there
-are no 'parent directory' pointers. If Alice creates a nested set of
-directories, "~alice/share-with-bob/subdir2", and gives a read-only URI to
-share-with-bob to Bob, then Bob will be unable to write to either
-share-with-bob/ or subdir2/.
-
-A suitable UI needs to be created to allow users to easily perform this
-sharing action: dragging a folder their vdrive to an IM or email user icon,
-for example. The UI will need to give the sending user an opportunity to
-indicate whether they want to grant read-write or read-only access to the
-recipient. The recipient then needs an interface to drag the new folder into
-their vdrive and give it a home.
-
-== Revocation ==
-
-When Alice decides that she no longer wants Bob to be able to access the
-shared directory, what should she do? Suppose she's shared this folder with
-both Bob and Carol, and now she wants Carol to retain access to it but Bob to
-be shut out. Ideally Carol should not have to do anything: her access should
-continue unabated.
-
-The current plan is to have her client create a deep copy of the folder in
-question, delegate access to the new folder to the remaining members of the
-group (Carol), asking the lucky survivors to replace their old reference with
-the new one. Bob may still have access to the old folder, but he is now the
-only one who cares: everyone else has moved on, and he will no longer be able
-to see their new changes. In a strict sense, this is the strongest form of
-revocation that can be accomplished: there is no point trying to force Bob to
-forget about the files that he read a moment before being kicked out. In
-addition it must be noted that anyone who can access the directory can proxy
-for Bob, reading files to him and accepting changes whenever he wants.
-Preventing delegation between communication parties is just as pointless as
-asking Bob to forget previously accessed files. However, there may be value
-to configuring the UI to ask Carol to not share files with Bob, or to
-removing all files from Bob's view at the same time his access is revoked.
-
diff --git a/docs/file-encoding.txt b/docs/file-encoding.txt
deleted file mode 100644 (file)
index 23862ea..0000000
+++ /dev/null
@@ -1,148 +0,0 @@
-
-== FileEncoding ==
-
-When the client wishes to upload an immutable file, the first step is to
-decide upon an encryption key. There are two methods: convergent or random.
-The goal of the convergent-key method is to make sure that multiple uploads
-of the same file will result in only one copy on the grid, whereas the
-random-key method does not provide this "convergence" feature.
-
-The convergent-key method computes the SHA-256d hash of a single-purpose tag,
-the encoding parameters, a "convergence secret", and the contents of the
-file. It uses a portion of the resulting hash as the AES encryption key.
-There are security concerns with using convergence this approach (the
-"partial-information guessing attack", please see ticket #365 for some
-references), so Tahoe uses a separate (randomly-generated) "convergence
-secret" for each node, stored in NODEDIR/private/convergence . The encoding
-parameters (k, N, and the segment size) are included in the hash to make sure
-that two different encodings of the same file will get different keys. This
-method requires an extra IO pass over the file, to compute this key, and
-encryption cannot be started until the pass is complete. This means that the
-convergent-key method will require at least two total passes over the file.
-
-The random-key method simply chooses a random encryption key. Convergence is
-disabled, however this method does not require a separate IO pass, so upload
-can be done with a single pass. This mode makes it easier to perform
-streaming upload.
-
-Regardless of which method is used to generate the key, the plaintext file is
-encrypted (using AES in CTR mode) to produce a ciphertext. This ciphertext is
-then erasure-coded and uploaded to the servers. Two hashes of the ciphertext
-are generated as the encryption proceeds: a flat hash of the whole
-ciphertext, and a Merkle tree. These are used to verify the correctness of
-the erasure decoding step, and can be used by a "verifier" process to make
-sure the file is intact without requiring the decryption key.
-
-The encryption key is hashed (with SHA-256d and a single-purpose tag) to
-produce the "Storage Index". This Storage Index (or SI) is used to identify
-the shares produced by the method described below. The grid can be thought of
-as a large table that maps Storage Index to a ciphertext. Since the
-ciphertext is stored as erasure-coded shares, it can also be thought of as a
-table that maps SI to shares.
-
-Anybody who knows a Storage Index can retrieve the associated ciphertext:
-ciphertexts are not secret.
-
-
-[[Image(file-encoding1.png)]]
-
-The ciphertext file is then broken up into segments. The last segment is
-likely to be shorter than the rest. Each segment is erasure-coded into a
-number of "blocks". This takes place one segment at a time. (In fact,
-encryption and erasure-coding take place at the same time, once per plaintext
-segment). Larger segment sizes result in less overhead overall, but increase
-both the memory footprint and the "alacrity" (the number of bytes we have to
-receive before we can deliver validated plaintext to the user). The current
-default segment size is 128KiB.
-
-One block from each segment is sent to each shareholder (aka leaseholder,
-aka landlord, aka storage node, aka peer). The "share" held by each remote
-shareholder is nominally just a collection of these blocks. The file will
-be recoverable when a certain number of shares have been retrieved.
-
-[[Image(file-encoding2.png)]]
-
-The blocks are hashed as they are generated and transmitted. These
-block hashes are put into a Merkle hash tree. When the last share has been
-created, the merkle tree is completed and delivered to the peer. Later, when
-we retrieve these blocks, the peer will send many of the merkle hash tree
-nodes ahead of time, so we can validate each block independently.
-
-The root of this block hash tree is called the "block root hash" and
-used in the next step.
-
-[[Image(file-encoding3.png)]]
-
-There is a higher-level Merkle tree called the "share hash tree". Its leaves
-are the block root hashes from each share. The root of this tree is called
-the "share root hash" and is included in the "URI Extension Block", aka UEB.
-The ciphertext hash and Merkle tree are also put here, along with the
-original file size, and the encoding parameters. The UEB contains all the
-non-secret values that could be put in the URI, but would have made the URI
-too big. So instead, the UEB is stored with the share, and the hash of the
-UEB is put in the URI.
-
-The URI then contains the secret encryption key and the UEB hash. It also
-contains the basic encoding parameters (k and N) and the file size, to make
-download more efficient (by knowing the number of required shares ahead of
-time, sufficient download queries can be generated in parallel).
-
-The URI (also known as the immutable-file read-cap, since possessing it
-grants the holder the capability to read the file's plaintext) is then
-represented as a (relatively) short printable string like so:
-
- URI:CHK:auxet66ynq55naiy2ay7cgrshm:6rudoctmbxsmbg7gwtjlimd6umtwrrsxkjzthuldsmo4nnfoc6fa:3:10:1000000
-
-[[Image(file-encoding4.png)]]
-
-During download, when a peer begins to transmit a share, it first transmits
-all of the parts of the share hash tree that are necessary to validate its
-block root hash. Then it transmits the portions of the block hash tree
-that are necessary to validate the first block. Then it transmits the
-first block. It then continues this loop: transmitting any portions of the
-block hash tree to validate block#N, then sending block#N.
-
-[[Image(file-encoding5.png)]]
-
-So the "share" that is sent to the remote peer actually consists of three
-pieces, sent in a specific order as they become available, and retrieved
-during download in a different order according to when they are needed.
-
-The first piece is the blocks themselves, one per segment. The last
-block will likely be shorter than the rest, because the last segment is
-probably shorter than the rest. The second piece is the block hash tree,
-consisting of a total of two SHA-1 hashes per block. The third piece is a
-hash chain from the share hash tree, consisting of log2(numshares) hashes.
-
-During upload, all blocks are sent first, followed by the block hash
-tree, followed by the share hash chain. During download, the share hash chain
-is delivered first, followed by the block root hash. The client then uses
-the hash chain to validate the block root hash. Then the peer delivers
-enough of the block hash tree to validate the first block, followed by
-the first block itself. The block hash chain is used to validate the
-block, then it is passed (along with the first block from several other
-peers) into decoding, to produce the first segment of crypttext, which is
-then decrypted to produce the first segment of plaintext, which is finally
-delivered to the user.
-
-[[Image(file-encoding6.png)]]
-
-== Hashes ==
-
-All hashes use SHA-256d, as defined in Practical Cryptography (by Ferguson
-and Schneier). All hashes use a single-purpose tag, e.g. the hash that
-converts an encryption key into a storage index is defined as follows:
-
- SI = SHA256d(netstring("allmydata_immutable_key_to_storage_index_v1") + key)
-
-When two separate values need to be combined together in a hash, we wrap each
-in a netstring.
-
-Using SHA-256d (instead of plain SHA-256) guards against length-extension
-attacks. Using the tag protects our Merkle trees against attacks in which the
-hash of a leaf is confused with a hash of two children (allowing an attacker
-to generate corrupted data that nevertheless appears to be valid), and is
-simply good "cryptograhic hygiene". The "Chosen Protocol Attack" by Kelsey,
-Schneier, and Wagner (http://www.schneier.com/paper-chosen-protocol.html) is
-relevant. Putting the tag in a netstring guards against attacks that seek to
-confuse the end of the tag with the beginning of the subsequent value.
diff --git a/docs/file-encoding1.svg b/docs/file-encoding1.svg
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diff --git a/docs/mutable.txt b/docs/mutable.txt
deleted file mode 100644 (file)
index 40a5374..0000000
+++ /dev/null
@@ -1,648 +0,0 @@
-
-This describes the "RSA-based mutable files" which were shipped in Tahoe v0.8.0.
-
-= Mutable Files =
-
-Mutable File Slots are places with a stable identifier that can hold data
-that changes over time. In contrast to CHK slots, for which the
-URI/identifier is derived from the contents themselves, the Mutable File Slot
-URI remains fixed for the life of the slot, regardless of what data is placed
-inside it.
-
-Each mutable slot is referenced by two different URIs. The "read-write" URI
-grants read-write access to its holder, allowing them to put whatever
-contents they like into the slot. The "read-only" URI is less powerful, only
-granting read access, and not enabling modification of the data. The
-read-write URI can be turned into the read-only URI, but not the other way
-around.
-
-The data in these slots is distributed over a number of servers, using the
-same erasure coding that CHK files use, with 3-of-10 being a typical choice
-of encoding parameters. The data is encrypted and signed in such a way that
-only the holders of the read-write URI will be able to set the contents of
-the slot, and only the holders of the read-only URI will be able to read
-those contents. Holders of either URI will be able to validate the contents
-as being written by someone with the read-write URI. The servers who hold the
-shares cannot read or modify them: the worst they can do is deny service (by
-deleting or corrupting the shares), or attempt a rollback attack (which can
-only succeed with the cooperation of at least k servers).
-
-== Consistency vs Availability ==
-
-There is an age-old battle between consistency and availability. Epic papers
-have been written, elaborate proofs have been established, and generations of
-theorists have learned that you cannot simultaneously achieve guaranteed
-consistency with guaranteed reliability. In addition, the closer to 0 you get
-on either axis, the cost and complexity of the design goes up.
-
-Tahoe's design goals are to largely favor design simplicity, then slightly
-favor read availability, over the other criteria.
-
-As we develop more sophisticated mutable slots, the API may expose multiple
-read versions to the application layer. The tahoe philosophy is to defer most
-consistency recovery logic to the higher layers. Some applications have
-effective ways to merge multiple versions, so inconsistency is not
-necessarily a problem (i.e. directory nodes can usually merge multiple "add
-child" operations).
-
-== The Prime Coordination Directive: "Don't Do That" ==
-
-The current rule for applications which run on top of Tahoe is "do not
-perform simultaneous uncoordinated writes". That means you need non-tahoe
-means to make sure that two parties are not trying to modify the same mutable
-slot at the same time. For example:
-
- * don't give the read-write URI to anyone else. Dirnodes in a private
-   directory generally satisfy this case, as long as you don't use two
-   clients on the same account at the same time
- * if you give a read-write URI to someone else, stop using it yourself. An
-   inbox would be a good example of this.
- * if you give a read-write URI to someone else, call them on the phone
-   before you write into it
- * build an automated mechanism to have your agents coordinate writes.
-   For example, we expect a future release to include a FURL for a
-   "coordination server" in the dirnodes. The rule can be that you must
-   contact the coordination server and obtain a lock/lease on the file
-   before you're allowed to modify it.
-
-If you do not follow this rule, Bad Things will happen. The worst-case Bad
-Thing is that the entire file will be lost. A less-bad Bad Thing is that one
-or more of the simultaneous writers will lose their changes. An observer of
-the file may not see monotonically-increasing changes to the file, i.e. they
-may see version 1, then version 2, then 3, then 2 again.
-
-Tahoe takes some amount of care to reduce the badness of these Bad Things.
-One way you can help nudge it from the "lose your file" case into the "lose
-some changes" case is to reduce the number of competing versions: multiple
-versions of the file that different parties are trying to establish as the
-one true current contents. Each simultaneous writer counts as a "competing
-version", as does the previous version of the file. If the count "S" of these
-competing versions is larger than N/k, then the file runs the risk of being
-lost completely. [TODO] If at least one of the writers remains running after
-the collision is detected, it will attempt to recover, but if S>(N/k) and all
-writers crash after writing a few shares, the file will be lost.
-
-Note that Tahoe uses serialization internally to make sure that a single
-Tahoe node will not perform simultaneous modifications to a mutable file. It
-accomplishes this by using a weakref cache of the MutableFileNode (so that
-there will never be two distinct MutableFileNodes for the same file), and by
-forcing all mutable file operations to obtain a per-node lock before they
-run. The Prime Coordination Directive therefore applies to inter-node
-conflicts, not intra-node ones.
-
-
-== Small Distributed Mutable Files ==
-
-SDMF slots are suitable for small (<1MB) files that are editing by rewriting
-the entire file. The three operations are:
-
- * allocate (with initial contents)
- * set (with new contents)
- * get (old contents)
-
-The first use of SDMF slots will be to hold directories (dirnodes), which map
-encrypted child names to rw-URI/ro-URI pairs.
-
-=== SDMF slots overview ===
-
-Each SDMF slot is created with a public/private key pair. The public key is
-known as the "verification key", while the private key is called the
-"signature key". The private key is hashed and truncated to 16 bytes to form
-the "write key" (an AES symmetric key). The write key is then hashed and
-truncated to form the "read key". The read key is hashed and truncated to
-form the 16-byte "storage index" (a unique string used as an index to locate
-stored data).
-
-The public key is hashed by itself to form the "verification key hash".
-
-The write key is hashed a different way to form the "write enabler master".
-For each storage server on which a share is kept, the write enabler master is
-concatenated with the server's nodeid and hashed, and the result is called
-the "write enabler" for that particular server. Note that multiple shares of
-the same slot stored on the same server will all get the same write enabler,
-i.e. the write enabler is associated with the "bucket", rather than the
-individual shares.
-
-The private key is encrypted (using AES in counter mode) by the write key,
-and the resulting crypttext is stored on the servers. so it will be
-retrievable by anyone who knows the write key. The write key is not used to
-encrypt anything else, and the private key never changes, so we do not need
-an IV for this purpose.
-
-The actual data is encrypted (using AES in counter mode) with a key derived
-by concatenating the readkey with the IV, the hashing the results and
-truncating to 16 bytes. The IV is randomly generated each time the slot is
-updated, and stored next to the encrypted data.
-
-The read-write URI consists of the write key and the verification key hash.
-The read-only URI contains the read key and the verification key hash. The
-verify-only URI contains the storage index and the verification key hash.
-
- URI:SSK-RW:b2a(writekey):b2a(verification_key_hash)
- URI:SSK-RO:b2a(readkey):b2a(verification_key_hash)
- URI:SSK-Verify:b2a(storage_index):b2a(verification_key_hash)
-
-Note that this allows the read-only and verify-only URIs to be derived from
-the read-write URI without actually retrieving the public keys. Also note
-that it means the read-write agent must validate both the private key and the
-public key when they are first fetched. All users validate the public key in
-exactly the same way.
-
-The SDMF slot is allocated by sending a request to the storage server with a
-desired size, the storage index, and the write enabler for that server's
-nodeid. If granted, the write enabler is stashed inside the slot's backing
-store file. All further write requests must be accompanied by the write
-enabler or they will not be honored. The storage server does not share the
-write enabler with anyone else.
-
-The SDMF slot structure will be described in more detail below. The important
-pieces are:
-
-  * a sequence number
-  * a root hash "R"
-  * the encoding parameters (including k, N, file size, segment size)
-  * a signed copy of [seqnum,R,encoding_params], using the signature key
-  * the verification key (not encrypted)
-  * the share hash chain (part of a Merkle tree over the share hashes)
-  * the block hash tree (Merkle tree over blocks of share data)
-  * the share data itself (erasure-coding of read-key-encrypted file data)
-  * the signature key, encrypted with the write key
-
-The access pattern for read is:
- * hash read-key to get storage index
- * use storage index to locate 'k' shares with identical 'R' values
-   * either get one share, read 'k' from it, then read k-1 shares
-   * or read, say, 5 shares, discover k, either get more or be finished
-   * or copy k into the URIs
- * read verification key
- * hash verification key, compare against verification key hash
- * read seqnum, R, encoding parameters, signature
- * verify signature against verification key
- * read share data, compute block-hash Merkle tree and root "r"
- * read share hash chain (leading from "r" to "R")
- * validate share hash chain up to the root "R"
- * submit share data to erasure decoding
- * decrypt decoded data with read-key
- * submit plaintext to application
-
-The access pattern for write is:
- * hash write-key to get read-key, hash read-key to get storage index
- * use the storage index to locate at least one share
- * read verification key and encrypted signature key
- * decrypt signature key using write-key
- * hash signature key, compare against write-key
- * hash verification key, compare against verification key hash
- * encrypt plaintext from application with read-key
-   * application can encrypt some data with the write-key to make it only
-     available to writers (use this for transitive read-onlyness of dirnodes)
- * erasure-code crypttext to form shares
- * split shares into blocks
- * compute Merkle tree of blocks, giving root "r" for each share
- * compute Merkle tree of shares, find root "R" for the file as a whole
- * create share data structures, one per server:
-   * use seqnum which is one higher than the old version
-   * share hash chain has log(N) hashes, different for each server
-   * signed data is the same for each server
- * now we have N shares and need homes for them
- * walk through peers
-   * if share is not already present, allocate-and-set
-   * otherwise, try to modify existing share:
-   * send testv_and_writev operation to each one
-   * testv says to accept share if their(seqnum+R) <= our(seqnum+R)
-   * count how many servers wind up with which versions (histogram over R)
-   * keep going until N servers have the same version, or we run out of servers
-     * if any servers wound up with a different version, report error to
-       application
-     * if we ran out of servers, initiate recovery process (described below)
-
-=== Server Storage Protocol ===
-
-The storage servers will provide a mutable slot container which is oblivious
-to the details of the data being contained inside it. Each storage index
-refers to a "bucket", and each bucket has one or more shares inside it. (In a
-well-provisioned network, each bucket will have only one share). The bucket
-is stored as a directory, using the base32-encoded storage index as the
-directory name. Each share is stored in a single file, using the share number
-as the filename.
-
-The container holds space for a container magic number (for versioning), the
-write enabler, the nodeid which accepted the write enabler (used for share
-migration, described below), a small number of lease structures, the embedded
-data itself, and expansion space for additional lease structures.
-
- #   offset    size    name
- 1   0         32      magic verstr "tahoe mutable container v1" plus binary
- 2   32        20      write enabler's nodeid
- 3   52        32      write enabler
- 4   84        8       data size (actual share data present) (a)
- 5   92        8       offset of (8) count of extra leases (after data)
- 6   100       368     four leases, 92 bytes each
-                        0    4   ownerid (0 means "no lease here")
-                        4    4   expiration timestamp
-                        8   32   renewal token
-                        40  32   cancel token
-                        72  20   nodeid which accepted the tokens
- 7   468       (a)     data
- 8   ??        4       count of extra leases
- 9   ??        n*92    extra leases
-
-The "extra leases" field must be copied and rewritten each time the size of
-the enclosed data changes. The hope is that most buckets will have four or
-fewer leases and this extra copying will not usually be necessary.
-
-The (4) "data size" field contains the actual number of bytes of data present
-in field (7), such that a client request to read beyond 504+(a) will result
-in an error. This allows the client to (one day) read relative to the end of
-the file. The container size (that is, (8)-(7)) might be larger, especially
-if extra size was pre-allocated in anticipation of filling the container with
-a lot of data.
-
-The offset in (5) points at the *count* of extra leases, at (8). The actual
-leases (at (9)) begin 4 bytes later. If the container size changes, both (8)
-and (9) must be relocated by copying.
-
-The server will honor any write commands that provide the write token and do
-not exceed the server-wide storage size limitations. Read and write commands
-MUST be restricted to the 'data' portion of the container: the implementation
-of those commands MUST perform correct bounds-checking to make sure other
-portions of the container are inaccessible to the clients.
-
-The two methods provided by the storage server on these "MutableSlot" share
-objects are:
-
- * readv(ListOf(offset=int, length=int))
-   * returns a list of bytestrings, of the various requested lengths
-   * offset < 0 is interpreted relative to the end of the data
-   * spans which hit the end of the data will return truncated data
-
- * testv_and_writev(write_enabler, test_vector, write_vector)
-   * this is a test-and-set operation which performs the given tests and only
-     applies the desired writes if all tests succeed. This is used to detect
-     simultaneous writers, and to reduce the chance that an update will lose
-     data recently written by some other party (written after the last time
-     this slot was read).
-   * test_vector=ListOf(TupleOf(offset, length, opcode, specimen))
-   * the opcode is a string, from the set [gt, ge, eq, le, lt, ne]
-   * each element of the test vector is read from the slot's data and 
-     compared against the specimen using the desired (in)equality. If all
-     tests evaluate True, the write is performed
-   * write_vector=ListOf(TupleOf(offset, newdata))
-     * offset < 0 is not yet defined, it probably means relative to the
-       end of the data, which probably means append, but we haven't nailed
-       it down quite yet
-     * write vectors are executed in order, which specifies the results of
-       overlapping writes
-   * return value:
-     * error: OutOfSpace
-     * error: something else (io error, out of memory, whatever)
-     * (True, old_test_data): the write was accepted (test_vector passed)
-     * (False, old_test_data): the write was rejected (test_vector failed)
-       * both 'accepted' and 'rejected' return the old data that was used
-         for the test_vector comparison. This can be used by the client
-         to detect write collisions, including collisions for which the
-         desired behavior was to overwrite the old version.
-
-In addition, the storage server provides several methods to access these
-share objects:
-
- * allocate_mutable_slot(storage_index, sharenums=SetOf(int))
-   * returns DictOf(int, MutableSlot)
- * get_mutable_slot(storage_index)
-   * returns DictOf(int, MutableSlot)
-   * or raises KeyError
-
-We intend to add an interface which allows small slots to allocate-and-write
-in a single call, as well as do update or read in a single call. The goal is
-to allow a reasonably-sized dirnode to be created (or updated, or read) in
-just one round trip (to all N shareholders in parallel).
-
-==== migrating shares ====
-
-If a share must be migrated from one server to another, two values become
-invalid: the write enabler (since it was computed for the old server), and
-the lease renew/cancel tokens.
-
-Suppose that a slot was first created on nodeA, and was thus initialized with
-WE(nodeA) (= H(WEM+nodeA)). Later, for provisioning reasons, the share is
-moved from nodeA to nodeB.
-
-Readers may still be able to find the share in its new home, depending upon
-how many servers are present in the grid, where the new nodeid lands in the
-permuted index for this particular storage index, and how many servers the
-reading client is willing to contact.
-
-When a client attempts to write to this migrated share, it will get a "bad
-write enabler" error, since the WE it computes for nodeB will not match the
-WE(nodeA) that was embedded in the share. When this occurs, the "bad write
-enabler" message must include the old nodeid (e.g. nodeA) that was in the
-share.
-
-The client then computes H(nodeB+H(WEM+nodeA)), which is the same as
-H(nodeB+WE(nodeA)). The client sends this along with the new WE(nodeB), which
-is H(WEM+nodeB). Note that the client only sends WE(nodeB) to nodeB, never to
-anyone else. Also note that the client does not send a value to nodeB that
-would allow the node to impersonate the client to a third node: everything
-sent to nodeB will include something specific to nodeB in it.
-
-The server locally computes H(nodeB+WE(nodeA)), using its own node id and the
-old write enabler from the share. It compares this against the value supplied
-by the client. If they match, this serves as proof that the client was able
-to compute the old write enabler. The server then accepts the client's new
-WE(nodeB) and writes it into the container.
-
-This WE-fixup process requires an extra round trip, and requires the error
-message to include the old nodeid, but does not require any public key
-operations on either client or server.
-
-Migrating the leases will require a similar protocol. This protocol will be
-defined concretely at a later date.
-
-=== Code Details ===
-
-The MutableFileNode class is used to manipulate mutable files (as opposed to
-ImmutableFileNodes). These are initially generated with
-client.create_mutable_file(), and later recreated from URIs with
-client.create_node_from_uri(). Instances of this class will contain a URI and
-a reference to the client (for peer selection and connection).
-
-NOTE: this section is out of date. Please see src/allmydata/interfaces.py
-(the section on IMutableFilesystemNode) for more accurate information.
-
-The methods of MutableFileNode are:
-
- * download_to_data() -> [deferred] newdata, NotEnoughSharesError
-   * if there are multiple retrieveable versions in the grid, get() returns
-     the first version it can reconstruct, and silently ignores the others.
-     In the future, a more advanced API will signal and provide access to
-     the multiple heads.
- * update(newdata) -> OK, UncoordinatedWriteError, NotEnoughSharesError
- * overwrite(newdata) -> OK, UncoordinatedWriteError, NotEnoughSharesError
-
-download_to_data() causes a new retrieval to occur, pulling the current
-contents from the grid and returning them to the caller. At the same time,
-this call caches information about the current version of the file. This
-information will be used in a subsequent call to update(), and if another
-change has occured between the two, this information will be out of date,
-triggering the UncoordinatedWriteError.
-
-update() is therefore intended to be used just after a download_to_data(), in
-the following pattern:
-
- d = mfn.download_to_data()
- d.addCallback(apply_delta)
- d.addCallback(mfn.update)
-
-If the update() call raises UCW, then the application can simply return an
-error to the user ("you violated the Prime Coordination Directive"), and they
-can try again later. Alternatively, the application can attempt to retry on
-its own. To accomplish this, the app needs to pause, download the new
-(post-collision and post-recovery) form of the file, reapply their delta,
-then submit the update request again. A randomized pause is necessary to
-reduce the chances of colliding a second time with another client that is
-doing exactly the same thing:
-
- d = mfn.download_to_data()
- d.addCallback(apply_delta)
- d.addCallback(mfn.update)
- def _retry(f):
-   f.trap(UncoordinatedWriteError)
-   d1 = pause(random.uniform(5, 20))
-   d1.addCallback(lambda res: mfn.download_to_data())
-   d1.addCallback(apply_delta)
-   d1.addCallback(mfn.update)
-   return d1
- d.addErrback(_retry)
-
-Enthusiastic applications can retry multiple times, using a randomized
-exponential backoff between each. A particularly enthusiastic application can
-retry forever, but such apps are encouraged to provide a means to the user of
-giving up after a while.
-
-UCW does not mean that the update was not applied, so it is also a good idea
-to skip the retry-update step if the delta was already applied:
-
- d = mfn.download_to_data()
- d.addCallback(apply_delta)
- d.addCallback(mfn.update)
- def _retry(f):
-   f.trap(UncoordinatedWriteError)
-   d1 = pause(random.uniform(5, 20))
-   d1.addCallback(lambda res: mfn.download_to_data())
-   def _maybe_apply_delta(contents):
-     new_contents = apply_delta(contents)
-     if new_contents != contents:
-       return mfn.update(new_contents)
-   d1.addCallback(_maybe_apply_delta)
-   return d1
- d.addErrback(_retry)
-
-update() is the right interface to use for delta-application situations, like
-directory nodes (in which apply_delta might be adding or removing child
-entries from a serialized table).
-
-Note that any uncoordinated write has the potential to lose data. We must do
-more analysis to be sure, but it appears that two clients who write to the
-same mutable file at the same time (even if both eventually retry) will, with
-high probability, result in one client observing UCW and the other silently
-losing their changes. It is also possible for both clients to observe UCW.
-The moral of the story is that the Prime Coordination Directive is there for
-a reason, and that recovery/UCW/retry is not a subsitute for write
-coordination.
-
-overwrite() tells the client to ignore this cached version information, and
-to unconditionally replace the mutable file's contents with the new data.
-This should not be used in delta application, but rather in situations where
-you want to replace the file's contents with completely unrelated ones. When
-raw files are uploaded into a mutable slot through the tahoe webapi (using
-POST and the ?mutable=true argument), they are put in place with overwrite().
-
-
-
-The peer-selection and data-structure manipulation (and signing/verification)
-steps will be implemented in a separate class in allmydata/mutable.py .
-
-=== SMDF Slot Format ===
-
-This SMDF data lives inside a server-side MutableSlot container. The server
-is oblivious to this format.
-
-This data is tightly packed. In particular, the share data is defined to run
-all the way to the beginning of the encrypted private key (the encprivkey
-offset is used both to terminate the share data and to begin the encprivkey).
-
- #    offset   size    name
- 1    0        1       version byte, \x00 for this format
- 2    1        8       sequence number. 2^64-1 must be handled specially, TBD
- 3    9        32      "R" (root of share hash Merkle tree)
- 4    41       16      IV (share data is AES(H(readkey+IV)) )
- 5    57       18      encoding parameters:
-       57       1        k
-       58       1        N
-       59       8        segment size
-       67       8        data length (of original plaintext)
- 6    75       32      offset table:
-       75       4        (8) signature
-       79       4        (9) share hash chain
-       83       4        (10) block hash tree
-       87       4        (11) share data
-       91       8        (12) encrypted private key
-       99       8        (13) EOF
- 7    107      436ish  verification key (2048 RSA key)
- 8    543ish   256ish  signature=RSAenc(sigkey, H(version+seqnum+r+IV+encparm))
- 9    799ish   (a)     share hash chain, encoded as:
-                        "".join([pack(">H32s", shnum, hash)
-                                 for (shnum,hash) in needed_hashes])
-10    (927ish) (b)     block hash tree, encoded as:
-                        "".join([pack(">32s",hash) for hash in block_hash_tree])
-11    (935ish) LEN     share data (no gap between this and encprivkey)
-12    ??       1216ish encrypted private key= AESenc(write-key, RSA-key)
-13    ??       --      EOF
-
-(a) The share hash chain contains ceil(log(N)) hashes, each 32 bytes long.
-    This is the set of hashes necessary to validate this share's leaf in the
-    share Merkle tree. For N=10, this is 4 hashes, i.e. 128 bytes.
-(b) The block hash tree contains ceil(length/segsize) hashes, each 32 bytes
-    long. This is the set of hashes necessary to validate any given block of
-    share data up to the per-share root "r". Each "r" is a leaf of the share
-    has tree (with root "R"), from which a minimal subset of hashes is put in
-    the share hash chain in (8).
-
-=== Recovery ===
-
-The first line of defense against damage caused by colliding writes is the
-Prime Coordination Directive: "Don't Do That".
-
-The second line of defense is to keep "S" (the number of competing versions)
-lower than N/k. If this holds true, at least one competing version will have
-k shares and thus be recoverable. Note that server unavailability counts
-against us here: the old version stored on the unavailable server must be
-included in the value of S.
-
-The third line of defense is our use of testv_and_writev() (described below),
-which increases the convergence of simultaneous writes: one of the writers
-will be favored (the one with the highest "R"), and that version is more
-likely to be accepted than the others. This defense is least effective in the
-pathological situation where S simultaneous writers are active, the one with
-the lowest "R" writes to N-k+1 of the shares and then dies, then the one with
-the next-lowest "R" writes to N-2k+1 of the shares and dies, etc, until the
-one with the highest "R" writes to k-1 shares and dies. Any other sequencing
-will allow the highest "R" to write to at least k shares and establish a new
-revision.
-
-The fourth line of defense is the fact that each client keeps writing until
-at least one version has N shares. This uses additional servers, if
-necessary, to make sure that either the client's version or some
-newer/overriding version is highly available.
-
-The fifth line of defense is the recovery algorithm, which seeks to make sure
-that at least *one* version is highly available, even if that version is
-somebody else's.
-
-The write-shares-to-peers algorithm is as follows:
-
- * permute peers according to storage index
- * walk through peers, trying to assign one share per peer
- * for each peer:
-   * send testv_and_writev, using "old(seqnum+R) <= our(seqnum+R)" as the test
-     * this means that we will overwrite any old versions, and we will
-       overwrite simultaenous writers of the same version if our R is higher.
-       We will not overwrite writers using a higher seqnum.
-   * record the version that each share winds up with. If the write was
-     accepted, this is our own version. If it was rejected, read the
-     old_test_data to find out what version was retained.
-   * if old_test_data indicates the seqnum was equal or greater than our
-     own, mark the "Simultanous Writes Detected" flag, which will eventually
-     result in an error being reported to the writer (in their close() call).
-   * build a histogram of "R" values
-   * repeat until the histogram indicate that some version (possibly ours)
-     has N shares. Use new servers if necessary.
-   * If we run out of servers:
-     * if there are at least shares-of-happiness of any one version, we're
-       happy, so return. (the close() might still get an error)
-     * not happy, need to reinforce something, goto RECOVERY
-
-RECOVERY:
- * read all shares, count the versions, identify the recoverable ones,
-   discard the unrecoverable ones.
- * sort versions: locate max(seqnums), put all versions with that seqnum
-   in the list, sort by number of outstanding shares. Then put our own
-   version. (TODO: put versions with seqnum <max but >us ahead of us?).
- * for each version:
-   * attempt to recover that version
-   * if not possible, remove it from the list, go to next one
-   * if recovered, start at beginning of peer list, push that version,
-     continue until N shares are placed
-   * if pushing our own version, bump up the seqnum to one higher than
-     the max seqnum we saw
-   * if we run out of servers:
-     * schedule retry and exponential backoff to repeat RECOVERY
-   * admit defeat after some period? presumeably the client will be shut down
-     eventually, maybe keep trying (once per hour?) until then.
-
-
-
-
-== Medium Distributed Mutable Files ==
-
-These are just like the SDMF case, but:
-
- * we actually take advantage of the Merkle hash tree over the blocks, by
-   reading a single segment of data at a time (and its necessary hashes), to
-   reduce the read-time alacrity
- * we allow arbitrary writes to the file (i.e. seek() is provided, and
-   O_TRUNC is no longer required)
- * we write more code on the client side (in the MutableFileNode class), to
-   first read each segment that a write must modify. This looks exactly like
-   the way a normal filesystem uses a block device, or how a CPU must perform
-   a cache-line fill before modifying a single word.
- * we might implement some sort of copy-based atomic update server call,
-   to allow multiple writev() calls to appear atomic to any readers.
-
-MDMF slots provide fairly efficient in-place edits of very large files (a few
-GB). Appending data is also fairly efficient, although each time a power of 2
-boundary is crossed, the entire file must effectively be re-uploaded (because
-the size of the block hash tree changes), so if the filesize is known in
-advance, that space ought to be pre-allocated (by leaving extra space between
-the block hash tree and the actual data).
-
-MDMF1 uses the Merkle tree to enable low-alacrity random-access reads. MDMF2
-adds cache-line reads to allow random-access writes.
-
-== Large Distributed Mutable Files ==
-
-LDMF slots use a fundamentally different way to store the file, inspired by
-Mercurial's "revlog" format. They enable very efficient insert/remove/replace
-editing of arbitrary spans. Multiple versions of the file can be retained, in
-a revision graph that can have multiple heads. Each revision can be
-referenced by a cryptographic identifier. There are two forms of the URI, one
-that means "most recent version", and a longer one that points to a specific
-revision.
-
-Metadata can be attached to the revisions, like timestamps, to enable rolling
-back an entire tree to a specific point in history.
-
-LDMF1 provides deltas but tries to avoid dealing with multiple heads. LDMF2
-provides explicit support for revision identifiers and branching.
-
-== TODO ==
-
-improve allocate-and-write or get-writer-buckets API to allow one-call (or
-maybe two-call) updates. The challenge is in figuring out which shares are on
-which machines. First cut will have lots of round trips.
-
-(eventually) define behavior when seqnum wraps. At the very least make sure
-it can't cause a security problem. "the slot is worn out" is acceptable.
-
-(eventually) define share-migration lease update protocol. Including the
-nodeid who accepted the lease is useful, we can use the same protocol as we
-do for updating the write enabler. However we need to know which lease to
-update.. maybe send back a list of all old nodeids that we find, then try all
-of them when we accept the update?
-
- We now do this in a specially-formatted IndexError exception:
-  "UNABLE to renew non-existent lease. I have leases accepted by " +
-  "nodeids: '12345','abcde','44221' ."
-
-confirm that a repairer can regenerate shares without the private key. Hmm,
-without the write-enabler they won't be able to write those shares to the
-servers.. although they could add immutable new shares to new servers.
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+         y="952.95337">B is derived from A by hashing, therefore B validates A</tspan></text>
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+       x="217.67101"
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+         y="978.66254"
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+         sodipodi:role="line"
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+         y="1006.0856">A is used as an index to retrieve data B</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:32px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#1bcc03;fill-opacity:1;stroke:#1bcc03;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
+       x="625.5899"
+       y="740.42419"
+       id="text3497"
+       sodipodi:linespacing="100%"><tspan
+         sodipodi:role="line"
+         id="tspan3499"
+         x="625.5899"
+         y="740.42419">SHARE</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:32px;font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;text-align:start;line-height:100%;writing-mode:lr-tb;text-anchor:start;fill:#000000;fill-opacity:1;stroke:none;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;font-family:Bitstream Charter"
+       x="249.20799"
+       y="30.851007"
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+         sodipodi:role="line"
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+         x="249.20799"
+         y="30.851007">CHK File Hashes</tspan></text>
+  </g>
+</svg>
diff --git a/docs/specifications/Makefile b/docs/specifications/Makefile
new file mode 100644 (file)
index 0000000..2625e04
--- /dev/null
@@ -0,0 +1,19 @@
+SOURCES = CHK-hashes.svg file-encoding1.svg file-encoding2.svg \
+       file-encoding3.svg file-encoding4.svg file-encoding5.svg \
+       file-encoding6.svg
+
+PNGS = $(patsubst %.svg,%.png,$(SOURCES))
+EPSS = $(patsubst %.svg,%.eps,$(SOURCES))
+
+.PHONY: images-png images-eps
+all: $(PNGS) $(EPSS)
+images-png: $(PNGS)
+images-eps: $(EPSS)
+
+%.png: %.svg
+       inkscape -b white -d 90 -D --export-png $@ $<
+%.eps: %.svg
+       inkscape --export-eps $@ $<
+
+clean:
+       rm -f *.png *.eps
diff --git a/docs/specifications/URI-extension.txt b/docs/specifications/URI-extension.txt
new file mode 100644 (file)
index 0000000..8ec383e
--- /dev/null
@@ -0,0 +1,61 @@
+
+"URI Extension Block"
+
+This block is a serialized dictionary with string keys and string values
+(some of which represent numbers, some of which are SHA-256 hashes). All
+buckets hold an identical copy. The hash of the serialized data is kept in
+the URI.
+
+The download process must obtain a valid copy of this data before any
+decoding can take place. The download process must also obtain other data
+before incremental validation can be performed. Full-file validation (for
+clients who do not wish to do incremental validation) can be performed solely
+with the data from this block.
+
+At the moment, this data block contains the following keys (and an estimate
+on their sizes):
+
+ size                5
+ segment_size        7
+ num_segments        2
+ needed_shares       2
+ total_shares        3
+
+ codec_name          3
+ codec_params        5+1+2+1+3=12
+ tail_codec_params   12
+
+ share_root_hash     32 (binary) or 52 (base32-encoded) each
+ plaintext_hash
+ plaintext_root_hash
+ crypttext_hash
+ crypttext_root_hash
+
+Some pieces are needed elsewhere (size should be visible without pulling the
+block, the Tahoe3 algorithm needs total_shares to find the right peers, all
+peer selection algorithms need needed_shares to ask a minimal set of peers).
+Some pieces are arguably redundant but are convenient to have present
+(test_encode.py makes use of num_segments).
+
+The rule for this data block is that it should be a constant size for all
+files, regardless of file size. Therefore hash trees (which have a size that
+depends linearly upon the number of segments) are stored elsewhere in the
+bucket, with only the hash tree root stored in this data block.
+
+This block will be serialized as follows:
+
+ assert that all keys match ^[a-zA-z_\-]+$
+ sort all the keys lexicographically
+ for k in keys:
+  write("%s:" % k)
+  write(netstring(data[k]))
+
+
+Serialized size:
+
+ dense binary (but decimal) packing: 160+46=206
+ including 'key:' (185) and netstring (6*3+7*4=46) on values: 231
+ including 'key:%d\n' (185+13=198) and printable values (46+5*52=306)=504
+
+We'll go with the 231-sized block, and provide a tool to dump it as text if
+we really want one.
diff --git a/docs/specifications/dirnodes.txt b/docs/specifications/dirnodes.txt
new file mode 100644 (file)
index 0000000..adc8fca
--- /dev/null
@@ -0,0 +1,433 @@
+
+= Tahoe Directory Nodes =
+
+As explained in the architecture docs, Tahoe can be roughly viewed as a
+collection of three layers. The lowest layer is the distributed filestore, or
+DHT: it provides operations that accept files and upload them to the mesh,
+creating a URI in the process which securely references the file's contents.
+The middle layer is the filesystem, creating a structure of directories and
+filenames resembling the traditional unix/windows filesystems. The top layer
+is the application layer, which uses the lower layers to provide useful
+services to users, like a backup application, or a way to share files with
+friends.
+
+This document examines the middle layer, the "filesystem".
+
+== DHT Primitives ==
+
+In the lowest layer (DHT), there are two operations that reference immutable
+data (which we refer to as "CHK URIs" or "CHK read-capabilities" or "CHK
+read-caps"). One puts data into the grid (but only if it doesn't exist
+already), the other retrieves it:
+
+ chk_uri = put(data)
+ data = get(chk_uri)
+
+We also have three operations which reference mutable data (which we refer to
+as "mutable slots", or "mutable write-caps and read-caps", or sometimes "SSK
+slots"). One creates a slot with some initial contents, a second replaces the
+contents of a pre-existing slot, and the third retrieves the contents:
+
+ mutable_uri = create(initial_data)
+ replace(mutable_uri, new_data)
+ data = get(mutable_uri)
+
+== Filesystem Goals ==
+
+The main goal for the middle (filesystem) layer is to give users a way to
+organize the data that they have uploaded into the mesh. The traditional way
+to do this in computer filesystems is to put this data into files, give those
+files names, and collect these names into directories.
+
+Each directory is a series of name-value pairs, which maps "child name" to an
+object of some kind. Those child objects might be files, or they might be
+other directories.
+
+The directory structure is therefore a directed graph of nodes, in which each
+node might be a directory node or a file node. All file nodes are terminal
+nodes.
+
+== Dirnode Goals ==
+
+What properties might be desirable for these directory nodes? In no
+particular order:
+
+ 1: functional. Code which does not work doesn't count.
+ 2: easy to document, explain, and understand
+ 3: confidential: it should not be possible for others to see the contents of
+                  a directory
+ 4: integrity: it should not be possible for others to modify the contents
+               of a directory
+ 5: available: directories should survive host failure, just like files do
+ 6: efficient: in storage, communication bandwidth, number of round-trips
+ 7: easy to delegate individual directories in a flexible way
+ 8: updateness: everybody looking at a directory should see the same contents
+ 9: monotonicity: everybody looking at a directory should see the same
+                  sequence of updates
+
+Some of these goals are mutually exclusive. For example, availability and
+consistency are opposing, so it is not possible to achieve #5 and #8 at the
+same time. Moreover, it takes a more complex architecture to get close to the
+available-and-consistent ideal, so #2/#6 is in opposition to #5/#8.
+
+Tahoe-0.7.0 introduced distributed mutable files, which use public key
+cryptography for integrity, and erasure coding for availability. These
+achieve roughly the same properties as immutable CHK files, but their
+contents can be replaced without changing their identity. Dirnodes are then
+just a special way of interpreting the contents of a specific mutable file.
+Earlier releases used a "vdrive server": this server was abolished in the
+0.7.0 release.
+
+For details of how mutable files work, please see "mutable.txt" in this
+directory.
+
+For the current 0.7.0 release, we achieve most of our desired properties. The
+integrity and availability of dirnodes is equivalent to that of regular
+(immutable) files, with the exception that there are more simultaneous-update
+failure modes for mutable slots. Delegation is quite strong: you can give
+read-write or read-only access to any subtree, and the data format used for
+dirnodes is such that read-only access is transitive: i.e. if you grant Bob
+read-only access to a parent directory, then Bob will get read-only access
+(and *not* read-write access) to its children.
+
+Relative to the previous "vdrive-server" based scheme, the current
+distributed dirnode approach gives better availability, but cannot guarantee
+updateness quite as well, and requires far more network traffic for each
+retrieval and update. Mutable files are somewhat less available than
+immutable files, simply because of the increased number of combinations
+(shares of an immutable file are either present or not, whereas there are
+multiple versions of each mutable file, and you might have some shares of
+version 1 and other shares of version 2). In extreme cases of simultaneous
+update, mutable files might suffer from non-monotonicity.
+
+
+== Dirnode secret values ==
+
+As mentioned before, dirnodes are simply a special way to interpret the
+contents of a mutable file, so the secret keys and capability strings
+described in "mutable.txt" are all the same. Each dirnode contains an RSA
+public/private keypair, and the holder of the "write capability" will be able
+to retrieve the private key (as well as the AES encryption key used for the
+data itself). The holder of the "read capability" will be able to obtain the
+public key and the AES data key, but not the RSA private key needed to modify
+the data.
+
+The "write capability" for a dirnode grants read-write access to its
+contents. This is expressed on concrete form as the "dirnode write cap": a
+printable string which contains the necessary secrets to grant this access.
+Likewise, the "read capability" grants read-only access to a dirnode, and can
+be represented by a "dirnode read cap" string.
+
+For example,
+URI:DIR2:swdi8ge1s7qko45d3ckkyw1aac%3Aar8r5j99a4mezdojejmsfp4fj1zeky9gjigyrid4urxdimego68o
+is a write-capability URI, while
+URI:DIR2-RO:buxjqykt637u61nnmjg7s8zkny:ar8r5j99a4mezdojejmsfp4fj1zeky9gjigyrid4urxdimego68o
+is a read-capability URI, both for the same dirnode.
+
+
+== Dirnode storage format ==
+
+Each dirnode is stored in a single mutable file, distributed in the Tahoe
+grid. The contents of this file are a serialized list of netstrings, one per
+child. Each child is a list of four netstrings: (name, rocap, rwcap,
+metadata). (remember that the contents of the mutable file are encrypted by
+the read-cap, so this section describes the plaintext contents of the mutable
+file, *after* it has been decrypted by the read-cap).
+
+The name is simple a UTF-8 -encoded child name. The 'rocap' is a read-only
+capability URI to that child, either an immutable (CHK) file, a mutable file,
+or a directory. The 'rwcap' is a read-write capability URI for that child,
+encrypted with the dirnode's write-cap: this enables the "transitive
+readonlyness" property, described further below. The 'metadata' is a
+JSON-encoded dictionary of type,value metadata pairs. Some metadata keys are
+pre-defined, the rest are left up to the application.
+
+Each rwcap is stored as IV + ciphertext + MAC. The IV is a 16-byte random
+value. The ciphertext is obtained by using AES in CTR mode on the rwcap URI
+string, using a key that is formed from a tagged hash of the IV and the
+dirnode's writekey. The MAC is a 32-byte SHA-256 -based HMAC (using that same
+AES key) over the (IV+ciphertext) pair.
+
+If Bob has read-only access to the 'bar' directory, and he adds it as a child
+to the 'foo' directory, then he will put the read-only cap for 'bar' in both
+the rwcap and rocap slots (encrypting the rwcap contents as described above).
+If he has full read-write access to 'bar', then he will put the read-write
+cap in the 'rwcap' slot, and the read-only cap in the 'rocap' slot. Since
+other users who have read-only access to 'foo' will be unable to decrypt its
+rwcap slot, this limits those users to read-only access to 'bar' as well,
+thus providing the transitive readonlyness that we desire.
+
+=== Dirnode sizes, mutable-file initial read sizes ===
+
+How big are dirnodes? When reading dirnode data out of mutable files, how
+large should our initial read be? If we guess exactly, we can read a dirnode
+in a single round-trip, and update one in two RTT. If we guess too high,
+we'll waste some amount of bandwidth. If we guess low, we need to make a
+second pass to get the data (or the encrypted privkey, for writes), which
+will cost us at least another RTT.
+
+Assuming child names are between 10 and 99 characters long, how long are the
+various pieces of a dirnode?
+
+ netstring(name) ~= 4+len(name)
+ chk-cap = 97 (for 4-char filesizes)
+ dir-rw-cap = 88
+ dir-ro-cap = 91
+ netstring(cap) = 4+len(cap)
+ encrypted(cap) = 16+cap+32
+ JSON({}) = 2
+ JSON({ctime=float,mtime=float}): 57
+ netstring(metadata) = 4+57 = 61
+
+so a CHK entry is:
+ 5+ 4+len(name) + 4+97 + 5+16+97+32 + 4+57
+And a 15-byte filename gives a 336-byte entry. When the entry points at a
+subdirectory instead of a file, the entry is a little bit smaller. So an
+empty directory uses 0 bytes, a directory with one child uses about 336
+bytes, a directory with two children uses about 672, etc.
+
+When the dirnode data is encoding using our default 3-of-10, that means we
+get 112ish bytes of data in each share per child.
+
+The pubkey, signature, and hashes form the first 935ish bytes of the
+container, then comes our data, then about 1216 bytes of encprivkey. So if we
+read the first:
+
+ 1kB: we get 65bytes of dirnode data : only empty directories
+ 1kiB: 89bytes of dirnode data : maybe one short-named subdir
+ 2kB: 1065bytes: about 9 entries
+ 3kB: 2065bytes: about 18 entries, or 7.5 entries plus the encprivkey
+ 4kB: 3065bytes: about 27 entries, or about 16.5 plus the encprivkey
+
+So we've written the code to do an initial read of 2kB from each share when
+we read the mutable file, which should give good performance (one RTT) for
+small directories.
+
+
+== Design Goals, redux ==
+
+How well does this design meet the goals?
+
+ #1 functional: YES: the code works and has extensive unit tests
+ #2 documentable: YES: this document is the existence proof
+ #3 confidential: YES: see below
+ #4 integrity: MOSTLY: a coalition of storage servers can rollback individual
+                       mutable files, but not a single one. No server can
+                       substitute fake data as genuine.
+ #5 availability: YES: as long as 'k' storage servers are present and have
+                       the same version of the mutable file, the dirnode will
+                       be available.
+ #6 efficient: MOSTLY:
+      network: single dirnode lookup is very efficient, since clients can
+               fetch specific keys rather than being required to get or set
+               the entire dirnode each time. Traversing many directories
+               takes a lot of roundtrips, and these can't be collapsed with
+               promise-pipelining because the intermediate values must only
+               be visible to the client. Modifying many dirnodes at once
+               (e.g. importing a large pre-existing directory tree) is pretty
+               slow, since each graph edge must be created independently.
+      storage: each child has a separate IV, which makes them larger than
+               if all children were aggregated into a single encrypted string
+ #7 delegation: VERY: each dirnode is a completely independent object,
+                to which clients can be granted separate read-write or
+                read-only access
+ #8 updateness: VERY: with only a single point of access, and no caching,
+                each client operation starts by fetching the current
+                value, so there are no opportunities for staleness
+ #9 monotonicity: VERY: the single point of access also protects against
+                  retrograde motion
+     
+
+
+=== Confidentiality leaks in the vdrive server ===
+
+Dirnode (and the mutable files upon which they are based) are very private
+against other clients: traffic between the client and the storage servers is
+protected by the Foolscap SSL connection, so they can observe very little.
+Storage index values are hashes of secrets and thus unguessable, and they are
+not made public, so other clients cannot snoop through encrypted dirnodes
+that they have not been told about.
+
+Storage servers can observe access patterns and see ciphertext, but they
+cannot see the plaintext (of child names, metadata, or URIs). If an attacker
+operates a significant number of storage servers, they can infer the shape of
+the directory structure by assuming that directories are usually accessed
+from root to leaf in rapid succession. Since filenames are usually much
+shorter than read-caps and write-caps, the attacker can use the length of the
+ciphertext to guess the number of children of each node, and might be able to
+guess the length of the child names (or at least their sum). From this, the
+attacker may be able to build up a graph with the same shape as the plaintext
+filesystem, but with unlabeled edges and unknown file contents.
+
+
+=== Integrity failures in the vdrive server ===
+
+The mutable file's integrity mechanism (RSA signature on the hash of the file
+contents) prevents the storage server from modifying the dirnode's contents
+without detection. Therefore the storage servers can make the dirnode
+unavailable, but not corrupt it.
+
+A sufficient number of colluding storage servers can perform a rollback
+attack: replace all shares of the whole mutable file with an earlier version.
+TODO: To prevent this, when retrieving the contents of a mutable file, the
+client should query more servers than necessary and use the highest available
+version number. This insures that one or two misbehaving storage servers
+cannot cause this rollback on their own.
+
+
+=== Improving the efficiency of dirnodes ===
+
+The current mutable-file -based dirnode scheme suffers from certain
+inefficiencies. A very large directory (with thousands or millions of
+children) will take a significant time to extract any single entry, because
+the whole file must be downloaded first, then parsed and searched to find the
+desired child entry. Likewise, modifying a single child will require the
+whole file to be re-uploaded.
+
+The current design assumes (and in some cases, requires) that dirnodes remain
+small. The mutable files on which dirnodes are based are currently using
+"SDMF" ("Small Distributed Mutable File") design rules, which state that the
+size of the data shall remain below one megabyte. More advanced forms of
+mutable files (MDMF and LDMF) are in the design phase to allow efficient
+manipulation of larger mutable files. This would reduce the work needed to
+modify a single entry in a large directory.
+
+Judicious caching may help improve the reading-large-directory case. Some
+form of mutable index at the beginning of the dirnode might help as well. The
+MDMF design rules allow for efficient random-access reads from the middle of
+the file, which would give the index something useful to point at.
+
+The current SDMF design generates a new RSA public/private keypair for each
+directory. This takes considerable time and CPU effort, generally one or two
+seconds per directory. We have designed (but not yet built) a DSA-based
+mutable file scheme which will use shared parameters to reduce the
+directory-creation effort to a bare minimum (picking a random number instead
+of generating two random primes).
+
+
+When a backup program is run for the first time, it needs to copy a large
+amount of data from a pre-existing filesystem into reliable storage. This
+means that a large and complex directory structure needs to be duplicated in
+the dirnode layer. With the one-object-per-dirnode approach described here,
+this requires as many operations as there are edges in the imported
+filesystem graph.
+
+Another approach would be to aggregate multiple directories into a single
+storage object. This object would contain a serialized graph rather than a
+single name-to-child dictionary. Most directory operations would fetch the
+whole block of data (and presumeably cache it for a while to avoid lots of
+re-fetches), and modification operations would need to replace the whole
+thing at once. This "realm" approach would have the added benefit of
+combining more data into a single encrypted bundle (perhaps hiding the shape
+of the graph from a determined attacker), and would reduce round-trips when
+performing deep directory traversals (assuming the realm was already cached).
+It would also prevent fine-grained rollback attacks from working: a coalition
+of storage servers could change the entire realm to look like an earlier
+state, but it could not independently roll back individual directories.
+
+The drawbacks of this aggregation would be that small accesses (adding a
+single child, looking up a single child) would require pulling or pushing a
+lot of unrelated data, increasing network overhead (and necessitating
+test-and-set semantics for the modification side, which increases the chances
+that a user operation will fail, making it more challenging to provide
+promises of atomicity to the user). 
+
+It would also make it much more difficult to enable the delegation
+("sharing") of specific directories. Since each aggregate "realm" provides
+all-or-nothing access control, the act of delegating any directory from the
+middle of the realm would require the realm first be split into the upper
+piece that isn't being shared and the lower piece that is. This splitting
+would have to be done in response to what is essentially a read operation,
+which is not traditionally supposed to be a high-effort action. On the other
+hand, it may be possible to aggregate the ciphertext, but use distinct
+encryption keys for each component directory, to get the benefits of both
+schemes at once.
+
+
+=== Dirnode expiration and leases ===
+
+Dirnodes are created any time a client wishes to add a new directory. How
+long do they live? What's to keep them from sticking around forever, taking
+up space that nobody can reach any longer?
+
+Mutable files are created with limited-time "leases", which keep the shares
+alive until the last lease has expired or been cancelled. Clients which know
+and care about specific dirnodes can ask to keep them alive for a while, by
+renewing a lease on them (with a typical period of one month). Clients are
+expected to assist in the deletion of dirnodes by canceling their leases as
+soon as they are done with them. This means that when a client deletes a
+directory, it should also cancel its lease on that directory. When the lease
+count on a given share goes to zero, the storage server can delete the
+related storage. Multiple clients may all have leases on the same dirnode:
+the server may delete the shares only after all of the leases have gone away.
+
+We expect that clients will periodically create a "manifest": a list of
+so-called "refresh capabilities" for all of the dirnodes and files that they
+can reach. They will give this manifest to the "repairer", which is a service
+that keeps files (and dirnodes) alive on behalf of clients who cannot take on
+this responsibility for themselves. These refresh capabilities include the
+storage index, but do *not* include the readkeys or writekeys, so the
+repairer does not get to read the files or directories that it is helping to
+keep alive.
+
+After each change to the user's vdrive, the client creates a manifest and
+looks for differences from their previous version. Anything which was removed
+prompts the client to send out lease-cancellation messages, allowing the data
+to be deleted.
+
+
+== Starting Points: root dirnodes ==
+
+Any client can record the URI of a directory node in some external form (say,
+in a local file) and use it as the starting point of later traversal. Each
+Tahoe user is expected to create a new (unattached) dirnode when they first
+start using the grid, and record its URI for later use.
+
+== Mounting and Sharing Directories ==
+
+The biggest benefit of this dirnode approach is that sharing individual
+directories is almost trivial. Alice creates a subdirectory that she wants to
+use to share files with Bob. This subdirectory is attached to Alice's
+filesystem at "~alice/share-with-bob". She asks her filesystem for the
+read-write directory URI for that new directory, and emails it to Bob. When
+Bob receives the URI, he asks his own local vdrive to attach the given URI,
+perhaps at a place named "~bob/shared-with-alice". Every time either party
+writes a file into this directory, the other will be able to read it. If
+Alice prefers, she can give a read-only URI to Bob instead, and then Bob will
+be able to read files but not change the contents of the directory. Neither
+Alice nor Bob will get access to any files above the mounted directory: there
+are no 'parent directory' pointers. If Alice creates a nested set of
+directories, "~alice/share-with-bob/subdir2", and gives a read-only URI to
+share-with-bob to Bob, then Bob will be unable to write to either
+share-with-bob/ or subdir2/.
+
+A suitable UI needs to be created to allow users to easily perform this
+sharing action: dragging a folder their vdrive to an IM or email user icon,
+for example. The UI will need to give the sending user an opportunity to
+indicate whether they want to grant read-write or read-only access to the
+recipient. The recipient then needs an interface to drag the new folder into
+their vdrive and give it a home.
+
+== Revocation ==
+
+When Alice decides that she no longer wants Bob to be able to access the
+shared directory, what should she do? Suppose she's shared this folder with
+both Bob and Carol, and now she wants Carol to retain access to it but Bob to
+be shut out. Ideally Carol should not have to do anything: her access should
+continue unabated.
+
+The current plan is to have her client create a deep copy of the folder in
+question, delegate access to the new folder to the remaining members of the
+group (Carol), asking the lucky survivors to replace their old reference with
+the new one. Bob may still have access to the old folder, but he is now the
+only one who cares: everyone else has moved on, and he will no longer be able
+to see their new changes. In a strict sense, this is the strongest form of
+revocation that can be accomplished: there is no point trying to force Bob to
+forget about the files that he read a moment before being kicked out. In
+addition it must be noted that anyone who can access the directory can proxy
+for Bob, reading files to him and accepting changes whenever he wants.
+Preventing delegation between communication parties is just as pointless as
+asking Bob to forget previously accessed files. However, there may be value
+to configuring the UI to ask Carol to not share files with Bob, or to
+removing all files from Bob's view at the same time his access is revoked.
+
diff --git a/docs/specifications/file-encoding.txt b/docs/specifications/file-encoding.txt
new file mode 100644 (file)
index 0000000..23862ea
--- /dev/null
@@ -0,0 +1,148 @@
+
+== FileEncoding ==
+
+When the client wishes to upload an immutable file, the first step is to
+decide upon an encryption key. There are two methods: convergent or random.
+The goal of the convergent-key method is to make sure that multiple uploads
+of the same file will result in only one copy on the grid, whereas the
+random-key method does not provide this "convergence" feature.
+
+The convergent-key method computes the SHA-256d hash of a single-purpose tag,
+the encoding parameters, a "convergence secret", and the contents of the
+file. It uses a portion of the resulting hash as the AES encryption key.
+There are security concerns with using convergence this approach (the
+"partial-information guessing attack", please see ticket #365 for some
+references), so Tahoe uses a separate (randomly-generated) "convergence
+secret" for each node, stored in NODEDIR/private/convergence . The encoding
+parameters (k, N, and the segment size) are included in the hash to make sure
+that two different encodings of the same file will get different keys. This
+method requires an extra IO pass over the file, to compute this key, and
+encryption cannot be started until the pass is complete. This means that the
+convergent-key method will require at least two total passes over the file.
+
+The random-key method simply chooses a random encryption key. Convergence is
+disabled, however this method does not require a separate IO pass, so upload
+can be done with a single pass. This mode makes it easier to perform
+streaming upload.
+
+Regardless of which method is used to generate the key, the plaintext file is
+encrypted (using AES in CTR mode) to produce a ciphertext. This ciphertext is
+then erasure-coded and uploaded to the servers. Two hashes of the ciphertext
+are generated as the encryption proceeds: a flat hash of the whole
+ciphertext, and a Merkle tree. These are used to verify the correctness of
+the erasure decoding step, and can be used by a "verifier" process to make
+sure the file is intact without requiring the decryption key.
+
+The encryption key is hashed (with SHA-256d and a single-purpose tag) to
+produce the "Storage Index". This Storage Index (or SI) is used to identify
+the shares produced by the method described below. The grid can be thought of
+as a large table that maps Storage Index to a ciphertext. Since the
+ciphertext is stored as erasure-coded shares, it can also be thought of as a
+table that maps SI to shares.
+
+Anybody who knows a Storage Index can retrieve the associated ciphertext:
+ciphertexts are not secret.
+
+
+[[Image(file-encoding1.png)]]
+
+The ciphertext file is then broken up into segments. The last segment is
+likely to be shorter than the rest. Each segment is erasure-coded into a
+number of "blocks". This takes place one segment at a time. (In fact,
+encryption and erasure-coding take place at the same time, once per plaintext
+segment). Larger segment sizes result in less overhead overall, but increase
+both the memory footprint and the "alacrity" (the number of bytes we have to
+receive before we can deliver validated plaintext to the user). The current
+default segment size is 128KiB.
+
+One block from each segment is sent to each shareholder (aka leaseholder,
+aka landlord, aka storage node, aka peer). The "share" held by each remote
+shareholder is nominally just a collection of these blocks. The file will
+be recoverable when a certain number of shares have been retrieved.
+
+[[Image(file-encoding2.png)]]
+
+The blocks are hashed as they are generated and transmitted. These
+block hashes are put into a Merkle hash tree. When the last share has been
+created, the merkle tree is completed and delivered to the peer. Later, when
+we retrieve these blocks, the peer will send many of the merkle hash tree
+nodes ahead of time, so we can validate each block independently.
+
+The root of this block hash tree is called the "block root hash" and
+used in the next step.
+
+[[Image(file-encoding3.png)]]
+
+There is a higher-level Merkle tree called the "share hash tree". Its leaves
+are the block root hashes from each share. The root of this tree is called
+the "share root hash" and is included in the "URI Extension Block", aka UEB.
+The ciphertext hash and Merkle tree are also put here, along with the
+original file size, and the encoding parameters. The UEB contains all the
+non-secret values that could be put in the URI, but would have made the URI
+too big. So instead, the UEB is stored with the share, and the hash of the
+UEB is put in the URI.
+
+The URI then contains the secret encryption key and the UEB hash. It also
+contains the basic encoding parameters (k and N) and the file size, to make
+download more efficient (by knowing the number of required shares ahead of
+time, sufficient download queries can be generated in parallel).
+
+The URI (also known as the immutable-file read-cap, since possessing it
+grants the holder the capability to read the file's plaintext) is then
+represented as a (relatively) short printable string like so:
+
+ URI:CHK:auxet66ynq55naiy2ay7cgrshm:6rudoctmbxsmbg7gwtjlimd6umtwrrsxkjzthuldsmo4nnfoc6fa:3:10:1000000
+
+[[Image(file-encoding4.png)]]
+
+During download, when a peer begins to transmit a share, it first transmits
+all of the parts of the share hash tree that are necessary to validate its
+block root hash. Then it transmits the portions of the block hash tree
+that are necessary to validate the first block. Then it transmits the
+first block. It then continues this loop: transmitting any portions of the
+block hash tree to validate block#N, then sending block#N.
+
+[[Image(file-encoding5.png)]]
+
+So the "share" that is sent to the remote peer actually consists of three
+pieces, sent in a specific order as they become available, and retrieved
+during download in a different order according to when they are needed.
+
+The first piece is the blocks themselves, one per segment. The last
+block will likely be shorter than the rest, because the last segment is
+probably shorter than the rest. The second piece is the block hash tree,
+consisting of a total of two SHA-1 hashes per block. The third piece is a
+hash chain from the share hash tree, consisting of log2(numshares) hashes.
+
+During upload, all blocks are sent first, followed by the block hash
+tree, followed by the share hash chain. During download, the share hash chain
+is delivered first, followed by the block root hash. The client then uses
+the hash chain to validate the block root hash. Then the peer delivers
+enough of the block hash tree to validate the first block, followed by
+the first block itself. The block hash chain is used to validate the
+block, then it is passed (along with the first block from several other
+peers) into decoding, to produce the first segment of crypttext, which is
+then decrypted to produce the first segment of plaintext, which is finally
+delivered to the user.
+
+[[Image(file-encoding6.png)]]
+
+== Hashes ==
+
+All hashes use SHA-256d, as defined in Practical Cryptography (by Ferguson
+and Schneier). All hashes use a single-purpose tag, e.g. the hash that
+converts an encryption key into a storage index is defined as follows:
+
+ SI = SHA256d(netstring("allmydata_immutable_key_to_storage_index_v1") + key)
+
+When two separate values need to be combined together in a hash, we wrap each
+in a netstring.
+
+Using SHA-256d (instead of plain SHA-256) guards against length-extension
+attacks. Using the tag protects our Merkle trees against attacks in which the
+hash of a leaf is confused with a hash of two children (allowing an attacker
+to generate corrupted data that nevertheless appears to be valid), and is
+simply good "cryptograhic hygiene". The "Chosen Protocol Attack" by Kelsey,
+Schneier, and Wagner (http://www.schneier.com/paper-chosen-protocol.html) is
+relevant. Putting the tag in a netstring guards against attacks that seek to
+confuse the end of the tag with the beginning of the subsequent value.
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diff --git a/docs/specifications/mut.svg b/docs/specifications/mut.svg
new file mode 100644 (file)
index 0000000..3db01b8
--- /dev/null
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diff --git a/docs/specifications/mutable.txt b/docs/specifications/mutable.txt
new file mode 100644 (file)
index 0000000..40a5374
--- /dev/null
@@ -0,0 +1,648 @@
+
+This describes the "RSA-based mutable files" which were shipped in Tahoe v0.8.0.
+
+= Mutable Files =
+
+Mutable File Slots are places with a stable identifier that can hold data
+that changes over time. In contrast to CHK slots, for which the
+URI/identifier is derived from the contents themselves, the Mutable File Slot
+URI remains fixed for the life of the slot, regardless of what data is placed
+inside it.
+
+Each mutable slot is referenced by two different URIs. The "read-write" URI
+grants read-write access to its holder, allowing them to put whatever
+contents they like into the slot. The "read-only" URI is less powerful, only
+granting read access, and not enabling modification of the data. The
+read-write URI can be turned into the read-only URI, but not the other way
+around.
+
+The data in these slots is distributed over a number of servers, using the
+same erasure coding that CHK files use, with 3-of-10 being a typical choice
+of encoding parameters. The data is encrypted and signed in such a way that
+only the holders of the read-write URI will be able to set the contents of
+the slot, and only the holders of the read-only URI will be able to read
+those contents. Holders of either URI will be able to validate the contents
+as being written by someone with the read-write URI. The servers who hold the
+shares cannot read or modify them: the worst they can do is deny service (by
+deleting or corrupting the shares), or attempt a rollback attack (which can
+only succeed with the cooperation of at least k servers).
+
+== Consistency vs Availability ==
+
+There is an age-old battle between consistency and availability. Epic papers
+have been written, elaborate proofs have been established, and generations of
+theorists have learned that you cannot simultaneously achieve guaranteed
+consistency with guaranteed reliability. In addition, the closer to 0 you get
+on either axis, the cost and complexity of the design goes up.
+
+Tahoe's design goals are to largely favor design simplicity, then slightly
+favor read availability, over the other criteria.
+
+As we develop more sophisticated mutable slots, the API may expose multiple
+read versions to the application layer. The tahoe philosophy is to defer most
+consistency recovery logic to the higher layers. Some applications have
+effective ways to merge multiple versions, so inconsistency is not
+necessarily a problem (i.e. directory nodes can usually merge multiple "add
+child" operations).
+
+== The Prime Coordination Directive: "Don't Do That" ==
+
+The current rule for applications which run on top of Tahoe is "do not
+perform simultaneous uncoordinated writes". That means you need non-tahoe
+means to make sure that two parties are not trying to modify the same mutable
+slot at the same time. For example:
+
+ * don't give the read-write URI to anyone else. Dirnodes in a private
+   directory generally satisfy this case, as long as you don't use two
+   clients on the same account at the same time
+ * if you give a read-write URI to someone else, stop using it yourself. An
+   inbox would be a good example of this.
+ * if you give a read-write URI to someone else, call them on the phone
+   before you write into it
+ * build an automated mechanism to have your agents coordinate writes.
+   For example, we expect a future release to include a FURL for a
+   "coordination server" in the dirnodes. The rule can be that you must
+   contact the coordination server and obtain a lock/lease on the file
+   before you're allowed to modify it.
+
+If you do not follow this rule, Bad Things will happen. The worst-case Bad
+Thing is that the entire file will be lost. A less-bad Bad Thing is that one
+or more of the simultaneous writers will lose their changes. An observer of
+the file may not see monotonically-increasing changes to the file, i.e. they
+may see version 1, then version 2, then 3, then 2 again.
+
+Tahoe takes some amount of care to reduce the badness of these Bad Things.
+One way you can help nudge it from the "lose your file" case into the "lose
+some changes" case is to reduce the number of competing versions: multiple
+versions of the file that different parties are trying to establish as the
+one true current contents. Each simultaneous writer counts as a "competing
+version", as does the previous version of the file. If the count "S" of these
+competing versions is larger than N/k, then the file runs the risk of being
+lost completely. [TODO] If at least one of the writers remains running after
+the collision is detected, it will attempt to recover, but if S>(N/k) and all
+writers crash after writing a few shares, the file will be lost.
+
+Note that Tahoe uses serialization internally to make sure that a single
+Tahoe node will not perform simultaneous modifications to a mutable file. It
+accomplishes this by using a weakref cache of the MutableFileNode (so that
+there will never be two distinct MutableFileNodes for the same file), and by
+forcing all mutable file operations to obtain a per-node lock before they
+run. The Prime Coordination Directive therefore applies to inter-node
+conflicts, not intra-node ones.
+
+
+== Small Distributed Mutable Files ==
+
+SDMF slots are suitable for small (<1MB) files that are editing by rewriting
+the entire file. The three operations are:
+
+ * allocate (with initial contents)
+ * set (with new contents)
+ * get (old contents)
+
+The first use of SDMF slots will be to hold directories (dirnodes), which map
+encrypted child names to rw-URI/ro-URI pairs.
+
+=== SDMF slots overview ===
+
+Each SDMF slot is created with a public/private key pair. The public key is
+known as the "verification key", while the private key is called the
+"signature key". The private key is hashed and truncated to 16 bytes to form
+the "write key" (an AES symmetric key). The write key is then hashed and
+truncated to form the "read key". The read key is hashed and truncated to
+form the 16-byte "storage index" (a unique string used as an index to locate
+stored data).
+
+The public key is hashed by itself to form the "verification key hash".
+
+The write key is hashed a different way to form the "write enabler master".
+For each storage server on which a share is kept, the write enabler master is
+concatenated with the server's nodeid and hashed, and the result is called
+the "write enabler" for that particular server. Note that multiple shares of
+the same slot stored on the same server will all get the same write enabler,
+i.e. the write enabler is associated with the "bucket", rather than the
+individual shares.
+
+The private key is encrypted (using AES in counter mode) by the write key,
+and the resulting crypttext is stored on the servers. so it will be
+retrievable by anyone who knows the write key. The write key is not used to
+encrypt anything else, and the private key never changes, so we do not need
+an IV for this purpose.
+
+The actual data is encrypted (using AES in counter mode) with a key derived
+by concatenating the readkey with the IV, the hashing the results and
+truncating to 16 bytes. The IV is randomly generated each time the slot is
+updated, and stored next to the encrypted data.
+
+The read-write URI consists of the write key and the verification key hash.
+The read-only URI contains the read key and the verification key hash. The
+verify-only URI contains the storage index and the verification key hash.
+
+ URI:SSK-RW:b2a(writekey):b2a(verification_key_hash)
+ URI:SSK-RO:b2a(readkey):b2a(verification_key_hash)
+ URI:SSK-Verify:b2a(storage_index):b2a(verification_key_hash)
+
+Note that this allows the read-only and verify-only URIs to be derived from
+the read-write URI without actually retrieving the public keys. Also note
+that it means the read-write agent must validate both the private key and the
+public key when they are first fetched. All users validate the public key in
+exactly the same way.
+
+The SDMF slot is allocated by sending a request to the storage server with a
+desired size, the storage index, and the write enabler for that server's
+nodeid. If granted, the write enabler is stashed inside the slot's backing
+store file. All further write requests must be accompanied by the write
+enabler or they will not be honored. The storage server does not share the
+write enabler with anyone else.
+
+The SDMF slot structure will be described in more detail below. The important
+pieces are:
+
+  * a sequence number
+  * a root hash "R"
+  * the encoding parameters (including k, N, file size, segment size)
+  * a signed copy of [seqnum,R,encoding_params], using the signature key
+  * the verification key (not encrypted)
+  * the share hash chain (part of a Merkle tree over the share hashes)
+  * the block hash tree (Merkle tree over blocks of share data)
+  * the share data itself (erasure-coding of read-key-encrypted file data)
+  * the signature key, encrypted with the write key
+
+The access pattern for read is:
+ * hash read-key to get storage index
+ * use storage index to locate 'k' shares with identical 'R' values
+   * either get one share, read 'k' from it, then read k-1 shares
+   * or read, say, 5 shares, discover k, either get more or be finished
+   * or copy k into the URIs
+ * read verification key
+ * hash verification key, compare against verification key hash
+ * read seqnum, R, encoding parameters, signature
+ * verify signature against verification key
+ * read share data, compute block-hash Merkle tree and root "r"
+ * read share hash chain (leading from "r" to "R")
+ * validate share hash chain up to the root "R"
+ * submit share data to erasure decoding
+ * decrypt decoded data with read-key
+ * submit plaintext to application
+
+The access pattern for write is:
+ * hash write-key to get read-key, hash read-key to get storage index
+ * use the storage index to locate at least one share
+ * read verification key and encrypted signature key
+ * decrypt signature key using write-key
+ * hash signature key, compare against write-key
+ * hash verification key, compare against verification key hash
+ * encrypt plaintext from application with read-key
+   * application can encrypt some data with the write-key to make it only
+     available to writers (use this for transitive read-onlyness of dirnodes)
+ * erasure-code crypttext to form shares
+ * split shares into blocks
+ * compute Merkle tree of blocks, giving root "r" for each share
+ * compute Merkle tree of shares, find root "R" for the file as a whole
+ * create share data structures, one per server:
+   * use seqnum which is one higher than the old version
+   * share hash chain has log(N) hashes, different for each server
+   * signed data is the same for each server
+ * now we have N shares and need homes for them
+ * walk through peers
+   * if share is not already present, allocate-and-set
+   * otherwise, try to modify existing share:
+   * send testv_and_writev operation to each one
+   * testv says to accept share if their(seqnum+R) <= our(seqnum+R)
+   * count how many servers wind up with which versions (histogram over R)
+   * keep going until N servers have the same version, or we run out of servers
+     * if any servers wound up with a different version, report error to
+       application
+     * if we ran out of servers, initiate recovery process (described below)
+
+=== Server Storage Protocol ===
+
+The storage servers will provide a mutable slot container which is oblivious
+to the details of the data being contained inside it. Each storage index
+refers to a "bucket", and each bucket has one or more shares inside it. (In a
+well-provisioned network, each bucket will have only one share). The bucket
+is stored as a directory, using the base32-encoded storage index as the
+directory name. Each share is stored in a single file, using the share number
+as the filename.
+
+The container holds space for a container magic number (for versioning), the
+write enabler, the nodeid which accepted the write enabler (used for share
+migration, described below), a small number of lease structures, the embedded
+data itself, and expansion space for additional lease structures.
+
+ #   offset    size    name
+ 1   0         32      magic verstr "tahoe mutable container v1" plus binary
+ 2   32        20      write enabler's nodeid
+ 3   52        32      write enabler
+ 4   84        8       data size (actual share data present) (a)
+ 5   92        8       offset of (8) count of extra leases (after data)
+ 6   100       368     four leases, 92 bytes each
+                        0    4   ownerid (0 means "no lease here")
+                        4    4   expiration timestamp
+                        8   32   renewal token
+                        40  32   cancel token
+                        72  20   nodeid which accepted the tokens
+ 7   468       (a)     data
+ 8   ??        4       count of extra leases
+ 9   ??        n*92    extra leases
+
+The "extra leases" field must be copied and rewritten each time the size of
+the enclosed data changes. The hope is that most buckets will have four or
+fewer leases and this extra copying will not usually be necessary.
+
+The (4) "data size" field contains the actual number of bytes of data present
+in field (7), such that a client request to read beyond 504+(a) will result
+in an error. This allows the client to (one day) read relative to the end of
+the file. The container size (that is, (8)-(7)) might be larger, especially
+if extra size was pre-allocated in anticipation of filling the container with
+a lot of data.
+
+The offset in (5) points at the *count* of extra leases, at (8). The actual
+leases (at (9)) begin 4 bytes later. If the container size changes, both (8)
+and (9) must be relocated by copying.
+
+The server will honor any write commands that provide the write token and do
+not exceed the server-wide storage size limitations. Read and write commands
+MUST be restricted to the 'data' portion of the container: the implementation
+of those commands MUST perform correct bounds-checking to make sure other
+portions of the container are inaccessible to the clients.
+
+The two methods provided by the storage server on these "MutableSlot" share
+objects are:
+
+ * readv(ListOf(offset=int, length=int))
+   * returns a list of bytestrings, of the various requested lengths
+   * offset < 0 is interpreted relative to the end of the data
+   * spans which hit the end of the data will return truncated data
+
+ * testv_and_writev(write_enabler, test_vector, write_vector)
+   * this is a test-and-set operation which performs the given tests and only
+     applies the desired writes if all tests succeed. This is used to detect
+     simultaneous writers, and to reduce the chance that an update will lose
+     data recently written by some other party (written after the last time
+     this slot was read).
+   * test_vector=ListOf(TupleOf(offset, length, opcode, specimen))
+   * the opcode is a string, from the set [gt, ge, eq, le, lt, ne]
+   * each element of the test vector is read from the slot's data and 
+     compared against the specimen using the desired (in)equality. If all
+     tests evaluate True, the write is performed
+   * write_vector=ListOf(TupleOf(offset, newdata))
+     * offset < 0 is not yet defined, it probably means relative to the
+       end of the data, which probably means append, but we haven't nailed
+       it down quite yet
+     * write vectors are executed in order, which specifies the results of
+       overlapping writes
+   * return value:
+     * error: OutOfSpace
+     * error: something else (io error, out of memory, whatever)
+     * (True, old_test_data): the write was accepted (test_vector passed)
+     * (False, old_test_data): the write was rejected (test_vector failed)
+       * both 'accepted' and 'rejected' return the old data that was used
+         for the test_vector comparison. This can be used by the client
+         to detect write collisions, including collisions for which the
+         desired behavior was to overwrite the old version.
+
+In addition, the storage server provides several methods to access these
+share objects:
+
+ * allocate_mutable_slot(storage_index, sharenums=SetOf(int))
+   * returns DictOf(int, MutableSlot)
+ * get_mutable_slot(storage_index)
+   * returns DictOf(int, MutableSlot)
+   * or raises KeyError
+
+We intend to add an interface which allows small slots to allocate-and-write
+in a single call, as well as do update or read in a single call. The goal is
+to allow a reasonably-sized dirnode to be created (or updated, or read) in
+just one round trip (to all N shareholders in parallel).
+
+==== migrating shares ====
+
+If a share must be migrated from one server to another, two values become
+invalid: the write enabler (since it was computed for the old server), and
+the lease renew/cancel tokens.
+
+Suppose that a slot was first created on nodeA, and was thus initialized with
+WE(nodeA) (= H(WEM+nodeA)). Later, for provisioning reasons, the share is
+moved from nodeA to nodeB.
+
+Readers may still be able to find the share in its new home, depending upon
+how many servers are present in the grid, where the new nodeid lands in the
+permuted index for this particular storage index, and how many servers the
+reading client is willing to contact.
+
+When a client attempts to write to this migrated share, it will get a "bad
+write enabler" error, since the WE it computes for nodeB will not match the
+WE(nodeA) that was embedded in the share. When this occurs, the "bad write
+enabler" message must include the old nodeid (e.g. nodeA) that was in the
+share.
+
+The client then computes H(nodeB+H(WEM+nodeA)), which is the same as
+H(nodeB+WE(nodeA)). The client sends this along with the new WE(nodeB), which
+is H(WEM+nodeB). Note that the client only sends WE(nodeB) to nodeB, never to
+anyone else. Also note that the client does not send a value to nodeB that
+would allow the node to impersonate the client to a third node: everything
+sent to nodeB will include something specific to nodeB in it.
+
+The server locally computes H(nodeB+WE(nodeA)), using its own node id and the
+old write enabler from the share. It compares this against the value supplied
+by the client. If they match, this serves as proof that the client was able
+to compute the old write enabler. The server then accepts the client's new
+WE(nodeB) and writes it into the container.
+
+This WE-fixup process requires an extra round trip, and requires the error
+message to include the old nodeid, but does not require any public key
+operations on either client or server.
+
+Migrating the leases will require a similar protocol. This protocol will be
+defined concretely at a later date.
+
+=== Code Details ===
+
+The MutableFileNode class is used to manipulate mutable files (as opposed to
+ImmutableFileNodes). These are initially generated with
+client.create_mutable_file(), and later recreated from URIs with
+client.create_node_from_uri(). Instances of this class will contain a URI and
+a reference to the client (for peer selection and connection).
+
+NOTE: this section is out of date. Please see src/allmydata/interfaces.py
+(the section on IMutableFilesystemNode) for more accurate information.
+
+The methods of MutableFileNode are:
+
+ * download_to_data() -> [deferred] newdata, NotEnoughSharesError
+   * if there are multiple retrieveable versions in the grid, get() returns
+     the first version it can reconstruct, and silently ignores the others.
+     In the future, a more advanced API will signal and provide access to
+     the multiple heads.
+ * update(newdata) -> OK, UncoordinatedWriteError, NotEnoughSharesError
+ * overwrite(newdata) -> OK, UncoordinatedWriteError, NotEnoughSharesError
+
+download_to_data() causes a new retrieval to occur, pulling the current
+contents from the grid and returning them to the caller. At the same time,
+this call caches information about the current version of the file. This
+information will be used in a subsequent call to update(), and if another
+change has occured between the two, this information will be out of date,
+triggering the UncoordinatedWriteError.
+
+update() is therefore intended to be used just after a download_to_data(), in
+the following pattern:
+
+ d = mfn.download_to_data()
+ d.addCallback(apply_delta)
+ d.addCallback(mfn.update)
+
+If the update() call raises UCW, then the application can simply return an
+error to the user ("you violated the Prime Coordination Directive"), and they
+can try again later. Alternatively, the application can attempt to retry on
+its own. To accomplish this, the app needs to pause, download the new
+(post-collision and post-recovery) form of the file, reapply their delta,
+then submit the update request again. A randomized pause is necessary to
+reduce the chances of colliding a second time with another client that is
+doing exactly the same thing:
+
+ d = mfn.download_to_data()
+ d.addCallback(apply_delta)
+ d.addCallback(mfn.update)
+ def _retry(f):
+   f.trap(UncoordinatedWriteError)
+   d1 = pause(random.uniform(5, 20))
+   d1.addCallback(lambda res: mfn.download_to_data())
+   d1.addCallback(apply_delta)
+   d1.addCallback(mfn.update)
+   return d1
+ d.addErrback(_retry)
+
+Enthusiastic applications can retry multiple times, using a randomized
+exponential backoff between each. A particularly enthusiastic application can
+retry forever, but such apps are encouraged to provide a means to the user of
+giving up after a while.
+
+UCW does not mean that the update was not applied, so it is also a good idea
+to skip the retry-update step if the delta was already applied:
+
+ d = mfn.download_to_data()
+ d.addCallback(apply_delta)
+ d.addCallback(mfn.update)
+ def _retry(f):
+   f.trap(UncoordinatedWriteError)
+   d1 = pause(random.uniform(5, 20))
+   d1.addCallback(lambda res: mfn.download_to_data())
+   def _maybe_apply_delta(contents):
+     new_contents = apply_delta(contents)
+     if new_contents != contents:
+       return mfn.update(new_contents)
+   d1.addCallback(_maybe_apply_delta)
+   return d1
+ d.addErrback(_retry)
+
+update() is the right interface to use for delta-application situations, like
+directory nodes (in which apply_delta might be adding or removing child
+entries from a serialized table).
+
+Note that any uncoordinated write has the potential to lose data. We must do
+more analysis to be sure, but it appears that two clients who write to the
+same mutable file at the same time (even if both eventually retry) will, with
+high probability, result in one client observing UCW and the other silently
+losing their changes. It is also possible for both clients to observe UCW.
+The moral of the story is that the Prime Coordination Directive is there for
+a reason, and that recovery/UCW/retry is not a subsitute for write
+coordination.
+
+overwrite() tells the client to ignore this cached version information, and
+to unconditionally replace the mutable file's contents with the new data.
+This should not be used in delta application, but rather in situations where
+you want to replace the file's contents with completely unrelated ones. When
+raw files are uploaded into a mutable slot through the tahoe webapi (using
+POST and the ?mutable=true argument), they are put in place with overwrite().
+
+
+
+The peer-selection and data-structure manipulation (and signing/verification)
+steps will be implemented in a separate class in allmydata/mutable.py .
+
+=== SMDF Slot Format ===
+
+This SMDF data lives inside a server-side MutableSlot container. The server
+is oblivious to this format.
+
+This data is tightly packed. In particular, the share data is defined to run
+all the way to the beginning of the encrypted private key (the encprivkey
+offset is used both to terminate the share data and to begin the encprivkey).
+
+ #    offset   size    name
+ 1    0        1       version byte, \x00 for this format
+ 2    1        8       sequence number. 2^64-1 must be handled specially, TBD
+ 3    9        32      "R" (root of share hash Merkle tree)
+ 4    41       16      IV (share data is AES(H(readkey+IV)) )
+ 5    57       18      encoding parameters:
+       57       1        k
+       58       1        N
+       59       8        segment size
+       67       8        data length (of original plaintext)
+ 6    75       32      offset table:
+       75       4        (8) signature
+       79       4        (9) share hash chain
+       83       4        (10) block hash tree
+       87       4        (11) share data
+       91       8        (12) encrypted private key
+       99       8        (13) EOF
+ 7    107      436ish  verification key (2048 RSA key)
+ 8    543ish   256ish  signature=RSAenc(sigkey, H(version+seqnum+r+IV+encparm))
+ 9    799ish   (a)     share hash chain, encoded as:
+                        "".join([pack(">H32s", shnum, hash)
+                                 for (shnum,hash) in needed_hashes])
+10    (927ish) (b)     block hash tree, encoded as:
+                        "".join([pack(">32s",hash) for hash in block_hash_tree])
+11    (935ish) LEN     share data (no gap between this and encprivkey)
+12    ??       1216ish encrypted private key= AESenc(write-key, RSA-key)
+13    ??       --      EOF
+
+(a) The share hash chain contains ceil(log(N)) hashes, each 32 bytes long.
+    This is the set of hashes necessary to validate this share's leaf in the
+    share Merkle tree. For N=10, this is 4 hashes, i.e. 128 bytes.
+(b) The block hash tree contains ceil(length/segsize) hashes, each 32 bytes
+    long. This is the set of hashes necessary to validate any given block of
+    share data up to the per-share root "r". Each "r" is a leaf of the share
+    has tree (with root "R"), from which a minimal subset of hashes is put in
+    the share hash chain in (8).
+
+=== Recovery ===
+
+The first line of defense against damage caused by colliding writes is the
+Prime Coordination Directive: "Don't Do That".
+
+The second line of defense is to keep "S" (the number of competing versions)
+lower than N/k. If this holds true, at least one competing version will have
+k shares and thus be recoverable. Note that server unavailability counts
+against us here: the old version stored on the unavailable server must be
+included in the value of S.
+
+The third line of defense is our use of testv_and_writev() (described below),
+which increases the convergence of simultaneous writes: one of the writers
+will be favored (the one with the highest "R"), and that version is more
+likely to be accepted than the others. This defense is least effective in the
+pathological situation where S simultaneous writers are active, the one with
+the lowest "R" writes to N-k+1 of the shares and then dies, then the one with
+the next-lowest "R" writes to N-2k+1 of the shares and dies, etc, until the
+one with the highest "R" writes to k-1 shares and dies. Any other sequencing
+will allow the highest "R" to write to at least k shares and establish a new
+revision.
+
+The fourth line of defense is the fact that each client keeps writing until
+at least one version has N shares. This uses additional servers, if
+necessary, to make sure that either the client's version or some
+newer/overriding version is highly available.
+
+The fifth line of defense is the recovery algorithm, which seeks to make sure
+that at least *one* version is highly available, even if that version is
+somebody else's.
+
+The write-shares-to-peers algorithm is as follows:
+
+ * permute peers according to storage index
+ * walk through peers, trying to assign one share per peer
+ * for each peer:
+   * send testv_and_writev, using "old(seqnum+R) <= our(seqnum+R)" as the test
+     * this means that we will overwrite any old versions, and we will
+       overwrite simultaenous writers of the same version if our R is higher.
+       We will not overwrite writers using a higher seqnum.
+   * record the version that each share winds up with. If the write was
+     accepted, this is our own version. If it was rejected, read the
+     old_test_data to find out what version was retained.
+   * if old_test_data indicates the seqnum was equal or greater than our
+     own, mark the "Simultanous Writes Detected" flag, which will eventually
+     result in an error being reported to the writer (in their close() call).
+   * build a histogram of "R" values
+   * repeat until the histogram indicate that some version (possibly ours)
+     has N shares. Use new servers if necessary.
+   * If we run out of servers:
+     * if there are at least shares-of-happiness of any one version, we're
+       happy, so return. (the close() might still get an error)
+     * not happy, need to reinforce something, goto RECOVERY
+
+RECOVERY:
+ * read all shares, count the versions, identify the recoverable ones,
+   discard the unrecoverable ones.
+ * sort versions: locate max(seqnums), put all versions with that seqnum
+   in the list, sort by number of outstanding shares. Then put our own
+   version. (TODO: put versions with seqnum <max but >us ahead of us?).
+ * for each version:
+   * attempt to recover that version
+   * if not possible, remove it from the list, go to next one
+   * if recovered, start at beginning of peer list, push that version,
+     continue until N shares are placed
+   * if pushing our own version, bump up the seqnum to one higher than
+     the max seqnum we saw
+   * if we run out of servers:
+     * schedule retry and exponential backoff to repeat RECOVERY
+   * admit defeat after some period? presumeably the client will be shut down
+     eventually, maybe keep trying (once per hour?) until then.
+
+
+
+
+== Medium Distributed Mutable Files ==
+
+These are just like the SDMF case, but:
+
+ * we actually take advantage of the Merkle hash tree over the blocks, by
+   reading a single segment of data at a time (and its necessary hashes), to
+   reduce the read-time alacrity
+ * we allow arbitrary writes to the file (i.e. seek() is provided, and
+   O_TRUNC is no longer required)
+ * we write more code on the client side (in the MutableFileNode class), to
+   first read each segment that a write must modify. This looks exactly like
+   the way a normal filesystem uses a block device, or how a CPU must perform
+   a cache-line fill before modifying a single word.
+ * we might implement some sort of copy-based atomic update server call,
+   to allow multiple writev() calls to appear atomic to any readers.
+
+MDMF slots provide fairly efficient in-place edits of very large files (a few
+GB). Appending data is also fairly efficient, although each time a power of 2
+boundary is crossed, the entire file must effectively be re-uploaded (because
+the size of the block hash tree changes), so if the filesize is known in
+advance, that space ought to be pre-allocated (by leaving extra space between
+the block hash tree and the actual data).
+
+MDMF1 uses the Merkle tree to enable low-alacrity random-access reads. MDMF2
+adds cache-line reads to allow random-access writes.
+
+== Large Distributed Mutable Files ==
+
+LDMF slots use a fundamentally different way to store the file, inspired by
+Mercurial's "revlog" format. They enable very efficient insert/remove/replace
+editing of arbitrary spans. Multiple versions of the file can be retained, in
+a revision graph that can have multiple heads. Each revision can be
+referenced by a cryptographic identifier. There are two forms of the URI, one
+that means "most recent version", and a longer one that points to a specific
+revision.
+
+Metadata can be attached to the revisions, like timestamps, to enable rolling
+back an entire tree to a specific point in history.
+
+LDMF1 provides deltas but tries to avoid dealing with multiple heads. LDMF2
+provides explicit support for revision identifiers and branching.
+
+== TODO ==
+
+improve allocate-and-write or get-writer-buckets API to allow one-call (or
+maybe two-call) updates. The challenge is in figuring out which shares are on
+which machines. First cut will have lots of round trips.
+
+(eventually) define behavior when seqnum wraps. At the very least make sure
+it can't cause a security problem. "the slot is worn out" is acceptable.
+
+(eventually) define share-migration lease update protocol. Including the
+nodeid who accepted the lease is useful, we can use the same protocol as we
+do for updating the write enabler. However we need to know which lease to
+update.. maybe send back a list of all old nodeids that we find, then try all
+of them when we accept the update?
+
+ We now do this in a specially-formatted IndexError exception:
+  "UNABLE to renew non-existent lease. I have leases accepted by " +
+  "nodeids: '12345','abcde','44221' ."
+
+confirm that a repairer can regenerate shares without the private key. Hmm,
+without the write-enabler they won't be able to write those shares to the
+servers.. although they could add immutable new shares to new servers.
diff --git a/docs/specifications/uri.txt b/docs/specifications/uri.txt
new file mode 100644 (file)
index 0000000..5599fa1
--- /dev/null
@@ -0,0 +1,187 @@
+
+= Tahoe URIs =
+
+Each file and directory in a Tahoe filesystem is described by a "URI". There
+are different kinds of URIs for different kinds of objects, and there are
+different kinds of URIs to provide different kinds of access to those
+objects. Each URI is a string representation of a "capability" or "cap", and
+there are read-caps, write-caps, verify-caps, and others.
+
+Each URI provides both '''location''' and '''identification''' properties.
+'''location''' means that holding the URI is sufficient to locate the data it
+represents (this means it contains a storage index or a lookup key, whatever
+is necessary to find the place or places where the data is being kept).
+'''identification''' means that the URI also serves to validate the data: an
+attacker who wants to trick you into into using the wrong data will be
+limited in their abilities by the identification properties of the URI.
+
+Some URIs are subsets of others. In particular, if you know a URI which
+allows you to modify some object, you can produce a weaker read-only URI and
+give it to someone else, and they will be able to read that object but not
+modify it. Directories, for example, have a read-cap which is derived from
+the write-cap: anyone with read/write access to the directory can produce a
+limited URI that grants read-only access, but not the other way around.
+
+source:src/allmydata/uri.py is the main place where URIs are processed. It is
+the authoritative definition point for all the the URI types described
+herein.
+
+== File URIs ==
+
+The lowest layer of the Tahoe architecture (the "grid") is reponsible for
+mapping URIs to data. This is basically a distributed hash table, in which
+the URI is the key, and some sequence of bytes is the value.
+
+There are two kinds of entries in this table: immutable and mutable. For
+immutable entries, the URI represents a fixed chunk of data. The URI itself
+is derived from the data when it is uploaded into the grid, and can be used
+to locate and download that data from the grid at some time in the future.
+
+For mutable entries, the URI identifies a "slot" or "container", which can be
+filled with different pieces of data at different times.
+
+It is important to note that the "files" described by these URIs are just a
+bunch of bytes, and that __no__ filenames or other metadata is retained at
+this layer. The vdrive layer (which sits above the grid layer) is entirely
+responsible for directories and filenames and the like.
+
+=== CHI URIs ===
+
+CHK (Content Hash Keyed) files are immutable sequences of bytes. They are
+uploaded in a distributed fashion using a "storage index" (for the "location"
+property), and encrypted using a "read key". A secure hash of the data is
+computed to help validate the data afterwards (providing the "identification"
+property). All of these pieces, plus information about the file's size and
+the number of shares into which it has been distributed, are put into the
+"CHK" uri. The storage index is derived by hashing the read key (using a
+tagged SHA-256d hash, then truncated to 128 bits), so it does not need to be
+physically present in the URI.
+
+The current format for CHK URIs is the concatenation of the following
+strings:
+
+ URI:CHK:(key):(hash):(needed-shares):(total-shares):(size)
+
+Where (key) is the base32 encoding of the 16-byte AES read key, (hash) is the
+base32 encoding of the SHA-256 hash of the URI Extension Block,
+(needed-shares) is an ascii decimal representation of the number of shares
+required to reconstruct this file, (total-shares) is the same representation
+of the total number of shares created, and (size) is an ascii decimal
+representation of the size of the data represented by this URI. All base32
+encodings are expressed in lower-case, with the trailing '=' signs removed.
+
+For example, the following is a CHK URI, generated from the contents of the
+architecture.txt document that lives next to this one in the source tree:
+
+URI:CHK:ihrbeov7lbvoduupd4qblysj7a:bg5agsdt62jb34hxvxmdsbza6do64f4fg5anxxod2buttbo6udzq:3:10:28733
+
+Historical note: The name "CHK" is somewhat inaccurate and continues to be
+used for historical reasons. "Content Hash Key" means that the encryption key
+is derived by hashing the contents, which gives the useful property that
+encoding the same file twice will result in the same URI. However, this is an
+optional step: by passing a different flag to the appropriate API call, Tahoe
+will generate a random encryption key instead of hashing the file: this gives
+the useful property that the URI or storage index does not reveal anything
+about the file's contents (except filesize), which improves privacy. The
+URI:CHK: prefix really indicates that an immutable file is in use, without
+saying anything about how the key was derived.
+
+=== LIT URIs ===
+
+LITeral files are also an immutable sequence of bytes, but they are so short
+that the data is stored inside the URI itself. These are used for files of 55
+bytes or shorter, which is the point at which the LIT URI is the same length
+as a CHK URI would be.
+
+LIT URIs do not require an upload or download phase, as their data is stored
+directly in the URI.
+
+The format of a LIT URI is simply a fixed prefix concatenated with the base32
+encoding of the file's data:
+
+ URI:LIT:bjuw4y3movsgkidbnrwg26lemf2gcl3xmvrc6kropbuhi3lmbi
+
+The LIT URI for an empty file is "URI:LIT:", and the LIT URI for a 5-byte
+file that contains the string "hello" is "URI:LIT:nbswy3dp".
+
+=== Mutable File URIs ===
+
+The other kind of DHT entry is the "mutable slot", in which the URI names a
+container to which data can be placed and retrieved without changing the
+identity of the container.
+
+These slots have write-caps (which allow read/write access), read-caps (which
+only allow read-access), and verify-caps (which allow a file checker/repairer
+to confirm that the contents exist, but does not let it decrypt the
+contents).
+
+Mutable slots use public key technology to provide data integrity, and put a
+hash of the public key in the URI. As a result, the data validation is
+limited to confirming that the data retrieved matches _some_ data that was
+uploaded in the past, but not _which_ version of that data.
+
+The format of the write-cap for mutable files is:
+
+ URI:SSK:(writekey):(fingerprint)
+
+Where (writekey) is the base32 encoding of the 16-byte AES encryption key
+that is used to encrypt the RSA private key, and (fingerprint) is the base32
+encoded 32-byte SHA-256 hash of the RSA public key. For more details about
+the way these keys are used, please see docs/mutable.txt .
+
+The format for mutable read-caps is:
+
+ URI:SSK-RO:(readkey):(fingerprint)
+
+The read-cap is just like the write-cap except it contains the other AES
+encryption key: the one used for encrypting the mutable file's contents. This
+second key is derived by hashing the writekey, which allows the holder of a
+write-cap to produce a read-cap, but not the other way around. The
+fingerprint is the same in both caps.
+
+Historical note: the "SSK" prefix is a perhaps-inaccurate reference to
+"Sub-Space Keys" from the Freenet project, which uses a vaguely similar
+structure to provide mutable file access.
+
+== Directory URIs ==
+
+The grid layer provides a mapping from URI to data. To turn this into a graph
+of directories and files, the "vdrive" layer (which sits on top of the grid
+layer) needs to keep track of "directory nodes", or "dirnodes" for short.
+source:docs/dirnodes.txt describes how these work.
+
+Dirnodes are contained inside mutable files, and are thus simply a particular
+way to interpret the contents of these files. As a result, a directory
+write-cap looks a lot like a mutable-file write-cap:
+
+ URI:DIR2:(writekey):(fingerprint)
+
+Likewise directory read-caps (which provide read-only access to the
+directory) look much like mutable-file read-caps:
+
+ URI:DIR2-RO:(readkey):(fingerprint)
+
+Historical note: the "DIR2" prefix is used because the non-distributed
+dirnodes in earlier Tahoe releases had already claimed the "DIR" prefix.
+
+== Internal Usage of URIs ==
+
+The classes in source:src/allmydata/uri.py are used to pack and unpack these
+various kinds of URIs. Three Interfaces are defined (IURI, IFileURI, and
+IDirnodeURI) which are implemented by these classes, and string-to-URI-class
+conversion routines have been registered as adapters, so that code which
+wants to extract e.g. the size of a CHK or LIT uri can do:
+
+{{{
+print IFileURI(uri).get_size()
+}}}
+
+If the URI does not represent a CHK or LIT uri (for example, if it was for a
+directory instead), the adaptation will fail, raising a TypeError inside the
+IFileURI() call.
+
+Several utility methods are provided on these objects. The most important is
+{{{ to_string() }}}, which returns the string form of the URI. Therefore {{{
+IURI(uri).to_string == uri }}} is true for any valid URI. See the IURI class
+in source:src/allmydata/interfaces.py for more details.
+
diff --git a/docs/uri.txt b/docs/uri.txt
deleted file mode 100644 (file)
index 5599fa1..0000000
+++ /dev/null
@@ -1,187 +0,0 @@
-
-= Tahoe URIs =
-
-Each file and directory in a Tahoe filesystem is described by a "URI". There
-are different kinds of URIs for different kinds of objects, and there are
-different kinds of URIs to provide different kinds of access to those
-objects. Each URI is a string representation of a "capability" or "cap", and
-there are read-caps, write-caps, verify-caps, and others.
-
-Each URI provides both '''location''' and '''identification''' properties.
-'''location''' means that holding the URI is sufficient to locate the data it
-represents (this means it contains a storage index or a lookup key, whatever
-is necessary to find the place or places where the data is being kept).
-'''identification''' means that the URI also serves to validate the data: an
-attacker who wants to trick you into into using the wrong data will be
-limited in their abilities by the identification properties of the URI.
-
-Some URIs are subsets of others. In particular, if you know a URI which
-allows you to modify some object, you can produce a weaker read-only URI and
-give it to someone else, and they will be able to read that object but not
-modify it. Directories, for example, have a read-cap which is derived from
-the write-cap: anyone with read/write access to the directory can produce a
-limited URI that grants read-only access, but not the other way around.
-
-source:src/allmydata/uri.py is the main place where URIs are processed. It is
-the authoritative definition point for all the the URI types described
-herein.
-
-== File URIs ==
-
-The lowest layer of the Tahoe architecture (the "grid") is reponsible for
-mapping URIs to data. This is basically a distributed hash table, in which
-the URI is the key, and some sequence of bytes is the value.
-
-There are two kinds of entries in this table: immutable and mutable. For
-immutable entries, the URI represents a fixed chunk of data. The URI itself
-is derived from the data when it is uploaded into the grid, and can be used
-to locate and download that data from the grid at some time in the future.
-
-For mutable entries, the URI identifies a "slot" or "container", which can be
-filled with different pieces of data at different times.
-
-It is important to note that the "files" described by these URIs are just a
-bunch of bytes, and that __no__ filenames or other metadata is retained at
-this layer. The vdrive layer (which sits above the grid layer) is entirely
-responsible for directories and filenames and the like.
-
-=== CHI URIs ===
-
-CHK (Content Hash Keyed) files are immutable sequences of bytes. They are
-uploaded in a distributed fashion using a "storage index" (for the "location"
-property), and encrypted using a "read key". A secure hash of the data is
-computed to help validate the data afterwards (providing the "identification"
-property). All of these pieces, plus information about the file's size and
-the number of shares into which it has been distributed, are put into the
-"CHK" uri. The storage index is derived by hashing the read key (using a
-tagged SHA-256d hash, then truncated to 128 bits), so it does not need to be
-physically present in the URI.
-
-The current format for CHK URIs is the concatenation of the following
-strings:
-
- URI:CHK:(key):(hash):(needed-shares):(total-shares):(size)
-
-Where (key) is the base32 encoding of the 16-byte AES read key, (hash) is the
-base32 encoding of the SHA-256 hash of the URI Extension Block,
-(needed-shares) is an ascii decimal representation of the number of shares
-required to reconstruct this file, (total-shares) is the same representation
-of the total number of shares created, and (size) is an ascii decimal
-representation of the size of the data represented by this URI. All base32
-encodings are expressed in lower-case, with the trailing '=' signs removed.
-
-For example, the following is a CHK URI, generated from the contents of the
-architecture.txt document that lives next to this one in the source tree:
-
-URI:CHK:ihrbeov7lbvoduupd4qblysj7a:bg5agsdt62jb34hxvxmdsbza6do64f4fg5anxxod2buttbo6udzq:3:10:28733
-
-Historical note: The name "CHK" is somewhat inaccurate and continues to be
-used for historical reasons. "Content Hash Key" means that the encryption key
-is derived by hashing the contents, which gives the useful property that
-encoding the same file twice will result in the same URI. However, this is an
-optional step: by passing a different flag to the appropriate API call, Tahoe
-will generate a random encryption key instead of hashing the file: this gives
-the useful property that the URI or storage index does not reveal anything
-about the file's contents (except filesize), which improves privacy. The
-URI:CHK: prefix really indicates that an immutable file is in use, without
-saying anything about how the key was derived.
-
-=== LIT URIs ===
-
-LITeral files are also an immutable sequence of bytes, but they are so short
-that the data is stored inside the URI itself. These are used for files of 55
-bytes or shorter, which is the point at which the LIT URI is the same length
-as a CHK URI would be.
-
-LIT URIs do not require an upload or download phase, as their data is stored
-directly in the URI.
-
-The format of a LIT URI is simply a fixed prefix concatenated with the base32
-encoding of the file's data:
-
- URI:LIT:bjuw4y3movsgkidbnrwg26lemf2gcl3xmvrc6kropbuhi3lmbi
-
-The LIT URI for an empty file is "URI:LIT:", and the LIT URI for a 5-byte
-file that contains the string "hello" is "URI:LIT:nbswy3dp".
-
-=== Mutable File URIs ===
-
-The other kind of DHT entry is the "mutable slot", in which the URI names a
-container to which data can be placed and retrieved without changing the
-identity of the container.
-
-These slots have write-caps (which allow read/write access), read-caps (which
-only allow read-access), and verify-caps (which allow a file checker/repairer
-to confirm that the contents exist, but does not let it decrypt the
-contents).
-
-Mutable slots use public key technology to provide data integrity, and put a
-hash of the public key in the URI. As a result, the data validation is
-limited to confirming that the data retrieved matches _some_ data that was
-uploaded in the past, but not _which_ version of that data.
-
-The format of the write-cap for mutable files is:
-
- URI:SSK:(writekey):(fingerprint)
-
-Where (writekey) is the base32 encoding of the 16-byte AES encryption key
-that is used to encrypt the RSA private key, and (fingerprint) is the base32
-encoded 32-byte SHA-256 hash of the RSA public key. For more details about
-the way these keys are used, please see docs/mutable.txt .
-
-The format for mutable read-caps is:
-
- URI:SSK-RO:(readkey):(fingerprint)
-
-The read-cap is just like the write-cap except it contains the other AES
-encryption key: the one used for encrypting the mutable file's contents. This
-second key is derived by hashing the writekey, which allows the holder of a
-write-cap to produce a read-cap, but not the other way around. The
-fingerprint is the same in both caps.
-
-Historical note: the "SSK" prefix is a perhaps-inaccurate reference to
-"Sub-Space Keys" from the Freenet project, which uses a vaguely similar
-structure to provide mutable file access.
-
-== Directory URIs ==
-
-The grid layer provides a mapping from URI to data. To turn this into a graph
-of directories and files, the "vdrive" layer (which sits on top of the grid
-layer) needs to keep track of "directory nodes", or "dirnodes" for short.
-source:docs/dirnodes.txt describes how these work.
-
-Dirnodes are contained inside mutable files, and are thus simply a particular
-way to interpret the contents of these files. As a result, a directory
-write-cap looks a lot like a mutable-file write-cap:
-
- URI:DIR2:(writekey):(fingerprint)
-
-Likewise directory read-caps (which provide read-only access to the
-directory) look much like mutable-file read-caps:
-
- URI:DIR2-RO:(readkey):(fingerprint)
-
-Historical note: the "DIR2" prefix is used because the non-distributed
-dirnodes in earlier Tahoe releases had already claimed the "DIR" prefix.
-
-== Internal Usage of URIs ==
-
-The classes in source:src/allmydata/uri.py are used to pack and unpack these
-various kinds of URIs. Three Interfaces are defined (IURI, IFileURI, and
-IDirnodeURI) which are implemented by these classes, and string-to-URI-class
-conversion routines have been registered as adapters, so that code which
-wants to extract e.g. the size of a CHK or LIT uri can do:
-
-{{{
-print IFileURI(uri).get_size()
-}}}
-
-If the URI does not represent a CHK or LIT uri (for example, if it was for a
-directory instead), the adaptation will fail, raising a TypeError inside the
-IFileURI() call.
-
-Several utility methods are provided on these objects. The most important is
-{{{ to_string() }}}, which returns the string form of the URI. Therefore {{{
-IURI(uri).to_string == uri }}} is true for any valid URI. See the IURI class
-in source:src/allmydata/interfaces.py for more details.
-