From: Brian Warner Date: Tue, 2 Aug 2011 04:45:41 +0000 (-0700) Subject: add docs about timing-channel attacks X-Git-Url: https://git.rkrishnan.org/%5B?a=commitdiff_plain;h=4028908b2eb195f68de6b210ce541c13ec00118f;p=tahoe-lafs%2Ftahoe-lafs.git add docs about timing-channel attacks --- diff --git a/docs/cautions.rst b/docs/cautions.rst new file mode 100644 index 00000000..34ef2a53 --- /dev/null +++ b/docs/cautions.rst @@ -0,0 +1,52 @@ +Things To Be Careful About As We Venture Boldly Forth +===================================================== + +Timing Attacks +-------------- + +Asymmetric-key cryptography operations are particularly sensitive to +side-channel attacks. Unless the library is carefully hardened against timing +attacks, it is dangerous to allow an attacker to measure how long signature +and pubkey-derivation operations take. With enough samples, the attacker can +deduce the private signing key from these measurements. (Note that +verification operations are only sensitive if the verifying key is secret, +which is not the case for anything in Tahoe). + +We currently use private-key operations in mutable-file writes, and +anticipate using them in signed-introducer announcements and accounting +setup. + +Mutable-file writes can reveal timing information to the attacker because the +signature operation takes place in the middle of a read-modify-write cycle. +Modifying a directory requires downloading the old contents of the mutable +file, modifying the contents, signing the new contents, then uploading the +new contents. By observing the elapsed time between the receipt of the last +packet for the download, and the emission of the first packet of the upload, +the attacker will learn information about how long the signature took. The +attacker might ensure that they run one of the servers, and delay responding +to the download request so that their packet is the last one needed by the +client. They might also manage to be the first server to which a new upload +packet is sent. This attack gives the adversary timing information about one +signature operation per mutable-file write. Note that the UCWE +automatic-retry response (used by default in directory modification code) can +cause multiple mutable-file read-modify-write cycles per user-triggered +operation, giving the adversary a slightly higher multiplier. + +The signed-introducer announcement involves a signature made as the client +node is booting, before the first connection is established to the +Introducer. This might reveal timing information if any information is +revealed about the client's exact boot time: the signature operation starts a +fixed number of cycles after node startup, and the first packet to the +Introducer is sent a fixed number of cycles after the signature is made. An +adversary who can compare the node boot time against the transmission time of +the first packet will learn information about the signature operation, one +measurement per reboot. We currently do not provide boot-time information in +Introducer messages or other client-to-server data. + +In general, we are not worried about these leakages, because timing-channel +attacks typically require thousands or millions of measurements to detect the +(presumably) small timing variations exposed by our asymmetric crypto +operations, which would require thousands of mutable-file writes or thousands +of reboots to be of use to the adversary. However, future authors should take +care to not make changes that could provide additional information to +attackers.