From 48dcd6f1641f4eee0dcdfcbaa9af396f8e2d6a64 Mon Sep 17 00:00:00 2001
From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Fri, 24 Jul 2015 21:13:10 +0530
Subject: [PATCH] verify SHA1 hash of each piece after assembling the blocks

---
 src/FuncTorrent/Peer.hs | 43 +++++++++++++++++++++++++----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/src/FuncTorrent/Peer.hs b/src/FuncTorrent/Peer.hs
index ae768a8..7802b08 100644
--- a/src/FuncTorrent/Peer.hs
+++ b/src/FuncTorrent/Peer.hs
@@ -4,10 +4,10 @@ module FuncTorrent.Peer
      handlePeerMsgs
     ) where
 
-import Prelude hiding (lookup, concat, replicate, splitAt, writeFile)
+import Prelude hiding (lookup, concat, replicate, splitAt, writeFile, take)
 
 import System.IO (Handle, BufferMode(..), IOMode(..), SeekMode(..), withFile, hSeek, hSetBuffering)
-import Data.ByteString (ByteString, pack, unpack, concat, hGet, hPut, singleton, writeFile)
+import Data.ByteString (ByteString, pack, unpack, concat, hGet, hPut, singleton, writeFile, take)
 import Data.ByteString.Lazy (fromStrict, fromChunks, toStrict)
 import qualified Data.ByteString.Char8 as BC (replicate, pack, length)
 import Network (connectTo, PortID(..))
@@ -19,6 +19,7 @@ import Control.Applicative ((<$>), liftA3)
 import Data.Bits
 import Data.Word (Word8)
 import Data.Map (Map, fromList, toList, (!), mapWithKey, adjust)
+import qualified Crypto.Hash.SHA1 as SHA1 (hash)
 
 import FuncTorrent.Metainfo (Info(..), Metainfo(..))
 import FuncTorrent.Utils (splitN, splitNum)
@@ -213,9 +214,12 @@ msgLoop pState pieceStatus | meInterested pState == False &&
                                Nothing -> putStrLn "Nothing to download"
                                Just workPiece -> do
                                  let pLen = len (pieceStatus ! workPiece)
-                                 _ <- downloadPiece (handle pState) workPiece pLen
-                                 -- TODO: verify the hash
-                                 msgLoop pState (adjust (\pieceData -> pieceData { state = Have }) workPiece pieceStatus)
+                                 pBS <- downloadPiece (handle pState) workPiece pLen
+                                 if not $ verifyHash pBS (hash (pieceStatus ! workPiece))
+                                   then
+                                   putStrLn $ "Hash mismatch: " ++ show (hash (pieceStatus ! workPiece)) ++ " vs " ++ show (take 20 (SHA1.hash pBS))
+                                   else
+                                   msgLoop pState (adjust (\pieceData -> pieceData { state = Have }) workPiece pieceStatus)
                           | otherwise = do
                               msg <- getMsg (handle pState)
                               putStrLn $ "<-- " ++ show msg ++ "from peer: " ++ show (peer pState)
@@ -270,17 +274,22 @@ handlePeerMsgs p m peerId = do
       pieceStatus = mkPieceMap numPieces pieceHash (splitNum fileLen pLen)
   msgLoop state pieceStatus
   
-downloadPiece :: Handle -> Integer -> Integer -> IO [ByteString]
+downloadPiece :: Handle -> Integer -> Integer -> IO ByteString
 downloadPiece h index pieceLength = do
   let chunks = splitNum pieceLength 16384
-  forM (zip [0..] chunks) (\(i, pLen) -> do
-                              sendMsg h (RequestMsg index (i*pLen) pLen)
-                              putStrLn $ "--> " ++ "RequestMsg for Piece "
-                                ++ (show index) ++ ", part: " ++ show i ++ " of length: "
-                                ++ show pLen
-                              PieceMsg index begin block <- getMsg h
-                              putStrLn $ " <-- PieceMsg for Piece: "
-                                ++ show index
-                                ++ ", offset: "
-                                ++ show begin
-                              return block)
+  liftM concat $ forM (zip [0..] chunks) (\(i, pLen) -> do
+                                             sendMsg h (RequestMsg index (i*pLen) pLen)
+                                             putStrLn $ "--> " ++ "RequestMsg for Piece "
+                                               ++ (show index) ++ ", part: " ++ show i ++ " of length: "
+                                               ++ show pLen
+                                             -- TODO: non exhaustive pattern matching. :(
+                                             PieceMsg index begin block <- getMsg h
+                                             putStrLn $ " <-- PieceMsg for Piece: "
+                                               ++ show index
+                                               ++ ", offset: "
+                                               ++ show begin
+                                             return block)
+
+verifyHash :: ByteString -> ByteString -> Bool
+verifyHash bs pieceHash =
+  take 20 (SHA1.hash bs) == pieceHash
-- 
2.37.2