git-annex/Crypto.hs

{- git-annex crypto
 -
 - Currently using gpg; could later be modified to support different
 - crypto backends if neccessary.
 -
 - Copyright 2011 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module Crypto (
	Cipher,
	EncryptedCipher,
	genCipher,
	updateCipher,
	describeCipher,
	storeCipher,
	extractCipher,
	decryptCipher,		
	encryptKey,
	withEncryptedHandle,
	withDecryptedHandle,
	withEncryptedContent,
	withDecryptedContent,

	prop_hmacWithCipher_sane
) where

import qualified Data.ByteString.Lazy.Char8 as L
import qualified Data.Map as M
import Data.ByteString.Lazy.UTF8 (fromString)
import Data.Digest.Pure.SHA
import Control.Applicative

import Common.Annex
import qualified Utility.Gpg as Gpg
import Types.Key
import Types.Remote
import Utility.Base64
import Types.Crypto

{- The first half of a Cipher is used for HMAC; the remainder
 - is used as the GPG symmetric encryption passphrase.
 -
 - HMAC SHA1 needs only 64 bytes. The remainder is for expansion,
 - perhaps to HMAC SHA512, which needs 128 bytes (ideally).
 -
 - 256 is enough for gpg's symetric cipher; unlike weaker public key
 - crypto, the key does not need to be too large.
 -}
cipherHalf :: Int
cipherHalf = 256

cipherSize :: Int
cipherSize = cipherHalf * 2

cipherPassphrase :: Cipher -> String
cipherPassphrase (Cipher c) = drop cipherHalf c

cipherHmac :: Cipher -> String
cipherHmac (Cipher c) = take cipherHalf c

{- Creates a new Cipher, encrypted as specified in the remote's configuration -}
genCipher :: RemoteConfig -> IO EncryptedCipher
genCipher c = do
	ks <- configKeyIds c
	random <- genrandom
	encryptCipher (Cipher random) ks
	where
		genrandom = Gpg.readStrict
			-- Armor the random data, to avoid newlines,
			-- since gpg only reads ciphers up to the first
			-- newline.
			[ Params "--gen-random --armor"
			, Param $ show randomquality
			, Param $ show cipherSize
			]
		-- 1 is /dev/urandom; 2 is /dev/random
		randomquality = 1 :: Int

{- Updates an existing Cipher, re-encrypting it to add KeyIds specified in
 - the remote's configuration. -}
updateCipher :: RemoteConfig -> EncryptedCipher -> IO EncryptedCipher
updateCipher c encipher@(EncryptedCipher _ ks) = do
	ks' <- configKeyIds c
	cipher <- decryptCipher c encipher
	encryptCipher cipher (merge ks ks')
	where
		merge (KeyIds a) (KeyIds b) = KeyIds $ a ++ b

describeCipher :: EncryptedCipher -> String
describeCipher (EncryptedCipher _ (KeyIds ks)) =
	"with gpg " ++ keys ks ++ " " ++ unwords ks
	where
		keys [_] = "key"
		keys _ = "keys"

{- Stores an EncryptedCipher in a remote's configuration. -}
storeCipher :: RemoteConfig -> EncryptedCipher -> RemoteConfig
storeCipher c (EncryptedCipher t ks) = 
	M.insert "cipher" (toB64 t) $ M.insert "cipherkeys" (showkeys ks) c
	where
		showkeys (KeyIds l) = join "," l

{- Extracts an EncryptedCipher from a remote's configuration. -}
extractCipher :: RemoteConfig -> Maybe EncryptedCipher
extractCipher c = 
	case (M.lookup "cipher" c, M.lookup "cipherkeys" c) of
		(Just t, Just ks) -> Just $ EncryptedCipher (fromB64 t) (readkeys ks)
		_ -> Nothing
	where
		readkeys = KeyIds . split ","

{- Encrypts a Cipher to the specified KeyIds. -}
encryptCipher :: Cipher -> KeyIds -> IO EncryptedCipher
encryptCipher (Cipher c) (KeyIds ks) = do
	let ks' = nub $ sort ks -- gpg complains about duplicate recipient keyids
	encipher <- Gpg.pipeStrict (encrypt++recipients ks') c
	return $ EncryptedCipher encipher (KeyIds ks')
	where
		encrypt = [ Params "--encrypt" ]
		recipients l = force_recipients :
			concatMap (\k -> [Param "--recipient", Param k]) l
		-- Force gpg to only encrypt to the specified
		-- recipients, not configured defaults.
		force_recipients = Params "--no-encrypt-to --no-default-recipient"

{- Decrypting an EncryptedCipher is expensive; the Cipher should be cached. -}
decryptCipher :: RemoteConfig -> EncryptedCipher -> IO Cipher
decryptCipher _ (EncryptedCipher encipher _) = 
	Cipher <$> Gpg.pipeStrict decrypt encipher
	where
		decrypt = [ Param "--decrypt" ]

{- Generates an encrypted form of a Key. The encryption does not need to be
 - reversable, nor does it need to be the same type of encryption used
 - on content. It does need to be repeatable. -}
encryptKey :: Cipher -> Key -> Key
encryptKey c k = Key
	{ keyName = hmacWithCipher c (show k)
	, keyBackendName = "GPGHMACSHA1"
	, keySize = Nothing -- size and mtime omitted
	, keyMtime = Nothing -- to avoid leaking data
	}

{- Runs an action, passing it a handle from which it can 
 - stream encrypted content. -}
withEncryptedHandle :: Cipher -> IO L.ByteString -> (Handle -> IO a) -> IO a
withEncryptedHandle = Gpg.passphraseHandle [Params "--symmetric --force-mdc"] . cipherPassphrase

{- Runs an action, passing it a handle from which it can
 - stream decrypted content. -}
withDecryptedHandle :: Cipher -> IO L.ByteString -> (Handle -> IO a) -> IO a
withDecryptedHandle = Gpg.passphraseHandle [Param "--decrypt"] . cipherPassphrase

{- Streams encrypted content to an action. -}
withEncryptedContent :: Cipher -> IO L.ByteString -> (L.ByteString -> IO a) -> IO a
withEncryptedContent = pass withEncryptedHandle

{- Streams decrypted content to an action. -}
withDecryptedContent :: Cipher -> IO L.ByteString -> (L.ByteString -> IO a) -> IO a
withDecryptedContent = pass withDecryptedHandle

pass :: (Cipher -> IO L.ByteString -> (Handle -> IO a) -> IO a) 
      -> Cipher -> IO L.ByteString -> (L.ByteString -> IO a) -> IO a
pass to c i a = to c i $ \h -> a =<< L.hGetContents h

configKeyIds :: RemoteConfig -> IO KeyIds
configKeyIds c = Gpg.findPubKeys $ configGet c "encryption"

configGet :: RemoteConfig -> String -> String
configGet c key = fromMaybe missing $ M.lookup key c
	where
		missing = error $ "missing " ++ key ++ " in remote config"

hmacWithCipher :: Cipher -> String -> String
hmacWithCipher c = hmacWithCipher' (cipherHmac c) 
hmacWithCipher' :: String -> String -> String
hmacWithCipher' c s = showDigest $ hmacSha1 (fromString c) (fromString s)

{- Ensure that hmacWithCipher' returns the same thing forevermore. -}
prop_hmacWithCipher_sane :: Bool
prop_hmacWithCipher_sane = known_good == hmacWithCipher' "foo" "bar"
	where
		known_good = "46b4ec586117154dacd49d664e5d63fdc88efb51"