-- |
-- Module      : Crypto.Saltine.Core.Auth
-- Copyright   : (c) Joseph Abrahamson 2013
-- License     : MIT
--
-- Maintainer  : me@jspha.com
-- Stability   : experimental
-- Portability : non-portable
--
-- Secret-key message authentication:
-- "Crypto.Saltine.Core.Auth"
--
-- The 'auth' function authenticates a message 'ByteString' using a
-- secret key. The function returns an authenticator. The 'verify'
-- function checks if it's passed a correct authenticator of a message
-- under the given secret key.
--
-- The 'auth' function, viewed as a function of the message for a
-- uniform random key, is designed to meet the standard notion of
-- unforgeability. This means that an attacker cannot find
-- authenticators for any messages not authenticated by the sender,
-- even if the attacker has adaptively influenced the messages
-- authenticated by the sender. For a formal definition see, e.g.,
-- Section 2.4 of Bellare, Kilian, and Rogaway, \"The security of the
-- cipher block chaining message authentication code,\" Journal of
-- Computer and System Sciences 61 (2000), 362–399;
-- <http://www-cse.ucsd.edu/~mihir/papers/cbc.html>.
--
-- Saltine does not make any promises regarding \"strong\"
-- unforgeability; perhaps one valid authenticator can be converted
-- into another valid authenticator for the same message. NaCl also
-- does not make any promises regarding \"truncated unforgeability.\"
--
-- "Crypto.Saltine.Core.Auth" is currently an implementation of
-- HMAC-SHA-512-256, i.e., the first 256 bits of
-- HMAC-SHA-512. HMAC-SHA-512-256 is conjectured to meet the standard
-- notion of unforgeability.
--
-- This is version 2010.08.30 of the auth.html web page.
module Crypto.Saltine.Core.Auth (
  Key, Authenticator,
  newKey,
  auth, verify
  ) where

import Crypto.Saltine.Internal.Auth
            ( c_auth
            , c_auth_verify
            , Key(..)
            , Authenticator(..)
            )
import Crypto.Saltine.Internal.Util as U
import Data.ByteString              (ByteString)

import qualified Crypto.Saltine.Internal.Auth as Bytes

-- | Creates a random key of the correct size for 'auth' and 'verify'.
newKey :: IO Key
newKey :: IO Key
newKey = ByteString -> Key
Key (ByteString -> Key) -> IO ByteString -> IO Key
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Int -> IO ByteString
randomByteString Int
Bytes.auth_keybytes

-- | Computes an keyed authenticator 'ByteString' from a message. It
-- is infeasible to forge these authenticators without the key, even
-- if an attacker observes many authenticators and messages and has
-- the ability to influence the messages sent.
auth :: Key
     -> ByteString
     -- ^ Message
     -> Authenticator
auth :: Key -> ByteString -> Authenticator
auth (Key ByteString
key) ByteString
msg =
  ByteString -> Authenticator
Au (ByteString -> Authenticator)
-> ((Ptr CChar -> IO CInt) -> ByteString)
-> (Ptr CChar -> IO CInt)
-> Authenticator
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (CInt, ByteString) -> ByteString
forall a b. (a, b) -> b
snd ((CInt, ByteString) -> ByteString)
-> ((Ptr CChar -> IO CInt) -> (CInt, ByteString))
-> (Ptr CChar -> IO CInt)
-> ByteString
forall b c a. (b -> c) -> (a -> b) -> a -> c
. Int -> (Ptr CChar -> IO CInt) -> (CInt, ByteString)
forall b. Int -> (Ptr CChar -> IO b) -> (b, ByteString)
buildUnsafeByteString Int
Bytes.auth_bytes ((Ptr CChar -> IO CInt) -> Authenticator)
-> (Ptr CChar -> IO CInt) -> Authenticator
forall a b. (a -> b) -> a -> b
$ \Ptr CChar
pa ->
    [ByteString] -> ([CStringLen] -> IO CInt) -> IO CInt
forall b. [ByteString] -> ([CStringLen] -> IO b) -> IO b
constByteStrings [ByteString
key, ByteString
msg] (([CStringLen] -> IO CInt) -> IO CInt)
-> ([CStringLen] -> IO CInt) -> IO CInt
forall a b. (a -> b) -> a -> b
$ \[(Ptr CChar
pk, Int
_), (Ptr CChar
pm, Int
mlen)] ->
    Ptr CChar -> Ptr CChar -> CULLong -> Ptr CChar -> IO CInt
c_auth Ptr CChar
pa Ptr CChar
pm (Int -> CULLong
forall a b. (Integral a, Num b) => a -> b
fromIntegral Int
mlen) Ptr CChar
pk

-- | Checks to see if an authenticator is a correct proof that a
-- message was signed by some key.
verify :: Key
       -> Authenticator
       -> ByteString
       -- ^ Message
       -> Bool
       -- ^ Is this message authentic?
verify :: Key -> Authenticator -> ByteString -> Bool
verify (Key ByteString
key) (Au ByteString
a) ByteString
msg =
  IO CInt -> Bool
unsafeDidSucceed (IO CInt -> Bool) -> IO CInt -> Bool
forall a b. (a -> b) -> a -> b
$ [ByteString] -> ([CStringLen] -> IO CInt) -> IO CInt
forall b. [ByteString] -> ([CStringLen] -> IO b) -> IO b
constByteStrings [ByteString
key, ByteString
msg, ByteString
a] (([CStringLen] -> IO CInt) -> IO CInt)
-> ([CStringLen] -> IO CInt) -> IO CInt
forall a b. (a -> b) -> a -> b
$ \[(Ptr CChar
pk, Int
_), (Ptr CChar
pm, Int
mlen), (Ptr CChar
pa, Int
_)] ->
    CInt -> IO CInt
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return (CInt -> IO CInt) -> CInt -> IO CInt
forall a b. (a -> b) -> a -> b
$ Ptr CChar -> Ptr CChar -> CULLong -> Ptr CChar -> CInt
c_auth_verify Ptr CChar
pa Ptr CChar
pm (Int -> CULLong
forall a b. (Integral a, Num b) => a -> b
fromIntegral Int
mlen) Ptr CChar
pk