
-- | Password encoding and validation using bcrypt.
--
-- Example usasge:
--
-- >>> import Crypto.KDF.BCrypt (hashPassword, validatePassword)
-- >>> import qualified Data.ByteString.Char8 as B
-- >>>
-- >>> let bcryptHash = B.pack "$2a$10$MJJifxfaqQmbx1Mhsq3oq.YmMmfNhkyW4s/MS3K5rIMVfB7w0Q/OW"
-- >>> let password = B.pack "password"
-- >>> validatePassword password bcryptHash
-- >>> True
-- >>> let otherPassword = B.pack "otherpassword"
-- >>> otherHash <- hashPassword 12 otherPasssword :: IO B.ByteString
-- >>> validatePassword otherPassword otherHash
-- >>> True
--
-- See <https://www.usenix.org/conference/1999-usenix-annual-technical-conference/future-adaptable-password-scheme>
-- for details of the original algorithm.
--
-- The functions @hashPassword@ and @validatePassword@ should be all that
-- most users need.
--
-- Hashes are strings of the form
-- @$2a$10$MJJifxfaqQmbx1Mhsq3oq.YmMmfNhkyW4s/MS3K5rIMVfB7w0Q/OW@ which
-- encode a version number, an integer cost parameter and the concatenated
-- salt and hash bytes (each separately Base64 encoded. Incrementing the
-- cost parameter approximately doubles the time taken to calculate the hash.
--
-- The different version numbers have evolved because of bugs in the standard
-- C implementations. The most up to date version is @2b@ and this
-- implementation the @2b@ version prefix, but will also attempt to validate
-- against hashes with versions @2a@ and @2y@. Version @2@ or @2x@ will be
-- rejected. No attempt is made to differentiate between the different versions
-- when validating a password, but in practice this shouldn't cause any problems
-- if passwords are UTF-8 encoded (which they should be).
--
-- The cost parameter can be between 4 and 31 inclusive, but anything less than
-- 10 is probably not strong enough. High values may be prohibitively slow
-- depending on your hardware. Choose the highest value you can without having
-- an unacceptable impact on your users. The cost parameter can also be varied
-- depending on the account, since it is unique to an individual hash.

module Crypto.KDF.BCrypt
    ( hashPassword
    , validatePassword
    , validatePasswordEither
    , bcrypt
    )
where

import           Control.Monad (unless, when)
import           Crypto.Cipher.Blowfish.Primitive (eksBlowfish, encrypt)
import           Crypto.Random (MonadRandom, getRandomBytes)
import           Data.ByteArray (ByteArrayAccess, ByteArray, Bytes)
import qualified Data.ByteArray as B
import           Data.ByteArray.Encoding
import           Data.Char

data BCryptHash = BCH Char Int Bytes Bytes

-- | Create a bcrypt hash for a password with a provided cost value.
-- Typically used to create a hash when a new user account is registered
-- or when a user changes their password.
--
-- Each increment of the cost approximately doubles the time taken.
-- The 16 bytes of random salt will be generated internally.
hashPassword :: (MonadRandom m, ByteArray password, ByteArray hash)
             => Int
             -- ^ The cost parameter. Should be between 4 and 31 (inclusive).
             -- Values which lie outside this range will be adjusted accordingly.
             -> password
             -- ^ The password. Should be the UTF-8 encoded bytes of the password text.
             -> m hash
             -- ^ The bcrypt hash in standard format.
hashPassword cost password = do
    salt <- getRandomBytes 16
    return $ bcrypt cost (salt :: Bytes) password

-- | Create a bcrypt hash for a password with a provided cost value and salt.
bcrypt :: (ByteArray salt, ByteArray password, ByteArray output)
       => Int
       -- ^ The cost parameter. Should be between 4 and 31 (inclusive).
       -- Values which lie outside this range will be adjusted accordingly.
       -> salt
       -- ^ The salt. Must be 16 bytes in length or an error will be raised.
       -> password
       -- ^ The password. Should be the UTF-8 encoded bytes of the password text.
       -> output
       -- ^ The bcrypt hash in standard format.
bcrypt cost salt password = B.concat [header, B.snoc costBytes dollar, b64 salt, b64 hash]
  where
    hash   = rawHash 'b' realCost salt password
    header = B.pack [dollar, fromIntegral (ord '2'), fromIntegral (ord 'a'), dollar]
    dollar = fromIntegral (ord '$')
    zero   = fromIntegral (ord '0')
    costBytes  = B.pack [zero + fromIntegral (realCost `div` 10), zero + fromIntegral (realCost `mod` 10)]
    realCost
        | cost < 4  = 10 -- 4 is virtually pointless so go for 10
        | cost > 31 = 31
        | otherwise = cost

    b64 :: (ByteArray ba) => ba -> ba
    b64 = convertToBase Base64OpenBSD

-- | Check a password against a stored bcrypt hash when authenticating a user.
--
-- Returns @False@ if the password doesn't match the hash, or if the hash is
-- invalid or an unsupported version.
validatePassword :: (ByteArray password, ByteArray hash) => password -> hash -> Bool
validatePassword password bcHash = either (const False) id (validatePasswordEither password bcHash)

-- | Check a password against a bcrypt hash
--
-- As for @validatePassword@ but will provide error information if the hash is invalid or
-- an unsupported version.
validatePasswordEither :: (ByteArray password, ByteArray hash) => password -> hash -> Either String Bool
validatePasswordEither password bcHash = do
    BCH version cost salt hash <- parseBCryptHash bcHash
    return $ (rawHash version cost salt password :: Bytes) `B.constEq` hash

rawHash :: (ByteArrayAccess salt, ByteArray password, ByteArray output) => Char -> Int -> salt -> password -> output
rawHash _ cost salt password = B.take 23 hash -- Another compatibility bug. Ignore last byte of hash
  where
    hash = loop (0 :: Int) orpheanBeholder

    loop i input
        | i < 64    = loop (i+1) (encrypt ctx input)
        | otherwise = input

    -- Truncate the password if necessary and append a null byte for C compatibility
    key = B.snoc (B.take 72 password) 0

    ctx = eksBlowfish cost salt key

    -- The BCrypt plaintext: "OrpheanBeholderScryDoubt"
    orpheanBeholder = B.pack [79,114,112,104,101,97,110,66,101,104,111,108,100,101,114,83,99,114,121,68,111,117,98,116]

-- "$2a$10$XajjQvNhvvRt5GSeFk1xFeyqRrsxkhBkUiQeg0dt.wU1qD4aFDcga"
parseBCryptHash :: (ByteArray ba) => ba -> Either String BCryptHash
parseBCryptHash bc = do
    unless (B.length bc == 60      &&
            B.index bc 0 == dollar &&
            B.index bc 1 == fromIntegral (ord '2') &&
            B.index bc 3 == dollar &&
            B.index bc 6 == dollar) (Left "Invalid hash format")
    unless (version == 'b' || version == 'a' || version == 'y') (Left ("Unsupported minor version: " ++ [version]))
    when (costTens > 3 || cost > 31 || cost < 4)  (Left "Invalid bcrypt cost")
    (salt, hash) <- decodeSaltHash (B.drop 7 bc)
    return (BCH version cost salt hash)
  where
    dollar    = fromIntegral (ord '$')
    zero      = ord '0'
    costTens  = fromIntegral (B.index bc 4) - zero
    costUnits = fromIntegral (B.index bc 5) - zero
    version   = chr (fromIntegral (B.index bc 2))
    cost      = costUnits + (if costTens == 0 then 0 else 10^costTens) :: Int

    decodeSaltHash saltHash = do
        let (s, h) = B.splitAt 22 saltHash
        salt <- convertFromBase Base64OpenBSD s
        hash <- convertFromBase Base64OpenBSD h
        return (salt, hash)
