From 67b5043c69648f0e4b9d9138a03cce154c20e38d Mon Sep 17 00:00:00 2001
From: Benjamin Barenblat <bbaren@google.com>
Date: Tue, 4 Sep 2018 12:27:10 -0500
Subject: Documentation improvements

---
 src/BTLS/Result.hs          |  1 +
 src/BTLS/Types.hs           |  6 ++++-
 src/Codec/Crypto/HKDF.hs    | 65 ++++++++++++++++++++++++++++++++++++++++-----
 src/Data/Digest.hs          | 60 +++++++++++++++++++++++++++++++++++------
 src/Data/HMAC.hs            | 39 ++++++++++++++++++++++-----
 src/System/Random/Crypto.hs |  9 ++++++-
 tests/Data/HMACTests.hs     |  6 ++---
 7 files changed, 160 insertions(+), 26 deletions(-)

diff --git a/src/BTLS/Result.hs b/src/BTLS/Result.hs
index 63626bd..4f0238a 100644
--- a/src/BTLS/Result.hs
+++ b/src/BTLS/Result.hs
@@ -40,6 +40,7 @@ requireSuccess r = when (r /= 1) $ ioError (userError "BoringSSL failure")
 
 type Result = Either [Error]
 
+-- | An error which occurred during processing.
 data Error = Error
   { err :: Err
   , file :: FilePath
diff --git a/src/BTLS/Types.hs b/src/BTLS/Types.hs
index 06b5173..44b29bc 100644
--- a/src/BTLS/Types.hs
+++ b/src/BTLS/Types.hs
@@ -31,7 +31,10 @@ newtype Algorithm = Algorithm (Ptr EVPMD)
 newtype AssociatedData = AssociatedData ByteString
   deriving (Eq, Ord, Show)
 
--- | The result of a hash operation.
+-- | The result of a hash operation. Equality comparisons on this type are
+-- variable-time.
+--
+-- The 'Show' instance for this type displays the digest as a hexadecimal string.
 newtype Digest = Digest ByteString
   deriving (Eq, Ord)
 
@@ -46,6 +49,7 @@ instance Show Digest where
 newtype Salt = Salt ByteString
   deriving (Eq, Ord, Show)
 
+-- | A special value used to request that no salt be used.
 noSalt :: Salt
 noSalt = Salt ByteString.empty
 
diff --git a/src/Codec/Crypto/HKDF.hs b/src/Codec/Crypto/HKDF.hs
index f40f707..bc1ea61 100644
--- a/src/Codec/Crypto/HKDF.hs
+++ b/src/Codec/Crypto/HKDF.hs
@@ -12,9 +12,44 @@
 -- License for the specific language governing permissions and limitations under
 -- the License.
 
+{-|
+  Module: Codec.Crypto.HKDF
+  Description: Hash-based key derivation
+  Copyright: 2018 Google LLC
+  License: Apache License, version 2.0
+
+  The hash-based key derivation function (HKDF), as specified in
+  [RFC 5869](https://tools.ietf.org/html/rfc5869).
+-}
 module Codec.Crypto.HKDF
-  ( AssociatedData(AssociatedData), Salt(Salt), SecretKey(SecretKey), noSalt
-  , hkdf, extract, expand
+  ( -- * Computing keys
+    SecretKey(SecretKey)
+  , hkdf
+  , extract
+  , expand
+
+    -- * Cryptographic hash algorithms
+  , Algorithm
+  , sha1
+
+    -- ** SHA-2 family
+    -- | The SHA-2 family of hash functions is defined in
+    -- [FIPS 180-4](https://csrc.nist.gov/publications/detail/fips/180/4/final).
+  , sha224, sha256, sha384, sha512
+
+    -- * Salt
+
+    -- | You may salt the hash used to generate the key. If you do not wish to
+    -- do so, specify 'noSalt' as the salt.
+  , Salt(Salt), noSalt
+
+    -- * Associated data
+    -- | You may mix in arbitrary data when generating a key. If you do not wish
+    -- to do so, specify the empty string as the associated data.
+  , AssociatedData(AssociatedData)
+
+    -- * Legacy functions
+  , md5
   ) where
 
 import Foreign (allocaArray)
@@ -27,12 +62,23 @@ import BTLS.Types
   ( Algorithm(Algorithm), AssociatedData(AssociatedData), Salt(Salt)
   , SecretKey(SecretKey), noSalt
   )
+import Data.Digest (md5, sha1, sha224, sha256, sha384, sha512)
 
--- | Computes an HKDF as specified by RFC 5869.
-hkdf :: Algorithm -> Salt -> AssociatedData -> Int -> SecretKey -> SecretKey
+-- | Computes an HKDF. It is defined by
+--
+-- prop> hkdf md salt info len = expand md info len . extract md salt
+--
+-- but may be faster than calling the two functions individually.
+hkdf ::
+     Algorithm
+  -> Salt
+  -> AssociatedData
+  -> Int -- ^ The length of the derived key, in bytes.
+  -> SecretKey
+  -> SecretKey
 hkdf md salt info outLen = expand md info outLen . extract md salt
 
--- | Computes an HKDF pseudorandom key (PRK) as specified by RFC 5869.
+-- | Computes an HKDF pseudorandom key (PRK).
 extract :: Algorithm -> Salt -> SecretKey -> SecretKey
 extract (Algorithm md) (Salt salt) (SecretKey secret) =
   SecretKey $
@@ -40,8 +86,13 @@ extract (Algorithm md) (Salt salt) (SecretKey secret) =
       onBufferOfMaxSize evpMaxMDSize $ \pOutKey pOutLen -> do
         hkdfExtract pOutKey pOutLen md secret salt
 
--- | Computes HKDF output key material (OKM) as specified by RFC 5869.
-expand :: Algorithm -> AssociatedData -> Int -> SecretKey -> SecretKey
+-- | Computes HKDF output key material (OKM).
+expand ::
+     Algorithm
+  -> AssociatedData
+  -> Int -- ^ The length of the OKM, in bytes.
+  -> SecretKey
+  -> SecretKey
 expand (Algorithm md) (AssociatedData info) outLen (SecretKey secret) =
   SecretKey $
     unsafeLocalState $
diff --git a/src/Data/Digest.hs b/src/Data/Digest.hs
index 336b38d..a17c438 100644
--- a/src/Data/Digest.hs
+++ b/src/Data/Digest.hs
@@ -12,15 +12,33 @@
 -- License for the specific language governing permissions and limitations under
 -- the License.
 
+{-|
+  Module: Data.Digest
+  Description: Cryptographic hash functions
+  Copyright: 2017 Google LLC
+  License: Apache License, version 2.0
+
+  Cryptographic hash functions.
+-}
 module Data.Digest
-  ( Algorithm
-  , Digest
+  ( -- * Computing digests
+    Digest
   , hash
+
+    -- * Digest algorithms
+  , Algorithm
+
+    -- ** SHA-2 family
+    -- | The SHA-2 family of hash functions is defined in
+    -- [FIPS 180-4](https://csrc.nist.gov/publications/detail/fips/180/4/final).
+  , sha224, sha256, sha384, sha512
+
+    -- * Legacy functions
   , md5
   , sha1
-  , sha224, sha256, sha384, sha512
   ) where
 
+import qualified Data.ByteString.Lazy as Lazy (ByteString)
 import qualified Data.ByteString.Lazy as ByteString.Lazy
 import Foreign (withForeignPtr)
 import Foreign.Marshal.Unsafe (unsafeLocalState)
@@ -30,18 +48,44 @@ import BTLS.BoringSSL.Digest
 import BTLS.Buffer (onBufferOfMaxSize)
 import BTLS.Types (Algorithm(Algorithm), Digest(Digest))
 
-type LazyByteString = ByteString.Lazy.ByteString
+-- | Message Digest 5, a 128-bit digest defined in
+-- [RFC 1321](https://tools.ietf.org/html/rfc1321). This algorithm is
+-- cryptographically broken; do not use it except to interface with legacy
+-- applications.
+md5 :: Algorithm
+md5 = Algorithm evpMD5
+
+-- | Secure Hash Algorithm 1, a 160-bit digest defined in
+-- [FIPS 180-4](https://csrc.nist.gov/publications/detail/fips/180/4/final).
+-- Hashing with this algorithm is cryptographically broken, although
+-- constructing HMACs with it is safe.
+sha1 :: Algorithm
+sha1 = Algorithm evpSHA1
 
-md5, sha1, sha224, sha256, sha384, sha512 :: Algorithm
-md5    = Algorithm evpMD5
-sha1   = Algorithm evpSHA1
+-- | The SHA224 digest, a 224-bit digest and Secure Hash Algorithm 2 family
+-- member.
+sha224 :: Algorithm
 sha224 = Algorithm evpSHA224
+
+-- | The SHA256 digest, a 256-bit digest and Secure Hash Algorithm 2 family
+-- member. Prefer this algorithm on 32-bit CPUs; it will run faster than
+-- 'sha384' or 'sha512'.
+sha256 :: Algorithm
 sha256 = Algorithm evpSHA256
+
+-- | The SHA384 digest, a 384-bit digest and Secure Hash Algorithm 2 family
+-- member.
+sha384 :: Algorithm
 sha384 = Algorithm evpSHA384
+
+-- | The SHA512 digest, a 512-bit digest and Secure Hash Algorithm 2 family
+-- member. Prefer this algorithm on 64-bit CPUs; it will run faster than
+-- 'sha224' or 'sha256'.
+sha512 :: Algorithm
 sha512 = Algorithm evpSHA512
 
 -- | Hashes according to the given 'Algorithm'.
-hash :: Algorithm -> LazyByteString -> Digest
+hash :: Algorithm -> Lazy.ByteString -> Digest
 hash (Algorithm md) bytes =
   unsafeLocalState $ do
     ctxFP <- mallocEVPMDCtx
diff --git a/src/Data/HMAC.hs b/src/Data/HMAC.hs
index bf1bef8..fb67817 100644
--- a/src/Data/HMAC.hs
+++ b/src/Data/HMAC.hs
@@ -12,15 +12,43 @@
 -- License for the specific language governing permissions and limitations under
 -- the License.
 
+{-|
+  Module: Data.HMAC
+  Description: Hash-based message authentication codes
+  Copyright: 2018 Google LLC
+  License: Apache License, version 2.0
+
+  Hash-based message authentication codes (HMACs). An HMAC guarantees
+  authenticity but not confidentiality.
+-}
 module Data.HMAC
-  ( SecretKey(SecretKey)
-  , HMAC, Result
+  ( -- * Computing HMACs
+    HMAC
   , hmac
+
+    -- * Cryptographic hash algorithms
+  , Algorithm
+  , sha1
+
+    -- ** SHA-2 family
+    -- | The SHA-2 family of hash functions is defined in
+    -- [FIPS 180-4](https://csrc.nist.gov/publications/detail/fips/180/4/final).
+  , sha224, sha256, sha384, sha512
+
+    -- * Keys
+  , SecretKey(SecretKey)
+
+    -- * Error handling
+  , Error
+
+    -- * Legacy functions
+  , md5
   ) where
 
 import Control.Monad.Trans.Class (lift)
 import Control.Monad.Trans.Except (runExceptT)
 import Data.ByteString (ByteString)
+import qualified Data.ByteString.Lazy as Lazy (ByteString)
 import qualified Data.ByteString.Lazy as ByteString.Lazy
 import qualified Data.ByteString.Unsafe as ByteString
 import Foreign (withForeignPtr)
@@ -31,10 +59,9 @@ import BTLS.BoringSSL.Digest (evpMaxMDSize)
 import BTLS.BoringSSL.HMAC
 import BTLS.BoringSSL.Mem (cryptoMemcmp)
 import BTLS.Buffer (onBufferOfMaxSize)
-import BTLS.Result (Result, check)
+import BTLS.Result (Error, check)
 import BTLS.Types (Algorithm(Algorithm), Digest(Digest), SecretKey(SecretKey))
-
-type LazyByteString = ByteString.Lazy.ByteString
+import Data.Digest (md5, sha1, sha224, sha256, sha384, sha512)
 
 -- | A hash-based message authentication code. Equality comparisons on this type
 -- are constant-time.
@@ -51,7 +78,7 @@ instance Show HMAC where
   show (HMAC m) = show (Digest m)
 
 -- | Creates an HMAC according to the given 'Algorithm'.
-hmac :: Algorithm -> SecretKey -> LazyByteString -> Result HMAC
+hmac :: Algorithm -> SecretKey -> Lazy.ByteString -> Either [Error] HMAC
 hmac (Algorithm md) (SecretKey key) bytes =
   unsafeLocalState $ do
     ctxFP <- mallocHMACCtx
diff --git a/src/System/Random/Crypto.hs b/src/System/Random/Crypto.hs
index ed5706d..464555e 100644
--- a/src/System/Random/Crypto.hs
+++ b/src/System/Random/Crypto.hs
@@ -12,6 +12,12 @@
 -- License for the specific language governing permissions and limitations under
 -- the License.
 
+{-|
+  Module: System.Random.Crypto
+  Description: Cryptographically secure pseudorandom number generator
+  Copyright: 2018 Google LLC
+  License: Apache License, version 2.0
+-}
 module System.Random.Crypto
   ( randomBytes
   ) where
@@ -22,7 +28,8 @@ import Foreign (allocaArray)
 import BTLS.BoringSSL.Rand (randBytes)
 import BTLS.Buffer (packCUStringLen)
 
--- | Generates a cryptographically random buffer of the specified size.
+-- | Generates a cryptographically random buffer of the specified size (in
+-- bytes).
 randomBytes :: Int -> IO ByteString
 randomBytes len =
   allocaArray len $ \pBuf -> do
diff --git a/tests/Data/HMACTests.hs b/tests/Data/HMACTests.hs
index ea786c0..bcdd7a6 100644
--- a/tests/Data/HMACTests.hs
+++ b/tests/Data/HMACTests.hs
@@ -23,7 +23,7 @@ import Test.Tasty (TestTree, testGroup)
 import Test.Tasty.HUnit ((@?=), testCase)
 
 import Data.Digest (md5, sha1, sha224, sha256, sha384, sha512)
-import Data.HMAC (Result, SecretKey(SecretKey), hmac)
+import Data.HMAC (Error, SecretKey(SecretKey), hmac)
 
 type LazyByteString = ByteString.Lazy.ByteString
 
@@ -35,7 +35,7 @@ tests = testGroup "Data.HMAC"
   ]
 
 tableTestCase ::
-     (SecretKey -> LazyByteString -> Result String)
+     (SecretKey -> LazyByteString -> Either [Error] String)
   -> (SecretKey, LazyByteString, String)
   -> TestTree
 tableTestCase f (key, input, output) =
@@ -176,7 +176,7 @@ testRFC4231 = testGroup "RFC 4231" $
 truncatedRFC4231Test =
   let key = SecretKey (ByteString.replicate 20 0x0c)
       input = "Test With Truncation" :: LazyByteString
-      t f = take 32 <$> f key input :: Result String
+      t f = take 32 <$> f key input :: Either [Error] String
   in testGroup (abbreviate input)
        [ testCase "SHA-224" (t hmacSha224 @?= Right "0e2aea68a90c8d37c988bcdb9fca6fa8")
        , testCase "SHA-256" (t hmacSha256 @?= Right "a3b6167473100ee06e0c796c2955552b")
-- 
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