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+-- Copyright 2018 Google LLC
+--
+-- Licensed under the Apache License, Version 2.0 (the "License"); you may not
+-- use this file except in compliance with the License. You may obtain a copy of
+-- the License at
+--
+--     https://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+-- WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+-- License for the specific language governing permissions and limitations under
+-- the License.
+
+{-# OPTIONS_GHC -Wno-missing-methods #-}
+{-# OPTIONS_GHC -Wno-orphans #-}
+
+module Internal.Hmac
+  ( mallocHmacCtx
+  , hmacInitEx
+  , hmacUpdate
+  , hmacFinal
+  ) where
+
+import Foreign
+  (FinalizerPtr, ForeignPtr, Ptr, Storable(alignment, sizeOf),
+   addForeignPtrFinalizer, mallocForeignPtr, withForeignPtr)
+import Foreign.C.Types
+
+import Foreign.Ptr.Cast (asVoidPtr)
+{#import Internal.Base#}
+import Result
+
+#include <openssl/hmac.h>
+
+-- | Memory-safe allocator for 'HmacCtx'.
+mallocHmacCtx :: IO (ForeignPtr HmacCtx)
+mallocHmacCtx = do
+  fp <- mallocForeignPtr
+  withForeignPtr fp {#call HMAC_CTX_init as ^#}
+  addForeignPtrFinalizer hmacCtxCleanup fp
+  return fp
+
+foreign import ccall "&HMAC_CTX_cleanup"
+  hmacCtxCleanup :: FinalizerPtr HmacCtx
+
+hmacInitEx :: Ptr HmacCtx -> Ptr a -> CULong -> Ptr EvpMd -> Ptr Engine -> IO ()
+hmacInitEx ctx bytes size md engine =
+  requireSuccess $
+    {#call HMAC_Init_ex as ^#} ctx (asVoidPtr bytes) size md engine
+
+hmacUpdate :: Ptr HmacCtx -> Ptr CUChar -> CULong -> IO ()
+hmacUpdate ctx bytes size =
+  alwaysSucceeds $ {#call HMAC_Update as ^#} ctx bytes size
+
+hmacFinal :: Ptr HmacCtx -> Ptr CUChar -> Ptr CUInt -> IO ()
+hmacFinal ctx out outSize =
+  requireSuccess $ {#call HMAC_Final as ^#} ctx out outSize
+
+instance Storable HmacCtx where
+  sizeOf _ = {#sizeof HMAC_CTX#}
+  alignment _ = {#alignof HMAC_CTX#}