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diff --git a/src/BTLS/ConstantTimeEquals.chs b/src/BTLS/ConstantTimeEquals.chs
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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.
+
+module BTLS.ConstantTimeEquals where
+
+import Foreign (Ptr)
+import Foreign.C.Types
+
+import BTLS.Cast (asVoidPtr)
+
+#include <openssl/mem.h>
+
+-- | Directly compares two buffers for equality. This operation takes an amount
+-- of time dependent on the specified size but independent of either buffer's
+-- contents.
+constantTimeEquals :: Ptr a -> Ptr a -> Int -> IO Bool
+constantTimeEquals a b size =
+  let size' = fromIntegral size :: CULong in
+  (== 0) <$> {#call CRYPTO_memcmp as ^#} (asVoidPtr a) (asVoidPtr b) size'