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diff --git a/src/Data/Digest/Internal.hs b/src/Data/Digest/Internal.hs
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+-- Copyright 2017 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 Data.Digest.Internal where
+
+import Data.Bits (Bits((.&.)), shiftR)
+import Data.ByteString (ByteString)
+import qualified Data.ByteString as ByteString
+import Data.Char (intToDigit)
+import Data.Word (Word8)
+import Foreign (Ptr)
+
+import Internal.Base (EvpMd)
+
+-- | A cryptographic hash function.
+newtype Algorithm = Algorithm (Ptr EvpMd)
+
+-- | The result of a hash operation.
+newtype Digest =
+  Digest ByteString
+  deriving (Eq, Ord)
+
+instance Show Digest where
+  show (Digest d) = ByteString.foldr showHexPadded [] d
+    where
+      showHexPadded b xs =
+        hexit (b `shiftR` 4 .&. 0x0f) : hexit (b .&. 0x0f) : xs
+      hexit = intToDigit . fromIntegral :: Word8 -> Char