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diff --git a/src/BTLS/Types.hs b/src/BTLS/Types.hs
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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.Types
+  ( AssociatedData(AssociatedData)
+  , Salt(Salt), noSalt
+  , SecretKey(SecretKey)
+  ) where
+
+import Data.ByteString (ByteString)
+import qualified Data.ByteString as ByteString
+
+-- | Context or application-specific information. Equality comparisons on this
+-- type are variable-time.
+newtype AssociatedData = AssociatedData ByteString
+  deriving (Eq, Ord, Show)
+
+-- | A salt. Equality comparisons on this type are variable-time.
+newtype Salt = Salt ByteString
+  deriving (Eq, Ord, Show)
+
+noSalt :: Salt
+noSalt = Salt ByteString.empty
+
+-- | A secret key used as input to a cipher or HMAC. Equality comparisons on
+-- this type are variable-time.
+newtype SecretKey = SecretKey ByteString
+  deriving (Eq, Ord, Show)