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diff --git a/include/axolotl/cipher.hh b/include/axolotl/cipher.hh
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+/* Copyright 2015 OpenMarket Ltd
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+#ifndef AXOLOTL_CIPHER_HH_
+#define AXOLOTL_CIPHER_HH_
+
+#include <cstdint>
+
+namespace axolotl {
+
+class Cipher {
+public:
+    virtual ~Cipher();
+
+    /**
+     * Returns the length of the message authentication code that will be
+     * appended to the output.
+     */
+    virtual std::size_t mac_length() const = 0;
+
+    /**
+     * Returns the length of cipher-text for a given length of plain-text.
+     */
+    virtual std::size_t encrypt_ciphertext_length(
+        std::size_t plaintext_length
+    ) const = 0;
+
+    /*
+     * Encrypts the plain-text into the output buffer and authenticates the
+     * contents of the output buffer covering both cipher-text and any other
+     * associated data in the output buffer.
+     *
+     *  |---------------------------------------output_length-->|
+     *  output  |--ciphertext_length-->|       |---mac_length-->|
+     *          ciphertext
+     *
+     * Returns std::size_t(-1) if the length of the cipher-text or the output
+     * buffer is too small. Otherwise returns the length of the output buffer.
+     */
+    virtual std::size_t encrypt(
+        std::uint8_t const * key, std::size_t key_length,
+        std::uint8_t const * plaintext, std::size_t plaintext_length,
+        std::uint8_t * ciphertext, std::size_t ciphertext_length,
+        std::uint8_t * output, std::size_t output_length
+    ) const = 0;
+
+    /**
+     * Returns the maximum length of plain-text that a given length of
+     * cipher-text can contain.
+     */
+    virtual std::size_t decrypt_max_plaintext_length(
+        std::size_t ciphertext_length
+    ) const = 0;
+
+    /**
+     * Authenticates the input and decrypts the cipher-text into the plain-text
+     * buffer.
+     *
+     *  |----------------------------------------input_length-->|
+     *  input   |--ciphertext_length-->|       |---mac_length-->|
+     *          ciphertext
+     *
+     *  Returns std::size_t(-1) if the length of the plain-text buffer is too
+     *  small or if the authentication check fails. Otherwise returns the length
+     *  of the plain text.
+     */
+    virtual std::size_t decrypt(
+       std::uint8_t const * key, std::size_t key_length,
+       std::uint8_t const * input, std::size_t input_length,
+       std::uint8_t const * ciphertext, std::size_t ciphertext_length,
+       std::uint8_t * plaintext, std::size_t max_plaintext_length
+    ) const = 0;
+};
+
+
+class CipherAesSha256 : public Cipher {
+public:
+    CipherAesSha256(
+        std::uint8_t const * kdf_info, std::size_t kdf_info_length
+    );
+
+    virtual std::size_t mac_length() const;
+
+    virtual std::size_t encrypt_ciphertext_length(
+        std::size_t plaintext_length
+    ) const;
+
+    virtual std::size_t encrypt(
+        std::uint8_t const * key, std::size_t key_length,
+        std::uint8_t const * plaintext, std::size_t plaintext_length,
+        std::uint8_t * ciphertext, std::size_t ciphertext_length,
+        std::uint8_t * output, std::size_t output_length
+    ) const;
+
+    virtual std::size_t decrypt_max_plaintext_length(
+        std::size_t ciphertext_length
+    ) const;
+
+    virtual std::size_t decrypt(
+        std::uint8_t const * key, std::size_t key_length,
+        std::uint8_t const * input, std::size_t input_length,
+        std::uint8_t const * ciphertext, std::size_t ciphertext_length,
+        std::uint8_t * plaintext, std::size_t max_plaintext_length
+    ) const;
+
+private:
+    std::uint8_t const * kdf_info;
+    std::size_t kdf_info_length;
+};
+
+
+} // namespace
+
+
+#endif /* AXOLOTL_CIPHER_HH_ */