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diff --git a/include/axolotl/ratchet.hh b/include/axolotl/ratchet.hh
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+++ b/include/axolotl/ratchet.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.
+ */
+
+#include "axolotl/crypto.hh"
+#include "axolotl/list.hh"
+
+namespace axolotl {
+
+typedef std::uint8_t SharedKey[32];
+
+
+struct ChainKey {
+    std::uint32_t index;
+    SharedKey key;
+};
+
+
+struct MessageKey {
+    std::uint32_t index;
+    Aes256Key cipher_key;
+    SharedKey mac_key;
+    Aes256Iv iv;
+};
+
+
+struct SenderChain {
+    Curve25519KeyPair ratchet_key;
+    ChainKey chain_key;
+};
+
+
+struct ReceiverChain {
+    Curve25519PublicKey ratchet_key;
+    ChainKey chain_key;
+};
+
+
+struct SkippedMessageKey {
+    Curve25519PublicKey ratchet_key;
+    MessageKey message_key;
+};
+
+
+enum struct ErrorCode {
+    SUCCESS = 0, /*!< There wasn't an error */
+    NOT_ENOUGH_RANDOM = 1,  /*!< Not enough entropy was supplied */
+    OUTPUT_BUFFER_TOO_SMALL = 2, /*!< Supplied output buffer is too small */
+    BAD_MESSAGE_VERSION = 3,  /*!< The message version is unsupported */
+    BAD_MESSAGE_FORMAT = 4, /*!< The message couldn't be decoded */
+    BAD_MESSAGE_MAC = 5, /*!< The message couldn't be decrypted */
+};
+
+
+static std::size_t const MAX_RECEIVER_CHAINS = 5;
+static std::size_t const MAX_SKIPPED_MESSAGE_KEYS = 40;
+
+
+struct KdfInfo {
+    std::uint8_t const * root_info;
+    std::size_t root_info_length;
+    std::uint8_t const * ratchet_info;
+    std::size_t ratchet_info_length;
+    std::uint8_t const * message_info;
+    std::size_t message_info_length;
+};
+
+
+struct Session {
+
+    Session(
+        KdfInfo const & kdf_info
+    );
+
+    /** A some strings identifing the application to feed into the KDF. */
+    KdfInfo kdf_info;
+
+    /** The last error that happened encypting or decrypting a message. */
+    ErrorCode last_error;
+
+    /** The root key is used to generate chain keys from the ephemeral keys.
+     * A new root_key derived each time a chain key is derived. */
+    SharedKey root_key;
+
+    /** The sender chain is used to send messages. Each time a new ephemeral
+     * key is received from the remote server we generate a new sender chain
+     * with a new empheral key when we next send a message. */
+    List<SenderChain, 1> sender_chain;
+
+    /** The receiver chain is used to decrypt recieved messages. We store the
+     * last few chains so we can decrypt any out of order messages we haven't
+     * received yet. */
+    List<ReceiverChain, MAX_RECEIVER_CHAINS> receiver_chains;
+
+    /** List of message keys we've skipped over when advancing the receiver
+     * chain. */
+    List<SkippedMessageKey, MAX_SKIPPED_MESSAGE_KEYS> skipped_message_keys;
+
+    /** Initialise the session using a shared secret and the public part of the
+     * remote's first ratchet key */
+    void initialise_as_bob(
+        std::uint8_t const * shared_secret, std::size_t shared_secret_length,
+        Curve25519PublicKey const & their_ratchet_key
+    );
+
+    /** Intialise the session using a shared secret and the public/private key
+     * pair for the first ratchet key */
+    void initialise_as_alice(
+        std::uint8_t const * shared_secret, std::size_t shared_secret_length,
+        Curve25519KeyPair const & our_ratchet_key
+    );
+
+    /** The maximum number of bytes of output the encrypt method will write for
+     * a given message length. */
+    std::size_t encrypt_max_output_length(
+        std::size_t plaintext_length
+    );
+
+    /** The number of bytes of random data the encrypt method will need to
+     * encrypt a message. This will be 32 bytes if the session needs to
+     * generate a new ephemeral key, or will be 0 bytes otherwise.*/
+    std::size_t encrypt_random_length();
+
+    /** Encrypt some plaintext. Returns the length of the encrypted message
+     * or std::size_t(-1) on failure. On failure last_error will be set with
+     * an error code. The last_error will be NOT_ENOUGH_RANDOM if the number
+     * of random bytes is too small. The last_error will be
+     * OUTPUT_BUFFER_TOO_SMALL if the output buffer is too small. */
+    std::size_t encrypt(
+        std::uint8_t const * plaintext, std::size_t plaintext_length,
+        std::uint8_t const * random, std::size_t random_length,
+        std::uint8_t * output, std::size_t max_output_length
+    );
+
+    /** An upper bound on the number of bytes of plaintext the decrypt method
+     * will write for a given input message length. */
+    std::size_t decrypt_max_plaintext_length(
+        std::size_t input_length
+    );
+
+    /** Decrypt a message. Returns the length of the decrypted plaintext or
+     * std::size_t(-1) on failure. On failure last_error will be set with an
+     * error code. The last_error will be OUTPUT_BUFFER_TOO_SMALL if the
+     * plaintext buffer is too small. The last_error will be
+     * BAD_MESSAGE_VERSION if the message was encrypted with an unsupported
+     * version of the protocol. The last_error will be BAD_MESSAGE_FORMAT if
+     * the message headers could not be decoded. The last_error will be
+     * BAD_MESSAGE_MAC if the message could not be verified */
+    std::size_t decrypt(
+        std::uint8_t const * input, std::size_t input_length,
+        std::uint8_t * plaintext, std::size_t max_plaintext_length
+    );
+};
+
+
+} // namespace axolotl