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diff --git a/include/olm/outbound_group_session.h b/include/olm/outbound_group_session.h
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+++ b/include/olm/outbound_group_session.h
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+/* Copyright 2016 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 OLM_OUTBOUND_GROUP_SESSION_H_
+#define OLM_OUTBOUND_GROUP_SESSION_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct OlmOutboundGroupSession OlmOutboundGroupSession;
+
+/** get the size of an outbound group session, in bytes. */
+size_t olm_outbound_group_session_size();
+
+/**
+ * Initialise an outbound group session object using the supplied memory
+ * The supplied memory should be at least olm_outbound_group_session_size()
+ * bytes.
+ */
+OlmOutboundGroupSession * olm_outbound_group_session(
+    void *memory
+);
+
+/**
+ * A null terminated string describing the most recent error to happen to a
+ * group session */
+const char *olm_outbound_group_session_last_error(
+    const OlmOutboundGroupSession *session
+);
+
+/** Clears the memory used to back this group session */
+size_t olm_clear_outbound_group_session(
+    OlmOutboundGroupSession *session
+);
+
+/** Returns the number of bytes needed to store an outbound group session */
+size_t olm_pickle_outbound_group_session_length(
+    const OlmOutboundGroupSession *session
+);
+
+/**
+ * Stores a group session as a base64 string. Encrypts the session using the
+ * supplied key. Returns the length of the session on success.
+ *
+ * Returns olm_error() on failure. If the pickle output buffer
+ * is smaller than olm_pickle_outbound_group_session_length() then
+ * olm_outbound_group_session_last_error() will be "OUTPUT_BUFFER_TOO_SMALL"
+ */
+size_t olm_pickle_outbound_group_session(
+    OlmOutboundGroupSession *session,
+    void const * key, size_t key_length,
+    void * pickled, size_t pickled_length
+);
+
+/**
+ * Loads a group session from a pickled base64 string. Decrypts the session
+ * using the supplied key.
+ *
+ * Returns olm_error() on failure. If the key doesn't match the one used to
+ * encrypt the account then olm_outbound_group_session_last_error() will be
+ * "BAD_ACCOUNT_KEY". If the base64 couldn't be decoded then
+ * olm_outbound_group_session_last_error() will be "INVALID_BASE64". The input
+ * pickled buffer is destroyed
+ */
+size_t olm_unpickle_outbound_group_session(
+    OlmOutboundGroupSession *session,
+    void const * key, size_t key_length,
+    void * pickled, size_t pickled_length
+);
+
+
+/** The number of random bytes needed to create an outbound group session */
+size_t olm_init_outbound_group_session_random_length(
+    const OlmOutboundGroupSession *session
+);
+
+/**
+ * Start a new outbound group session. Returns olm_error() 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 was too small.
+ */
+size_t olm_init_outbound_group_session(
+    OlmOutboundGroupSession *session,
+    uint8_t const * random, size_t random_length
+);
+
+/**
+ * The number of bytes that will be created by encrypting a message
+ */
+size_t olm_group_encrypt_message_length(
+    OlmOutboundGroupSession *session,
+    size_t plaintext_length
+);
+
+/**
+ * Encrypt some plain-text. Returns the length of the encrypted message or
+ * olm_error() on failure. On failure last_error will be set with an
+ * error code. The last_error will be OUTPUT_BUFFER_TOO_SMALL if the output
+ * buffer is too small.
+ */
+size_t olm_group_encrypt(
+    OlmOutboundGroupSession *session,
+    uint8_t const * plaintext, size_t plaintext_length,
+    uint8_t * message, size_t message_length
+);
+
+
+/**
+ * Get the number of bytes returned by olm_outbound_group_session_id()
+ */
+size_t olm_outbound_group_session_id_length(
+    const OlmOutboundGroupSession *session
+);
+
+/**
+ * Get a base64-encoded identifier for this session.
+ *
+ * Returns the length of the session id on success or olm_error() on
+ * failure. On failure last_error will be set with an error code. The
+ * last_error will be OUTPUT_BUFFER_TOO_SMALL if the id buffer was too
+ * small.
+ */
+size_t olm_outbound_group_session_id(
+    OlmOutboundGroupSession *session,
+    uint8_t * id, size_t id_length
+);
+
+/**
+ * Get the current message index for this session.
+ *
+ * Each message is sent with an increasing index; this returns the index for
+ * the next message.
+ */
+uint32_t olm_outbound_group_session_message_index(
+    OlmOutboundGroupSession *session
+);
+
+/**
+ * Get the number of bytes returned by olm_outbound_group_session_key()
+ */
+size_t olm_outbound_group_session_key_length(
+    const OlmOutboundGroupSession *session
+);
+
+/**
+ * Get the base64-encoded current ratchet key for this session.
+ *
+ * Each message is sent with a different ratchet key. This function returns the
+ * ratchet key that will be used for the next message.
+ *
+ * Returns the length of the ratchet key on success or olm_error() on
+ * failure. On failure last_error will be set with an error code. The
+ * last_error will be OUTPUT_BUFFER_TOO_SMALL if the buffer was too small.
+ */
+size_t olm_outbound_group_session_key(
+    OlmOutboundGroupSession *session,
+    uint8_t * key, size_t key_length
+);
+
+
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif /* OLM_OUTBOUND_GROUP_SESSION_H_ */