2 files changed, 56 insertions, 0 deletions
diff --git a/include/olm/account.hh b/include/olm/account.hh
index 7e58ca3..826b26d 100644
--- a/include/olm/account.hh
+++ b/include/olm/account.hh
@@ -43,6 +43,8 @@ struct Account {
     Account();
     IdentityKeys identity_keys;
     List<OneTimeKey, MAX_ONE_TIME_KEYS> one_time_keys;
+    OneTimeKey current_fallback_key;
+    OneTimeKey prev_fallback_key;
     std::uint32_t next_one_time_key_id;
     OlmErrorCode last_error;
 
@@ -126,6 +128,35 @@ struct Account {
         std::uint8_t const * random, std::size_t random_length
     );
 
+    /** The number of random bytes needed to generate a fallback key. */
+    std::size_t generate_fallback_key_random_length();
+
+    /** Generates a new fallback key. Returns std::size_t(-1) on error. If the
+     * number of random bytes is too small then last_error will be
+     * NOT_ENOUGH_RANDOM */
+    std::size_t generate_fallback_key(
+        std::uint8_t const * random, std::size_t random_length
+    );
+
+    /** Number of bytes needed to output the one time keys for this account */
+    std::size_t get_fallback_key_json_length();
+
+    /** Output the fallback key as JSON:
+     *
+     *  {"curve25519":
+     *  ["<6 byte key id>":"<43 base64 characters>"
+     *  ,"<6 byte key id>":"<43 base64 characters>"
+     *  ...
+     *  ]
+     *  }
+     *
+     * Returns the size of the JSON written or std::size_t(-1) on error.
+     * If the buffer is too small last_error will be OUTPUT_BUFFER_TOO_SMALL.
+     */
+    std::size_t get_fallback_key_json(
+        std::uint8_t * fallback_json, std::size_t fallback_json_length
+    );
+
     /** Lookup a one time key with the given public key */
     OneTimeKey const * lookup_key(
         _olm_curve25519_public_key const & public_key
diff --git a/include/olm/olm.h b/include/olm/olm.h
index 6a2b3fb..11d99b4 100644
--- a/include/olm/olm.h
+++ b/include/olm/olm.h
@@ -254,6 +254,31 @@ size_t olm_account_generate_one_time_keys(
     void * random, size_t random_length
 );
 
+/** The number of random bytes needed to generate a fallback key. */
+size_t olm_account_generate_fallback_key_random_length(
+    OlmAccount * account
+);
+
+/** Generates a new fallback key. Only one previous fallback key is
+ * stored. Returns olm_error() on error. If the number of random bytes is too
+ * small then olm_account_last_error() will be "NOT_ENOUGH_RANDOM". */
+size_t olm_account_generate_fallback_key(
+    OlmAccount * account,
+    void * random, size_t random_length
+);
+
+/** The number of bytes needed to hold the fallback key as returned by
+ * olm_account_fallback_key. */
+size_t olm_account_fallback_key_length(
+    OlmAccount * account
+);
+
+size_t olm_account_fallback_key(
+    OlmAccount * account,
+    void * fallback_key, size_t fallback_key_size
+);
+
+
 /** The number of random bytes needed to create an outbound session */
 size_t olm_create_outbound_session_random_length(
     OlmSession * session