4 files changed, 62 insertions, 0 deletions
diff --git a/include/olm/inbound_group_session.h b/include/olm/inbound_group_session.h
index 49992b2..0c5eb26 100644
--- a/include/olm/inbound_group_session.h
+++ b/include/olm/inbound_group_session.h
@@ -146,6 +146,27 @@ size_t olm_group_decrypt(
 );
 
 
+/**
+ * Get the number of bytes returned by olm_inbound_group_session_id()
+ */
+size_t olm_inbound_group_session_id_length(
+    const OlmInboundGroupSession *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_inbound_group_session_id(
+    OlmInboundGroupSession *session,
+    uint8_t * id, size_t id_length
+);
+
+
 #ifdef __cplusplus
 } // extern "C"
 #endif
diff --git a/javascript/olm_inbound_group_session.js b/javascript/olm_inbound_group_session.js
index 9722c31..01bfd26 100644
--- a/javascript/olm_inbound_group_session.js
+++ b/javascript/olm_inbound_group_session.js
@@ -89,4 +89,15 @@ InboundGroupSession.prototype['decrypt'] = restore_stack(function(
     return Pointer_stringify(plaintext_buffer);
 });
 
+InboundGroupSession.prototype['session_id'] = restore_stack(function() {
+    var length = inbound_group_session_method(
+        Module['_olm_inbound_group_session_id_length']
+    )(this.ptr);
+    var session_id = stack(length + NULL_BYTE_PADDING_LENGTH);
+    inbound_group_session_method(Module['_olm_inbound_group_session_id'])(
+        this.ptr, session_id, length
+    );
+    return Pointer_stringify(session_id);
+});
+
 olm_exports['InboundGroupSession'] = InboundGroupSession;
diff --git a/python/olm/inbound_group_session.py b/python/olm/inbound_group_session.py
index 6c01095..310620f 100644
--- a/python/olm/inbound_group_session.py
+++ b/python/olm/inbound_group_session.py
@@ -45,6 +45,9 @@ inbound_group_session_function(
     c_void_p, c_size_t, # plaintext
 )
 
+inbound_group_session_function(lib.olm_inbound_group_session_id_length)
+inbound_group_session_function(lib.olm_inbound_group_session_id, c_void_p, c_size_t)
+
 class InboundGroupSession(object):
     def __init__(self):
         self.buf = create_string_buffer(lib.olm_inbound_group_session_size())
@@ -84,3 +87,9 @@ class InboundGroupSession(object):
             plaintext_buffer, max_plaintext_length
         )
         return plaintext_buffer.raw[:plaintext_length]
+
+    def session_id(self):
+        id_length = lib.olm_inbound_group_session_id_length(self.ptr)
+        id_buffer = create_string_buffer(id_length)
+        lib.olm_inbound_group_session_id(self.ptr, id_buffer, id_length);
+        return id_buffer.raw
diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index 82ff66f..65c6018 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -29,6 +29,7 @@
 
 
 #define OLM_PROTOCOL_VERSION     3
+#define GROUP_SESSION_ID_LENGTH  ED25519_PUBLIC_KEY_LENGTH
 #define PICKLE_VERSION           1
 #define SESSION_KEY_VERSION      2
 
@@ -364,3 +365,23 @@ size_t olm_group_decrypt(
         plaintext, max_plaintext_length
     );
 }
+
+size_t olm_inbound_group_session_id_length(
+    const OlmInboundGroupSession *session
+) {
+    return _olm_encode_base64_length(GROUP_SESSION_ID_LENGTH);
+}
+
+size_t olm_inbound_group_session_id(
+    OlmInboundGroupSession *session,
+    uint8_t * id, size_t id_length
+) {
+    if (id_length < olm_inbound_group_session_id_length(session)) {
+        session->last_error = OLM_OUTPUT_BUFFER_TOO_SMALL;
+        return (size_t)-1;
+    }
+
+    return _olm_encode_base64(
+        session->signing_key.public_key, GROUP_SESSION_ID_LENGTH, id
+    );
+}