Add InboundGroupSession.session_key().

include/olm/inbound_group_session.h

@@ -172,6 +172,29 @@ uint32_t olm_inbound_group_session_first_known_index(
     const OlmInboundGroupSession *session
 );
 
+/**
+ * Get the number of bytes returned by olm_inbound_group_session_key()
+ */
+size_t olm_inbound_group_session_key_length(
+    const OlmInboundGroupSession *session
+);
+
+/**
+ * Get the base64-encoded ratchet key for this session, at the given index
+ *
+ * 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
+ *   * OLM_UNKNOWN_MESSAGE_INDEX  if we do not have a session key corresponding to the
+ *     given index (ie, it was sent before the session key was shared with
+ *     us)
+ */
+size_t olm_inbound_group_session_key(
+    OlmInboundGroupSession *session,
+    uint8_t * key, size_t key_length, uint32_t message_index
+);
+
 
 #ifdef __cplusplus
 } // extern "C"

javascript/olm_inbound_group_session.js

@@ -111,4 +111,15 @@ InboundGroupSession.prototype['first_known_index'] = restore_stack(function() {
     )(this.ptr);
 });
 
+InboundGroupSession.prototype['session_key'] = restore_stack(function(message_index) {
+    var key_length = inbound_group_session_method(
+        Module['_olm_inbound_group_session_key_length']
+    )(this.ptr);
+    var key = stack(key_length + NULL_BYTE_PADDING_LENGTH);
+    outbound_group_session_method(Module['_olm_inbound_group_session_key'])(
+        this.ptr, key, key_length, message_index
+    );
+    return Pointer_stringify(key);
+});
+
 olm_exports['InboundGroupSession'] = InboundGroupSession;

src/inbound_group_session.c

@@ -31,7 +31,6 @@
 #define OLM_PROTOCOL_VERSION     3
 #define GROUP_SESSION_ID_LENGTH  ED25519_PUBLIC_KEY_LENGTH
 #define PICKLE_VERSION           1
-#define SESSION_KEY_VERSION      2
 
 struct OlmInboundGroupSession {
     /** our earliest known ratchet value */
@@ -71,7 +70,7 @@ size_t olm_clear_inbound_group_session(
     return sizeof(OlmInboundGroupSession);
 }
 
-#define SESSION_KEY_RAW_LENGTH \
+#define V2_SESSION_KEY_RAW_LENGTH \
     (1 + 4 + MEGOLM_RATCHET_LENGTH + ED25519_PUBLIC_KEY_LENGTH\
         + ED25519_SIGNATURE_LENGTH)
 
@@ -83,7 +82,7 @@ static size_t _init_group_session_keys(
     const uint8_t *ptr = key_buf;
     size_t version = *ptr++;
 
-    if (version != SESSION_KEY_VERSION) {
+    if (version != 2) {
         session->last_error = OLM_BAD_SESSION_KEY;
         return (size_t)-1;
     }
@@ -116,7 +115,7 @@ size_t olm_init_inbound_group_session(
     OlmInboundGroupSession *session,
     const uint8_t * session_key, size_t session_key_length
 ) {
-    uint8_t key_buf[SESSION_KEY_RAW_LENGTH];
+    uint8_t key_buf[V2_SESSION_KEY_RAW_LENGTH];
     size_t raw_length = _olm_decode_base64_length(session_key_length);
     size_t result;
 
@@ -125,14 +124,14 @@ size_t olm_init_inbound_group_session(
         return (size_t)-1;
     }
 
-    if (raw_length != SESSION_KEY_RAW_LENGTH) {
+    if (raw_length != V2_SESSION_KEY_RAW_LENGTH) {
         session->last_error = OLM_BAD_SESSION_KEY;
         return (size_t)-1;
     }
 
     _olm_decode_base64(session_key, session_key_length, key_buf);
     result = _init_group_session_keys(session, key_buf);
-    _olm_unset(key_buf, SESSION_KEY_RAW_LENGTH);
+    _olm_unset(key_buf, V2_SESSION_KEY_RAW_LENGTH);
     return result;
 }
 
@@ -257,6 +256,32 @@ size_t olm_group_decrypt_max_plaintext_length(
     );
 }
 
+/**
+ * get a copy of the megolm ratchet, advanced
+ * to the relevant index. Returns 0 on success, -1 on error
+ */
+static size_t _get_megolm(
+    OlmInboundGroupSession *session, uint32_t message_index, Megolm *result
+) {
+    /* pick a megolm instance to use. If we're at or beyond the latest ratchet
+     * value, use that */
+    if ((message_index - session->latest_ratchet.counter) < (1U << 31)) {
+        megolm_advance_to(&session->latest_ratchet, message_index);
+        *result = session->latest_ratchet;
+        return 0;
+    } else if ((message_index - session->initial_ratchet.counter) >= (1U << 31)) {
+        /* the counter is before our intial ratchet - we can't decode this. */
+        session->last_error = OLM_UNKNOWN_MESSAGE_INDEX;
+        return (size_t)-1;
+    } else {
+        /* otherwise, start from the initial megolm. Take a copy so that we
+         * don't overwrite the initial megolm */
+        *result = session->initial_ratchet;
+        megolm_advance_to(result, message_index);
+        return 0;
+    }
+}
+
 /**
  * decrypt an un-base64-ed message
  */
@@ -268,8 +293,7 @@ static size_t _decrypt(
 ) {
     struct _OlmDecodeGroupMessageResults decoded_results;
     size_t max_length, r;
-    Megolm *megolm;
-    Megolm tmp_megolm;
+    Megolm megolm;
 
     _olm_decode_group_message(
         message, message_length,
@@ -316,33 +340,21 @@ static size_t _decrypt(
         return (size_t)-1;
     }
 
-    /* pick a megolm instance to use. If we're at or beyond the latest ratchet
-     * value, use that */
-    if ((decoded_results.message_index - session->latest_ratchet.counter) < (1U << 31)) {
-        megolm = &session->latest_ratchet;
-    } else if ((decoded_results.message_index - session->initial_ratchet.counter) >= (1U << 31)) {
-        /* the counter is before our intial ratchet - we can't decode this. */
-        session->last_error = OLM_UNKNOWN_MESSAGE_INDEX;
-        return (size_t)-1;
-    } else {
-        /* otherwise, start from the initial megolm. Take a copy so that we
-         * don't overwrite the initial megolm */
-        tmp_megolm = session->initial_ratchet;
-        megolm = &tmp_megolm;
+    r = _get_megolm(session, decoded_results.message_index, &megolm);
+    if (r == (size_t)-1) {
+        return r;
     }
 
-    megolm_advance_to(megolm, decoded_results.message_index);
-
     /* now try checking the mac, and decrypting */
     r = megolm_cipher->ops->decrypt(
         megolm_cipher,
-        megolm_get_data(megolm), MEGOLM_RATCHET_LENGTH,
+        megolm_get_data(&megolm), MEGOLM_RATCHET_LENGTH,
         message, message_length,
         decoded_results.ciphertext, decoded_results.ciphertext_length,
         plaintext, max_plaintext_length
     );
 
-    _olm_unset(&tmp_megolm, sizeof(tmp_megolm));
+    _olm_unset(&megolm, sizeof(megolm));
     if (r == (size_t)-1) {
         session->last_error = OLM_BAD_MESSAGE_MAC;
         return r;
@@ -397,3 +409,55 @@ uint32_t olm_inbound_group_session_first_known_index(
 ) {
     return session->initial_ratchet.counter;
 }
+
+// we don't have an Ed25519 signature to share; this is a version 3 message format
+#define SESSION_KEY_VERSION 3
+#define V3_SESSION_KEY_RAW_LENGTH \
+    (1 + 4 + MEGOLM_RATCHET_LENGTH + ED25519_PUBLIC_KEY_LENGTH)
+
+size_t olm_inbound_group_session_key_length(
+    const OlmInboundGroupSession *session
+) {
+    return _olm_encode_base64_length(V3_SESSION_KEY_RAW_LENGTH);
+}
+
+size_t olm_inbound_group_session_key(
+    OlmInboundGroupSession *session,
+    uint8_t * key, size_t key_length, uint32_t message_index
+) {
+    uint8_t *raw;
+    uint8_t *ptr;
+    Megolm megolm;
+    size_t r;
+    size_t encoded_length = olm_inbound_group_session_key_length(session);
+
+    if (key_length < encoded_length) {
+        session->last_error = OLM_OUTPUT_BUFFER_TOO_SMALL;
+        return (size_t)-1;
+    }
+
+    r = _get_megolm(session, message_index, &megolm);
+    if (r == (size_t)-1) {
+        return r;
+    }
+
+    /* put the raw data at the end of the output buffer. */
+    raw = ptr = key + encoded_length - V3_SESSION_KEY_RAW_LENGTH;
+    *ptr++ = SESSION_KEY_VERSION;
+
+    // Encode message index as a big endian 32-bit number.
+    for (unsigned i = 0; i < 4; i++) {
+        *ptr++ = 0xFF & (message_index >> 24); message_index <<= 8;
+    }
+
+    memcpy(ptr, megolm_get_data(&megolm), MEGOLM_RATCHET_LENGTH);
+    ptr += MEGOLM_RATCHET_LENGTH;
+
+    memcpy(
+        ptr, session->signing_key.public_key,
+        ED25519_PUBLIC_KEY_LENGTH
+    );
+    ptr += ED25519_PUBLIC_KEY_LENGTH;
+
+    return _olm_encode_base64(raw, V3_SESSION_KEY_RAW_LENGTH, key);
+}