Merge pull request #42 from matrix-org/rav/megolm_export

Export and import of megolm session data

1 files changed, 161 insertions, 30 deletions

diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index a54e55f..a78404d 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -30,8 +30,9 @@
 
 #define OLM_PROTOCOL_VERSION     3
 #define GROUP_SESSION_ID_LENGTH  ED25519_PUBLIC_KEY_LENGTH
-#define PICKLE_VERSION           1
+#define PICKLE_VERSION           2
 #define SESSION_KEY_VERSION      2
+#define SESSION_EXPORT_VERSION   1
 
 struct OlmInboundGroupSession {
     /** our earliest known ratchet value */
@@ -43,6 +44,17 @@ struct OlmInboundGroupSession {
     /** The ed25519 signing key */
     struct _olm_ed25519_public_key signing_key;
 
+    /**
+     * Have we ever seen any evidence that this is a valid session?
+     * (either because the original session share was signed, or because we
+     * have subsequently successfully decrypted a message)
+     *
+     * (We don't do anything with this currently, but we may want to bear it in
+     * mind when we consider handling key-shares for sessions we already know
+     * about.)
+     */
+    int signing_key_verified;
+
     enum OlmErrorCode last_error;
 };
 
@@ -71,19 +83,24 @@ size_t olm_clear_inbound_group_session(
     return sizeof(OlmInboundGroupSession);
 }
 
+#define SESSION_EXPORT_RAW_LENGTH \
+    (1 + 4 + MEGOLM_RATCHET_LENGTH + ED25519_PUBLIC_KEY_LENGTH)
+
 #define SESSION_KEY_RAW_LENGTH \
     (1 + 4 + MEGOLM_RATCHET_LENGTH + ED25519_PUBLIC_KEY_LENGTH\
         + ED25519_SIGNATURE_LENGTH)
 
-/** init the session keys from the un-base64-ed session keys */
 static size_t _init_group_session_keys(
     OlmInboundGroupSession *session,
-    const uint8_t *key_buf
+    const uint8_t *key_buf,
+    int export_format
 ) {
+    const uint8_t expected_version =
+        (export_format ? SESSION_EXPORT_VERSION : SESSION_KEY_VERSION);
     const uint8_t *ptr = key_buf;
     size_t version = *ptr++;
 
-    if (version != SESSION_KEY_VERSION) {
+    if (version != expected_version) {
         session->last_error = OLM_BAD_SESSION_KEY;
         return (size_t)-1;
     }
@@ -103,11 +120,15 @@ static size_t _init_group_session_keys(
     );
     ptr += ED25519_PUBLIC_KEY_LENGTH;
 
-    if (!_olm_crypto_ed25519_verify(
-        &session->signing_key, key_buf, ptr - key_buf, ptr
-    )) {
-        session->last_error = OLM_BAD_SIGNATURE;
-        return (size_t)-1;
+    if (!export_format) {
+        if (!_olm_crypto_ed25519_verify(&session->signing_key, key_buf,
+                                        ptr - key_buf, ptr)) {
+            session->last_error = OLM_BAD_SIGNATURE;
+            return (size_t)-1;
+        }
+
+        /* signed keyshare */
+        session->signing_key_verified = 1;
     }
     return 0;
 }
@@ -131,11 +152,35 @@ size_t olm_init_inbound_group_session(
     }
 
     _olm_decode_base64(session_key, session_key_length, key_buf);
-    result = _init_group_session_keys(session, key_buf);
+    result = _init_group_session_keys(session, key_buf, 0);
     _olm_unset(key_buf, SESSION_KEY_RAW_LENGTH);
     return result;
 }
 
+size_t olm_import_inbound_group_session(
+    OlmInboundGroupSession *session,
+    const uint8_t * session_key, size_t session_key_length
+) {
+    uint8_t key_buf[SESSION_EXPORT_RAW_LENGTH];
+    size_t raw_length = _olm_decode_base64_length(session_key_length);
+    size_t result;
+
+    if (raw_length == (size_t)-1) {
+        session->last_error = OLM_INVALID_BASE64;
+        return (size_t)-1;
+    }
+
+    if (raw_length != SESSION_EXPORT_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, 1);
+    _olm_unset(key_buf, SESSION_EXPORT_RAW_LENGTH);
+    return result;
+}
+
 static size_t raw_pickle_length(
     const OlmInboundGroupSession *session
 ) {
@@ -144,6 +189,7 @@ static size_t raw_pickle_length(
     length += megolm_pickle_length(&session->initial_ratchet);
     length += megolm_pickle_length(&session->latest_ratchet);
     length += _olm_pickle_ed25519_public_key_length(&session->signing_key);
+    length += _olm_pickle_bool_length(session->signing_key_verified);
     return length;
 }
 
@@ -171,6 +217,7 @@ size_t olm_pickle_inbound_group_session(
     pos = megolm_pickle(&session->initial_ratchet, pos);
     pos = megolm_pickle(&session->latest_ratchet, pos);
     pos = _olm_pickle_ed25519_public_key(pos, &session->signing_key);
+    pos = _olm_pickle_bool(pos, session->signing_key_verified);
 
     return _olm_enc_output(key, key_length, pickled, raw_length);
 }
@@ -194,7 +241,7 @@ size_t olm_unpickle_inbound_group_session(
     pos = pickled;
     end = pos + raw_length;
     pos = _olm_unpickle_uint32(pos, end, &pickle_version);
-    if (pickle_version != PICKLE_VERSION) {
+    if (pickle_version < 1 || pickle_version > PICKLE_VERSION) {
         session->last_error = OLM_UNKNOWN_PICKLE_VERSION;
         return (size_t)-1;
     }
@@ -202,6 +249,14 @@ size_t olm_unpickle_inbound_group_session(
     pos = megolm_unpickle(&session->latest_ratchet, pos, end);
     pos = _olm_unpickle_ed25519_public_key(pos, end, &session->signing_key);
 
+    if (pickle_version == 1) {
+        /* pickle v1 had no signing_key_verified field (all keyshares were
+         * verified at import time) */
+        session->signing_key_verified = 1;
+    } else {
+        pos = _olm_unpickle_bool(pos, end, &(session->signing_key_verified));
+    }
+
     if (end != pos) {
         /* We had the wrong number of bytes in the input. */
         session->last_error = OLM_CORRUPTED_PICKLE;
@@ -258,6 +313,32 @@ 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
  */
 static size_t _decrypt(
@@ -268,8 +349,7 @@ 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,
@@ -316,38 +396,30 @@ 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;
     }
 
+    /* once we have successfully decrypted a message, set a flag to say the
+     * session appears valid. */
+    session->signing_key_verified = 1;
+
     return r;
 }
 
@@ -391,3 +463,62 @@ size_t olm_inbound_group_session_id(
         session->signing_key.public_key, GROUP_SESSION_ID_LENGTH, id
     );
 }
+
+uint32_t olm_inbound_group_session_first_known_index(
+    const OlmInboundGroupSession *session
+) {
+    return session->initial_ratchet.counter;
+}
+
+int olm_inbound_group_session_is_verified(
+    const OlmInboundGroupSession *session
+) {
+    return session->signing_key_verified;
+}
+
+size_t olm_export_inbound_group_session_length(
+    const OlmInboundGroupSession *session
+) {
+    return _olm_encode_base64_length(SESSION_EXPORT_RAW_LENGTH);
+}
+
+size_t olm_export_inbound_group_session(
+    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_export_inbound_group_session_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 - SESSION_EXPORT_RAW_LENGTH;
+    *ptr++ = SESSION_EXPORT_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, SESSION_EXPORT_RAW_LENGTH, key);
+}