From 5fbeb3e29b6440a799d9320e871a1d4d509130b8 Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Fri, 6 Jan 2017 12:55:05 +0000 Subject: Enable exporting inbound group session keys A pair of functions which allow you to export the megolm keys for an inbound group session, so that an application can save/restore them. --- src/inbound_group_session.c | 108 ++++++++++++++++++++++++++++++++++++-------- 1 file changed, 89 insertions(+), 19 deletions(-) (limited to 'src/inbound_group_session.c') diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c index a54e55f..122c26c 100644 --- a/src/inbound_group_session.c +++ b/src/inbound_group_session.c @@ -32,6 +32,7 @@ #define GROUP_SESSION_ID_LENGTH ED25519_PUBLIC_KEY_LENGTH #define PICKLE_VERSION 1 #define SESSION_KEY_VERSION 2 +#define SESSION_EXPORT_VERSION 1 struct OlmInboundGroupSession { /** our earliest known ratchet value */ @@ -71,6 +72,9 @@ 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) @@ -257,6 +261,32 @@ 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 +298,7 @@ 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 +345,21 @@ 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; @@ -391,3 +408,56 @@ 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; +} + +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); +} -- cgit v1.2.3