Commit 5fbeb3e2 authored by Richard van der Hoff's avatar Richard van der Hoff
Browse files

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.
parent bd6ab72c
......@@ -165,6 +165,37 @@ size_t olm_inbound_group_session_id(
uint8_t * id, size_t id_length
);
/**
* Get the first message index we know how to decrypt.
*/
uint32_t olm_inbound_group_session_first_known_index(
const OlmInboundGroupSession *session
);
/**
* Get the number of bytes returned by olm_export_inbound_group_session()
*/
size_t olm_export_inbound_group_session_length(
const OlmInboundGroupSession *session
);
/**
* Export the base64-encoded ratchet key for this session, at the given index,
* in a format which can be used by olm_import_inbound_group_session
*
* 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_export_inbound_group_session(
OlmInboundGroupSession *session,
uint8_t * key, size_t key_length, uint32_t message_index
);
#ifdef __cplusplus
} // extern "C"
......
......@@ -123,4 +123,21 @@ InboundGroupSession.prototype['session_id'] = restore_stack(function() {
return Pointer_stringify(session_id);
});
InboundGroupSession.prototype['first_known_index'] = restore_stack(function() {
return inbound_group_session_method(
Module['_olm_inbound_group_session_first_known_index']
)(this.ptr);
});
InboundGroupSession.prototype['export_session'] = restore_stack(function(message_index) {
var key_length = inbound_group_session_method(
Module['_olm_export_inbound_group_session_length']
)(this.ptr);
var key = stack(key_length + NULL_BYTE_PADDING_LENGTH);
outbound_group_session_method(Module['_olm_export_inbound_group_session'])(
this.ptr, key, key_length, message_index
);
return Pointer_stringify(key);
});
olm_exports['InboundGroupSession'] = InboundGroupSession;
......@@ -277,6 +277,27 @@ def build_arg_parser():
type=argparse.FileType('wb'), nargs='?',
default=sys.stdout)
group_decrypt.set_defaults(func=do_group_decrypt)
export_inbound_group = commands.add_parser(
"export_inbound_group",
help="Export the keys for an inbound group session",
)
export_inbound_group.add_argument(
"session_file", help="Local inbound group session file",
)
export_inbound_group.add_argument(
"export_file", help="File to export to (default stdout)",
type=argparse.FileType('w'), nargs='?',
default=sys.stdout,
)
export_inbound_group.add_argument(
"--message_index",
help="Index to export session at. Defaults to the earliest known index",
type=int,
)
export_inbound_group.set_defaults(func=do_export_inbound_group)
return parser
def do_outbound_group(args):
......@@ -333,6 +354,15 @@ def do_group_decrypt(args):
f.write(session.pickle(args.key))
args.plaintext_file.write(plaintext)
def do_export_inbound_group(args):
session = InboundGroupSession()
session.unpickle(args.key, read_base64_file(args.session_file))
index = args.message_index
if index is None:
# default to first known index
index = session.first_known_index()
args.export_file.write(session.export_session(index))
if __name__ == '__main__':
parser = build_arg_parser()
args = parser.parse_args()
......
......@@ -49,6 +49,13 @@ inbound_group_session_function(
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)
lib.olm_inbound_group_session_first_known_index.argtypes = (c_void_p,)
lib.olm_inbound_group_session_first_known_index.restypes = c_uint32
inbound_group_session_function(lib.olm_export_inbound_group_session_length)
inbound_group_session_function(lib.olm_export_inbound_group_session, c_void_p, c_size_t, c_uint32)
class InboundGroupSession(object):
def __init__(self):
self.buf = create_string_buffer(lib.olm_inbound_group_session_size())
......@@ -95,5 +102,15 @@ class InboundGroupSession(object):
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);
lib.olm_inbound_group_session_id(self.ptr, id_buffer, id_length)
return id_buffer.raw
def first_known_index(self):
return lib.olm_inbound_group_session_first_known_index(self.ptr)
def export_session(self, message_index):
length = lib.olm_export_inbound_group_session_length(self.ptr)
buffer = create_string_buffer(length)
lib.olm_export_inbound_group_session(self.ptr, buffer, length,
message_index)
return buffer.raw
......@@ -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);
}
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