• poljar's avatar
    sas: Fix the base64 encoding of the MAC. · 4e927eb1
    poljar authored
    When calculating the MAC for a message using olm_sas_calculate_mac() and
    olm_sas_calculate_mac_long_kdf() the resulting MAC will be base64
    encoded using _olm_encode_base64().
    
    The _olm_encode_base64() method requires an input buffer and output
    buffer to be passed alongside the input length. The method is called
    with the same buffer, containing the MAC, for the input buffer as well
    as for the output buffer. This results in an incorrectly base64 encoded
    MAC.
    
    For example the byte array:
        [121, 105, 187, 19, 37, 94, 119, 248, 224, 34, 94, 29, 157, 5,
         15, 230, 246, 115, 236, 217, 80, 78, 56, 200, 80, 200, 82, 158,
         168, 179, 10, 230]
    
    will be encoded as  eWm7NyVeVmXgbVhnYlZobllsWm9ibGxzV205aWJHeHo
    instead of as       eWm7EyVed/jgIl4dnQUP5vZz7NlQTjjIUMhSnqizCuY
    
    Notice the different value at the 10th character.
    
    The correct result can be independently checked using Python for
    example:
    
    >>> from base64 import b64encode
    >>> mac = [121, 105, 187, 19, 37, 94, 119, 248, 224, 34, 94, 29, 157, \
               5, 15, 230, 246, 115, 236, 217, 80, 78, 56, 200, 80, 200, \
               82, 158, 168, 179, 10, 230]
    >>> b64encode(bytearray(mac)).rstrip(b"=")
    >>> b'eWm7EyVed/jgIl4dnQUP5vZz7NlQTjjIUMhSnqizCuY'
    
    This happens because the encode_base64() method that is used does not support
    in-place encoding in the general case. This is because the remainder for a 32
    bit input will always be 2 (32 % 6 == 2).
    
    The remainder will be used over here:
    https://gitlab.matrix.org/matrix-org/olm/-/blob/c01164f001d57fbe2297fe11954b58077a68dc0d/src/base64.cpp#L74
    
    The logic that gets executed if a remainder exists depends on the original input
    values, since those already got in-place encoded the whole block will behave
    differently if the input buffer is the same as the output buffer.
    4e927eb1
sas.c 4.88 KB