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| -rw-r--r-- | docs/olm.rst | 44 |
1 files changed, 41 insertions, 3 deletions
diff --git a/docs/olm.rst b/docs/olm.rst index db32cdb..8f1c612 100644 --- a/docs/olm.rst +++ b/docs/olm.rst @@ -8,9 +8,6 @@ https://github.com/trevp/axolotl/wiki. The Olm Algorithm ----------------- -.. figure:: Axolotl.svg - - Initial setup ~~~~~~~~~~~~~ @@ -153,3 +150,44 @@ with :math:`C_{i,j+1}` and stores the message keys that were skipped in the process so that they can decode out of order messages. If the receiver created a new receiver chain then they discard their current sender chain so that they will create a new chain when they next send a message. + + +The Olm Message Format +---------------------- + +Normal Messages +~~~~~~~~~~~~~~~ + +Olm messages start with a one byte version followed by a variable length +payload followed by a fixed length message authentication code. + + +.. code:: + + +--------------+------------------------------------+-----------+ + | Version Byte | Payload Bytes | MAC Bytes | + +--------------+------------------------------------+-----------+ + +The payload consists of key-value pairs where the keys are integers and the +values are integers and strings. The keys are encoded as a variable length +integer tag where the 3 lowest bits indicates the type of the value: +0 for integers, 2 for strings. If the value is an integer then the tag is +followed by the value encoded as a variable length integer. If the value is +a string then the tag is followed by the length of the string encoded as +a variable length integer followed by the string itself. + +Olm uses a variable length encoding for integers. Each integer is encoded as a +sequence of bytes with the high bit set followed by a byte with the high bit +clear. The seven low bits of each byte store the bits of the integer. The least +significant bits are stored in the first byte. + +=========== ===== ======== ================================================ + Name Tag Type Meaning +=========== ===== ======== ================================================ +Chain-Index 0x0A Integer The chain index, :math:`j`, of the message +Ratchet-Key 0x10 String The ratchet key, :math:`T_{i}`, of the message +Cipher-Text 0x22 String The cipher-text, :math:`X_{i,j}`, of the message +=========== ===== ======== ================================================ + + + |