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Start specifying the message format

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Mark Haines 2015-08-07 18:24:20 +01:00
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docs/olm.rst
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@ -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
=========== ===== ======== ================================================