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Start specifying the message format

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Mark Haines 2015-08-07 18:24:20 +01:00
parent 531a2fb426
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docs/olm.rst
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@ -8,9 +8,6 @@ https://github.com/trevp/axolotl/wiki.
The Olm Algorithm The Olm Algorithm
----------------- -----------------
.. figure:: Axolotl.svg
Initial setup 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 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 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. 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
=========== ===== ======== ================================================