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Document the pre-key messages

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Mark Haines 2015-08-11 13:12:51 +01:00
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docs/olm.rst
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@ -33,8 +33,8 @@ Curve25519 inputs: the previous ratchet key, :math:`T_{i-1}`, and the current
ratchet key :math:`T_i`. The even ratchet keys are generated by Alice.
The odd ratchet keys are generated by Bob. A shared secret is generated
using Diffie-Hellman on the ratchet keys. The next root key, :math:`R_i`, and
chain key, :math:`C_{i,0}`, are derived from the shared secret using an
HMAC-based Key Derivation Function (HKDF) using :math:`R_{i-1}` as the salt.
chain key, :math:`C_{i,0}`, are derived from the shared secret using an HKDF
using :math:`R_{i-1}` as the salt.
.. math::
\begin{align}
@ -151,7 +151,6 @@ 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
----------------------
@ -161,13 +160,14 @@ 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 version byte is ``"\x01"``.
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:
@ -184,10 +184,46 @@ 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
Ratchet-Key 0x0A String The ratchet key, :math:`T_{i}`, of the message
Chain-Index 0x10 Integer The chain index, :math:`j`, of the message
Cipher-Text 0x22 String The cipher-text, :math:`X_{i,j}`, of the message
=========== ===== ======== ================================================
The length of the MAC is determined by the authenticated encryption algorithm
being used. The MAC protects all of the bytes preceeding the MAC.
Pre-Key Messages
~~~~~~~~~~~~~~~~
Olm pre-key messages start with a one byte version followed by a variable
length payload.
.. code::
+--------------+------------------------------------+
| Version Byte | Payload Bytes |
+--------------+------------------------------------+
The version byte is ``"\x01"``.
The payload uses the same key-value format as for normal messages.
============ ===== ======== ================================================
Name Tag Type Meaning
============ ===== ======== ================================================
One-Time-Key 0x0A String Bob's single-use key, :math:`E_b`.
Base-Key 0x12 String Alice's single-use key, :math:`E_a`.
Identity-Key 0x1A String Alice's identity key, :math:`I_a`.
Message 0x22 String An embedded Olm message with its own version and
MAC.
============ ===== ======== ================================================
Olm Authenticated Encryption
----------------------------
Version 1
~~~~~~~~~
Version 1 of Olm uses AES-256 in CBC mode for encryption and HMAC-SHA-256 for
authentication. The 256 bit AES key, 256 bit HMAC key, and 128 bit AES IV are
derived from the message key using HKDF.