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Correct a couple of errors in the spec 

* We only use 8-byte MACs
* The version byte is \x03, not \x01
release-v1.0.0
Richard van der Hoff 2016-05-18 18:16:46 +01:00
parent 2a09ccbd63
commit 9ac6ab0f1c
1 changed files with 10 additions and 9 deletions
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19
docs/olm.rst
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@ -216,7 +216,7 @@ payload followed by a fixed length message authentication code.
| Version Byte | Payload Bytes | MAC Bytes |
+--------------+------------------------------------+-----------+
The version byte is ``"\x01"``.
The version byte is ``"\x03"``.
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
@ -241,7 +241,7 @@ 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. (Olm version 1 uses HMAC-SHA-256, giving a MAC of 32 bytes). The
being used. (Olm version 1 uses HMAC-SHA-256, truncated to 8 bytes). The
MAC protects all of the bytes preceding the MAC.
Pre-Key Messages
@ -256,7 +256,7 @@ length payload.
| Version Byte | Payload Bytes |
+--------------+------------------------------------+
The version byte is ``"\x01"``.
The version byte is ``"\x03"``.
The payload uses the same key-value format as for normal messages.
@ -280,9 +280,10 @@ Version 1
~~~~~~~~~
Version 1 of Olm uses AES-256_ in CBC_ mode with `PCKS#7`_ padding 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-SHA-256_ using the default salt and an info of ``"OLM_KEYS"``.
encryption and HMAC-SHA-256_ (truncated to 64 bits) for authentication. The
256 bit AES key, 256 bit HMAC key, and 128 bit AES IV are derived from the
message key using HKDF-SHA-256_ using the default salt and an info of
``"OLM_KEYS"``.
.. math::
@ -295,7 +296,7 @@ The plain-text is encrypted with AES-256, using the key :math:`AES\_KEY_{i,j}`
and the IV :math:`AES\_IV_{i,j}` to give the cipher-text, :math:`X_{i,j}`.
Then the entire message (including the Version Byte and all Payload Bytes) are
passed through HMAC-SHA-256, and the MAC is appended to the message.
passed through HMAC-SHA-256. The first 8 bytes of the MAC are appended to the message.
IPR
---
@ -311,8 +312,8 @@ Acknowledgements
----------------
The ratchet that Olm implements was designed by Trevor Perrin and Moxie
Marlinspike - details at https://github.com/trevp/axolotl/wiki. Olm is an
entirely new implementation written by the Matrix.org team.
Marlinspike - details at https://github.com/trevp/double_ratchet/wiki. Olm is
an entirely new implementation written by the Matrix.org team.
.. _`Curve25519`: http://cr.yp.to/ecdh.html
.. _`Triple Diffie-Hellman`: https://whispersystems.org/blog/simplifying-otr-deniability/