olm.rst

Olm: A Cryptographic Ratchet
============================

An implementation of the double cryptographic ratchet described by
https://github.com/trevp/double_ratchet/wiki.

Notation
--------

This document uses :math:`\parallel` to represent string concatenation. When
:math:`\parallel` appears on the right hand side of an :math:`=` it means that
the inputs are concatenated. When :math:`\parallel` appears on the left hand
side of an :math:`=` it means that the output is split.

When this document uses :math:`ECDH\left(K_A,\,K_B\right)` it means that each
party computes a Diffie-Hellman agreement using their private key and the
remote party's public key.
So party :math:`A` computes :math:`ECDH\left(K_B_public,\,K_A_private\right)`
and party :math:`B` computes :math:`ECDH\left(K_A_public,\,K_B_private\right)`.

Where this document uses :math:`HKDF\left(salt,\,IKM,\,info,\,L\right)` it
refers to the `HMAC-based key derivation function`_ with a salt value of
:math:`salt`, input key material of :math:`IKM`, context string :math:`info`,
and output keying material length of :math:`L` bytes.

The Olm Algorithm
-----------------

Initial setup
~~~~~~~~~~~~~

The setup takes four Curve25519_ inputs: Identity keys for Alice and Bob,
:math:`I_A` and :math:`I_B`, and ephemeral keys for Alice and Bob,
:math:`E_A` and :math:`E_B`. A shared secret, :math:`S`, is generated using
`Triple Diffie-Hellman`_. The initial 256 bit root key, :math:`R_0`, and 256
bit chain key, :math:`C_{0,0}`, are derived from the shared secret using an
HMAC-based Key Derivation Function using SHA-256_ as the hash function
(HKDF-SHA-256_) with default salt and ``"OLM_ROOT"`` as the info.

.. math::
    \begin{align}
        S&=ECDH\left(I_A,\,E_B\right)\;\parallel\;ECDH\left(E_A,\,I_B\right)\;
            \parallel\;ECDH\left(E_A,\,E_B\right)\\
        R_0\;\parallel\;C_{0,0}&=
             HKDF\left(0,\,S,\,\text{"OLM\_ROOT"},\,64\right)
    \end{align}

Advancing the root key
~~~~~~~~~~~~~~~~~~~~~~

Advancing a root key takes the previous root key, :math:`R_{i-1}`, and two
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
HKDF-SHA-256_ using :math:`R_{i-1}` as the salt and ``"OLM_RATCHET"`` as the
info.

.. math::
    \begin{align}
        R_i\;\parallel\;C_{i,0}&=HKDF\left(
            R_{i-1},\,
            ECDH\left(T_{i-1},\,T_i\right),\,
            \text{"OLM\_RATCHET"},\,
            64
        \right)
    \end{align}


Advancing the chain key
~~~~~~~~~~~~~~~~~~~~~~~

Advancing a chain key takes the previous chain key, :math:`C_{i,j-i}`. The next
chain key, :math:`C_{i,j}`, is the HMAC-SHA-256_ of ``"\x02"`` using the
previous chain key as the key.

.. math::
     \begin{align}
        C_{i,j}&=HMAC\left(C_{i,j-1},\,\text{"\textbackslash x02"}\right)
    \end{align}

Creating a message key
~~~~~~~~~~~~~~~~~~~~~~

Creating a message key takes the current chain key, :math:`C_{i,j}`. The
message key, :math:`M_{i,j}`, is the HMAC-SHA-256_ of ``"\x01"`` using the
current chain key as the key. The message keys where :math:`i` is even are used
by Alice to encrypt messages. The message keys where :math:`i` is odd are used
by Bob to encrypt messages.

.. math::
    \begin{align}
        M_{i,j}&=HMAC\left(C_{i,j},\,\text{"\textbackslash x01"}\right)
    \end{align}


The Olm Protocol
----------------

Creating an outbound session
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bob publishes the public parts of his identity key, :math:`I_B`, and some
single-use one-time keys :math:`E_B`.

Alice downloads Bob's identity key, :math:`I_B`, and a one-time key,
:math:`E_B`. She generates a new single-use key, :math:`E_A`, and computes a
root key, :math:`R_0`, and a chain key :math:`C_{0,0}`. She also generates a
new ratchet key :math:`T_0`.

Sending the first pre-key messages
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Alice computes a message key, :math:`M_{0,j}`, and a new chain key,
:math:`C_{0,j+1}`, using the current chain key. She replaces the current chain
key with the new one.

Alice encrypts her plain-text with the message key, :math:`M_{0,j}`, using an
authenticated encryption scheme (see below) to get a cipher-text,
:math:`X_{0,j}`.

She then sends the following to Bob:
 * The public part of her identity key, :math:`I_A`
 * The public part of her single-use key, :math:`E_A`
 * The public part of Bob's single-use key, :math:`E_B`
 * The current chain index, :math:`j`
 * The public part of her ratchet key, :math:`T_0`
 * The cipher-text, :math:`X_{0,j}`

Alice will continue to send pre-key messages until she receives a message from
Bob.

Creating an inbound session from a pre-key message
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bob receives a pre-key message as above.

Bob looks up the private part of his single-use key, :math:`E_B`. He can now
compute the root key, :math:`R_0`, and the chain key, :math:`C_{0,0}`, from
:math:`I_A`, :math:`E_A`, :math:`I_B`, and :math:`E_B`.

Bob then advances the chain key :math:`j` times, to compute the chain key used
by the message, :math:`C_{0,j}`. He now creates the
message key, :math:`M_{0,j}`, and attempts to decrypt the cipher-text,
:math:`X_{0,j}`. If the cipher-text's authentication is correct then Bob can
discard the private part of his single-use one-time key, :math:`E_B`.

Bob stores Alice's initial ratchet key, :math:`T_0`, until he wants to
send a message.

Sending normal messages
~~~~~~~~~~~~~~~~~~~~~~~

Once a message has been received from the other side, a session is considered
established, and a more compact form is used.

To send a message, the user checks if they have a sender chain key,
:math:`C_{i,j}`. Alice uses chain keys where :math:`i` is even. Bob uses chain
keys where :math:`i` is odd. If the chain key doesn't exist then a new ratchet
key :math:`T_i` is generated and a new root key :math:`R_i` and chain key
:math:`C_{i,0}` are computed using :math:`R_{i-1}`, :math:`T_{i-1}` and
:math:`T_i`.

A message key,
:math:`M_{i,j}` is computed from the current chain key, :math:`C_{i,j}`, and
the chain key is replaced with the next chain key, :math:`C_{i,j+1}`. The
plain-text is encrypted with :math:`M_{i,j}`, using an authenticated encryption
scheme (see below) to get a cipher-text, :math:`X_{i,j}`.

The user then sends the following to the recipient:
 * The current chain index, :math:`j`
 * The public part of the current ratchet key, :math:`T_i`
 * The cipher-text, :math:`X_{i,j}`

Receiving messages
~~~~~~~~~~~~~~~~~~

The user receives a message as above with the sender's current chain index, :math:`j`,
the sender's ratchet key, :math:`T_i`, and the cipher-text, :math:`X_{i,j}`.

The user checks if they have a receiver chain with the correct
:math:`i` by comparing the ratchet key, :math:`T_i`. If the chain doesn't exist
then they compute a new root key, :math:`R_i`, and a new receiver chain, with
chain key :math:`C_{i,0}`, using :math:`R_{i-1}`, :math:`T_{i-1}` and
:math:`T_i`.

If the :math:`j` of the message is less than
the current chain index on the receiver then the message may only be decrypted
if the receiver has stored a copy of the message key :math:`M_{i,j}`. Otherwise
the receiver computes the chain key, :math:`C_{i,j}`. The receiver computes the
message key, :math:`M_{i,j}`, from the chain key and attempts to decrypt the
cipher-text, :math:`X_{i,j}`.

If the decryption succeeds the receiver updates the chain key for :math:`T_i`
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
----------------------

Olm uses two types of messages. The underlying transport protocol must provide
a means for recipients to distinguish between them.

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 ``"\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
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
=========== ===== ======== ================================================
Ratchet-Key  0x0A String   The public part of 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. (Olm version 1 uses HMAC-SHA-256, truncated to 8 bytes). The
MAC protects all of the bytes preceding 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 ``"\x03"``.

The payload uses the same key-value format as for normal messages.

============ ===== ======== ================================================
    Name      Tag    Type                     Meaning
============ ===== ======== ================================================
One-Time-Key  0x0A String   The public part of Bob's single-use key,
                            :math:`E_b`.
Base-Key      0x12 String   The public part of Alice's single-use key,
                            :math:`E_a`.
Identity-Key  0x1A String   The public part of 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 with `PCKS#7`_ padding for
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::

    \begin{align}
    AES\_KEY_{i,j}\;\parallel\;HMAC\_KEY_{i,j}\;\parallel\;AES\_IV_{i,j}
        &= HKDF\left(0,\,M_{i,j},\text{"OLM\_KEYS"},\,80\right) \\
    \end{align}

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. The first 8 bytes of the MAC are appended to the message.

IPR
---

The Olm specification (this document) is hereby placed in the public domain.

Feedback
--------

Can be sent to mark at matrix.org.

Acknowledgements
----------------

The ratchet that Olm implements was designed by Trevor Perrin and Moxie
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/
.. _`HMAC-based key derivation function`: https://tools.ietf.org/html/rfc5869
.. _`HKDF-SHA-256`: https://tools.ietf.org/html/rfc5869
.. _`HMAC-SHA-256`: https://tools.ietf.org/html/rfc2104
.. _`SHA-256`: https://tools.ietf.org/html/rfc6234
.. _`AES-256`: http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf
.. _`CBC`: http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
.. _`PCKS#7`: https://tools.ietf.org/html/rfc2315