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Add some links to the spec for the various primitives being used.

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Mark Haines 2015-08-18 14:33:40 +01:00
parent 40e49ee173
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docs/olm.rst
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@ -11,13 +11,13 @@ The Olm Algorithm
Initial setup Initial setup
~~~~~~~~~~~~~ ~~~~~~~~~~~~~
The setup takes four Curve25519 inputs: Identity keys for Alice and Bob, The setup takes four Curve25519_ inputs: Identity keys for Alice and Bob,
:math:`I_A` and :math:`I_B`, and emphemeral keys for Alice and Bob, :math:`I_A` and :math:`I_B`, and emphemeral keys for Alice and Bob,
:math:`E_A` and :math:`E_B`. A shared secret, :math:`S`, is generated using :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 `Triple Diffie-Hellman`_. The initial 256 bit root key, :math:`R_0`, and 256
chain key, :math:`C_{0,0}`, are derived from the shared secret using an bit chain key, :math:`C_{0,0}`, are derived from the shared secret using an
HMAC-based Key Derivation Function (HKDF) with default salt and ``"OLM_ROOT"`` HMAC-based Key Derivation Function using SHA-256_ as the hash function
as the info. (HKDF-SHA-256_) with default salt and ``"OLM_ROOT"`` as the info.
.. math:: .. math::
\begin{align} \begin{align}
@ -34,8 +34,9 @@ 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. 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 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 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 HKDF chain key, :math:`C_{i,0}`, are derived from the shared secret using
using :math:`R_{i-1}` as the salt and ``"OLM_RATCHET"`` as the info. HKDF-SHA-256_ using :math:`R_{i-1}` as the salt and ``"OLM_RATCHET"`` as the
info.
.. math:: .. math::
\begin{align} \begin{align}
@ -51,8 +52,8 @@ Advancing the chain key
~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~
Advancing a root key takes the previous chain key, :math:`C_{i,j-i}`. The next Advancing a root key takes the previous chain key, :math:`C_{i,j-i}`. The next
chain key, :math:`C_{i,j}`, is the HMAC of ``"\x02"`` using the previous chain chain key, :math:`C_{i,j}`, is the HMAC-SHA-256_ of ``"\x02"`` using the
key as the key. previous chain key as the key.
.. math:: .. math::
\begin{align} \begin{align}
@ -63,9 +64,9 @@ Creating a message key
~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~
Creating a message key takes the current chain key, :math:`C_{i,j}`. The Creating a message key takes the current chain key, :math:`C_{i,j}`. The
message key, :math:`M_{i,j}`, is the HMAC of ``"\x01"`` using the current message key, :math:`M_{i,j}`, is the HMAC-SHA-256_ of ``"\x01"`` using the
chain key as the key. The message keys where :math:`i` is even are used by current chain key as the key. The message keys where :math:`i` is even are used
Alice to encrypt messages. The message keys where :math:`i` is odd are used by Alice to encrypt messages. The message keys where :math:`i` is odd are used
by Bob to encrypt messages. by Bob to encrypt messages.
.. math:: .. math::
@ -225,18 +226,27 @@ Olm Authenticated Encryption
Version 1 Version 1
~~~~~~~~~ ~~~~~~~~~
Version 1 of Olm uses AES-256 in CBC mode with PCKS#7 padding for encryption 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 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 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"``. the default salt and an info of ``"OLM_KEYS"``.
First the plain-text, :math:`P_{i,j}`, is encrypted to get the cipher-text First the plain-text is encrypted to get the cipher-text, :math:`X_{i,j}`.
:math:`X_{i,j}`. Then the entire message, both the headers and cipher-text, Then the entire message, both the headers and cipher-text, are HMAC'd and the
are HMAC'd and the MAC is appended to the message. MAC is appended to the message.
.. math:: .. math::
\begin{align} \begin{align}
AES\_KEY_{i,j}\;\parallel\;HMAC\_KEY_{i,j}\;\parallel\;AES\_IV_{i,j} AES\_KEY_{i,j}\;\parallel\;HMAC\_KEY_{i,j}\;\parallel\;AES\_IV_{i,j}
&= HKDF\left(M_{i,j},\,\text{"OLM\_RATCHET"}\right) \\ &= HKDF\left(M_{i,j},\,\text{"OLM\_KEYS"}\right) \\
\end{align} \end{align}
.. _`Curve25519`: http://cr.yp.to/ecdh.html
.. _`Triple Diffie-Hellman`: https://whispersystems.org/blog/simplifying-otr-deniability/
.. _`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