Various clarifications to the spec

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Richard van der Hoff 2016-04-26 17:55:42 +01:00
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commit 9b010290a4

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@ -1,7 +1,7 @@
Olm: A Cryptographic Ratchet Olm: A Cryptographic Ratchet
============================ ============================
An implementation of the cryptographic ratchet described by An implementation of the double cryptographic ratchet described by
https://github.com/trevp/axolotl/wiki. https://github.com/trevp/axolotl/wiki.
Notation Notation
@ -101,25 +101,32 @@ The Olm Protocol
Creating an outbound session Creating an outbound session
~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Bob publishes his identity key, :math:`I_B`, and some single-use one-time Bob publishes the public parts of his identity key, :math:`I_B`, and some
keys :math:`E_B`. single-use one-time keys :math:`E_B`.
Alice downloads Bob's identity key, :math:`I_B`, and a one-time key, Alice downloads Bob's identity key, :math:`I_B`, and a one-time key,
:math:`E_B`. Alice takes her identity key, :math:`I_A`, and generates a new :math:`E_B`. She generates a new single-use key, :math:`E_A`, and computes a
single-use key, :math:`E_A`. Alice computes a root key, :math:`R_0`, and a root key, :math:`R_0`, and a chain key :math:`C_{0,0}`. She also generates a
chain key :math:`C_{0,0}`. Alice generates a new ratchet key :math:`T_0`. new ratchet key :math:`T_0`.
Sending the first pre-key messages Sending the first pre-key messages
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Alice computes a message key, :math:`M_{0,j}`, using the current chain key, Alice computes a message key, :math:`M_{0,j}`, and a new chain key,
:math:`C_{0,j}`. Alice replaces the current chain key with :math:`C_{0,j+1}`. :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 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, authenticated encryption scheme (see below) to get a cipher-text,
:math:`X_{0,j}`. Alice sends her identity key, :math:`I_A`, her single-use key, :math:`X_{0,j}`.
:math:`E_A`, Bob's single-use key, :math:`E_B`, the current chain index,
:math:`j`, her ratchet key, :math:`T_0`, and the cipher-text, :math:`X_{0,j}`, She then sends the following to Bob:
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 Alice will continue to send pre-key messages until she receives a message from
Bob. Bob.
@ -127,10 +134,7 @@ Bob.
Creating an inbound session from a pre-key message Creating an inbound session from a pre-key message
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Bob receives a pre-key message with the public parts of Alice's identity key, Bob receives a pre-key message as above.
:math:`I_A`, Alice's single-use key, :math:`E_A`, Alice's ratchet key,
:math:`T_0`, and his own single-use key, :math:`E_B`, as well as the
current chain index, :math:`j`, and the cipher-text, :math:`X_{0,j}`.
Bob looks up the private part of his single-use key, :math:`E_B`. He can now 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 compute the root key, :math:`R_0`, and the chain key, :math:`C_{0,0}`, from
@ -145,8 +149,11 @@ 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 Bob stores Alice's initial ratchet key, :math:`T_0`, until he wants to
send a message. send a message.
Sending messages 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, 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 :math:`C_{i,j}`. Alice uses chain keys where :math:`i` is even. Bob uses chain
@ -159,14 +166,17 @@ A message key,
:math:`M_{i,j}` is computed from the current chain key, :math:`C_{i,j}`, and :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 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 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}`. Then user sends the scheme (see below) to get a cipher-text, :math:`X_{i,j}`.
current chain index, :math:`j`, the ratchet key, :math:`T_i`, and the
cipher-text, :math:`X_{i,j}`, to the other user. 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 Receiving messages
~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
The user receives a message with the sender's current chain index, :math:`j`, 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 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 The user checks if they have a receiver chain with the correct