mirror of
https://github.com/oxen-io/session-android.git
synced 2024-12-11 18:54:21 +00:00
86 lines
3.8 KiB
Markdown
86 lines
3.8 KiB
Markdown
|
|
# Overview
|
|
|
|
This is a ratcheting forward secrecy protocol that works in synchronous and asynchronous messaging
|
|
environments. The protocol overview is available [here](https://github.com/trevp/axolotl/wiki),
|
|
and the details of the wire format are available [here](https://github.com/WhisperSystems/TextSecure/wiki/ProtocolV2).
|
|
|
|
## PreKeys
|
|
|
|
This protocol uses a concept called 'PreKeys'. A PreKey is an ECPublicKey and an associated unique
|
|
ID which are stored together by a server. PreKeys can also be signed.
|
|
|
|
At install time, clients generate a single signed PreKey, as well as a large list of unsigned
|
|
PreKeys, and transmit all of them to the server.
|
|
|
|
## Sessions
|
|
|
|
The axolotl protocol is session-oriented. Clients establish a "session," which is then used for
|
|
all subsequent encrypt/decrypt operations. There is no need to ever tear down a session once one
|
|
has been established.
|
|
|
|
Sessions are established in one of three ways:
|
|
|
|
1. PreKeyBundles. A client that wishes to send a message to a recipient can establish a session by
|
|
retrieving a PreKeyBundle for that recipient from the server.
|
|
1. PreKeyWhisperMessages. A client can receive a PreKeyWhisperMessage from a recipient and use it
|
|
to establish a session.
|
|
1. KeyExchangeMessages. Two clients can exchange KeyExchange messages to establish a session.
|
|
|
|
## State
|
|
|
|
An established session encapsulates a lot of state between two clients. That state is maintained
|
|
in durable records which need to be kept for the life of the session.
|
|
|
|
State is kept in the following places:
|
|
|
|
1. Identity State. Clients will need to maintain the state of their own identity key pair, as well
|
|
as identity keys received from other clients.
|
|
1. PreKey State. Clients will need to maintain the state of their generated PreKeys.
|
|
1. Signed PreKey States. Clients will need to maintain the state of their signed PreKeys.
|
|
1. Session State. Clients will need to maintain the state of the sessions they have established.
|
|
|
|
# Using libaxolotl
|
|
|
|
## Install time
|
|
|
|
At install time, a libaxolotl client needs to generate its identity keys, registration id, and
|
|
prekeys.
|
|
|
|
IdentityKeyPair identityKeyPair = KeyHelper.generateIdentityKeyPair();
|
|
int registrationId = KeyHelper.generateRegistrationId();
|
|
List<PreKeyRecord> preKeys = KeyHelper.generatePreKeys(startId, 100);
|
|
PreKeyRecord lastResortKey = KeyHelper.generateLastResortKey();
|
|
SignedPreKeyRecord signedPreKey = KeyHelper.generateSignedPreKey(identityKeyPair, 5);
|
|
|
|
// Store identityKeyPair somewhere durable and safe.
|
|
// Store registrationId somewhere durable and safe.
|
|
|
|
// Store preKeys in PreKeyStore.
|
|
// Store signed prekey in SignedPreKeyStore.
|
|
|
|
## Building a session
|
|
|
|
A libaxolotl client needs to implement four interfaces: IdentityKeyStore, PreKeyStore,
|
|
SignedPreKeyStore, and SessionStore. These will manage loading and storing of identity,
|
|
prekeys, signed prekeys, and session state.
|
|
|
|
Once those are implemented, building a session is fairly straightforward:
|
|
|
|
SessionStore sessionStore = new MySessionStore();
|
|
PreKeyStore preKeyStore = new MyPreKeyStore();
|
|
SignedPreKeyStore signedPreKeyStore = new MySignedPreKeyStore();
|
|
IdentityKeyStore identityStore = new MyIdentityKeyStore();
|
|
|
|
// Instantiate a SessionBuilder for a remote recipientId + deviceId tuple.
|
|
SessionBuilder sessionBuilder = new SessionBuilder(sessionStore, preKeyStore, signedPreKeyStore,
|
|
identityStore, recipientId, deviceId);
|
|
|
|
// Build a session with a PreKey retrieved from the server.
|
|
sessionBuilder.process(retrievedPreKey);
|
|
|
|
SessionCipher sessionCipher = new SessionCipher(sessionStore, recipientId, deviceId);
|
|
CiphertextMessage message = sessionCipher.encrypt("Hello world!".getBytes("UTF-8"));
|
|
|
|
deliver(message.serialize());
|