From 60800e155612bea797eed93c67046a23d26054cc Mon Sep 17 00:00:00 2001
From: Moxie Marlinspike <moxie@thoughtcrime.org>
Date: Mon, 24 Nov 2014 12:54:30 -0800
Subject: Break out into separate repo.

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+# 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());
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