OTR

Version 15 (Adrian Georgescu, 09/13/2013 02:16 am)

1 3 Adrian Georgescu
h1. Blink OTR implementation
2 1 Adrian Georgescu
3 12 Adrian Georgescu
September 12th, 2013
4 12 Adrian Georgescu
5 15 Adrian Georgescu
"Blink SIP client for SIP2SIP edition":http://download.sip2sip.info/ is a multimedia SIP client that supports chat sessions using MSRP protocol (RFC4975 http://tools.ietf.org/html/rfc4975).  The client is designed to work with SIP2SIP service that implements MSRP relay protocol for NAT traversal of SIP sessions with MSRP media like Instant Messaging chat sessions. The functionality will be gradually deployed to other versions and variations of Blink client. 
6 2 Adrian Georgescu
7 7 Adrian Georgescu
Over the MSRP media chat session, Blink implements the OTR protocol. 
8 1 Adrian Georgescu
9 3 Adrian Georgescu
h2. OTR Protocol
10 3 Adrian Georgescu
11 9 Adrian Georgescu
"Off-the-Record (OTR) Messaging":http://www.cypherpunks.ca/otr/ allows two parties to have private conversations over instant messaging by providing:
12 3 Adrian Georgescu
13 1 Adrian Georgescu
h3. Encryption
14 1 Adrian Georgescu
15 1 Adrian Georgescu
No one else can read your instant messages.
16 1 Adrian Georgescu
17 1 Adrian Georgescu
h3. Authentication
18 3 Adrian Georgescu
19 1 Adrian Georgescu
You are assured the correspondent is who you think it is.
20 1 Adrian Georgescu
21 1 Adrian Georgescu
h3. Deniability
22 1 Adrian Georgescu
23 1 Adrian Georgescu
The messages you send do not have digital signatures that are checkable by a third party. Anyone can forge messages after a conversation to make them look like they came from you. However, during a conversation, your correspondent is assured the messages he sees are authentic and unmodified.
24 1 Adrian Georgescu
25 1 Adrian Georgescu
h3. Perfect forward secrecy
26 1 Adrian Georgescu
27 1 Adrian Georgescu
If you lose control of your private keys, no previous conversation is compromised.
28 3 Adrian Georgescu
29 3 Adrian Georgescu
h2. Implementation
30 1 Adrian Georgescu
31 13 Adrian Georgescu
The OTR implementation is backwards compatible with MSRP clients that do not support it (of course the encryption feature are not available in this case). OTR is not employed on the remote party is a multi-party conference server (advertised by is-focus Contact header parameter). The code is written in Python and is based on the "python-otr package":https://pypi.python.org/pypi/python-potr/1.0.0b5
32 7 Adrian Georgescu
33 4 Adrian Georgescu
h3. User input
34 4 Adrian Georgescu
35 1 Adrian Georgescu
 * Chat window has the Encryption toolbar icon, encryption features for each session can be controlled by clicking on this toolbar item, a contextual menu appears
36 14 Adrian Georgescu
 * Verification of remote identity can be performed using SMP protocol (in a separate window)
37 7 Adrian Georgescu
 * Each Contact can have encrypted related attributes saved (always use OTR, verification status and learned fingerprint)
38 10 Adrian Georgescu
 * Logging of Chat conversations can be toggled off (global setting)
39 13 Adrian Georgescu
 * History saves for each message the encryption status (encrypted/non-encrypted and fingerprint un/verified). This is rendered using a lock with a corespondent collor
40 1 Adrian Georgescu
41 4 Adrian Georgescu
h3. Notifications
42 4 Adrian Georgescu
43 7 Adrian Georgescu
When the remote party (identified by its SIP URI) has changed its encryption fingerprint, several visual and audible clues appear:
44 4 Adrian Georgescu
45 4 Adrian Georgescu
 * Chat window system message is displayed
46 4 Adrian Georgescu
 * Voice synthesiser speaks
47 1 Adrian Georgescu
 * System notification (OSX >=10.8) 
48 1 Adrian Georgescu
 * Growl notification
49 8 Adrian Georgescu
50 11 Adrian Georgescu
h3. Key storage
51 11 Adrian Georgescu
52 11 Adrian Georgescu
The private key is stored in the program configuration folder under a folder called *chat*. The database with the remote fingerprints is stored in a file under the same folder:
53 11 Adrian Georgescu
54 11 Adrian Georgescu
 * private_key.dsa
55 11 Adrian Georgescu
 * trusted_peers
56 11 Adrian Georgescu
57 8 Adrian Georgescu
58 8 Adrian Georgescu
h3. Interoperability
59 8 Adrian Georgescu
60 8 Adrian Georgescu
We are not aware of any other SIP/MSRP client implementation that supports OTR encryption today. Blink however interoperates well with XMPP clients when using SIP2SIP service built-in SIP/XMPP gateway. We were able to have bidirectional OTR encrypted chat sessions between Blink (using sIP/MSRP) and Jitsi (using XMPP).