Off-the-record Messaging
(
OTR
) is a
cryptographic protocol
that provides encryption for
instant messaging
conversations. OTR uses a combination of
AES
symmetric-key algorithm
with 128 bits key length, the
Diffie?Hellman key exchange
with 1536 bits group size, and the
SHA-1
hash function. In addition to
authentication
and
encryption
, OTR provides
forward secrecy
and
malleable encryption
.
The primary motivation behind the protocol was providing
deniable authentication
for the conversation participants while keeping conversations confidential, like a private conversation in real life, or
off the record
in
journalism sourcing
. This is in contrast with cryptography tools that produce output which can be later used as a verifiable record of the communication event and the identities of the participants. The initial introductory paper was named "Off-the-Record Communication, or, Why Not To Use
PGP
".
[1]
The OTR protocol was designed by cryptographers
Ian Goldberg
and
Nikita Borisov
and released on 26 October 2004.
[2]
They provide a client
library
to facilitate support for instant messaging client developers who want to implement the protocol. A
Pidgin
and
Kopete
plugin exists that allows OTR to be used over any IM protocol supported by Pidgin or Kopete, offering an
auto-detection
feature that starts the OTR session with the buddies that have it enabled, without interfering with regular, unencrypted conversations. Version 4 of the protocol
[3]
has been in development since 2017
[4]
by a team led by Sofia Celi, and reviewed by Nik Unger and Ian Goldberg. This version aims to provide online and offline deniability, to update the cryptographic primitives, and to support
out-of-order delivery
and asynchronous communication.
History
[
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]
OTR was presented in 2004 by Nikita Borisov, Ian Avrum Goldberg, and Eric A. Brewer as an improvement over the OpenPGP and the S/MIME system at the "Workshop on Privacy in the Electronic Society" (WPES).
[1]
The first version 0.8.0 of the reference implementation was published on 21 November 2004. In 2005 an analysis was presented by Mario Di Raimondo, Rosario Gennaro, and Hugo Krawczyk that called attention to several vulnerabilities and proposed appropriate fixes, most notably including a flaw in the key exchange.
[5]
As a result, version 2 of the OTR protocol was published in 2005 which implements a variation of the proposed modification that additionally hides the public keys. Moreover, the possibility to fragment OTR messages was introduced in order to deal with chat systems that have a limited message size, and a simpler method of verification against man-in-the-middle attacks was implemented.
[6]
In 2007
Olivier Goffart
published
mod_otr
[7]
for
ejabberd
, making it possible to perform
man-in-the-middle attacks
on OTR users who don't check key fingerprints. OTR developers countered this attack by introducing a
socialist millionaire protocol
implementation in libotr. Instead of comparing key checksums, knowledge of an arbitrary shared secret can be utilised for which relatively low
entropy
can be tolerated.
[8]
Version 3 of the protocol was published in 2012. As a measure against the repeated reestablishment of a session in case of several competing chat clients being signed on to the same user address at the same time, more precise identification labels for sending and receiving client instances were introduced in version 3. Moreover, an additional key is negotiated which can be used for another data channel.
[9]
Several solutions have been proposed for supporting conversations with multiple participants. A method proposed in 2007 by Jiang Bian, Remzi Seker, and Umit Topaloglu uses the system of one participant as a "virtual server".
[10]
The method called "Multi-party Off-the-Record Messaging" (mpOTR) which was published in 2009 works without a central management host and was introduced in
Cryptocat
by Ian Goldberg et al.
[11]
In 2013, the
Signal Protocol
was introduced, which is based on OTR Messaging and the
Silent Circle Instant Messaging Protocol
(SCIMP). It brought about support for asynchronous communication ("offline messages") as its major new feature, as well as better resilience with distorted order of messages and simpler support for conversations with multiple participants.
[12]
OMEMO
, introduced in an Android XMPP client called
Conversations
in 2015, integrates the
Double Ratchet Algorithm
used in Signal into the instant messaging protocol
XMPP
("Jabber") and also enables encryption of file transfers. In the autumn of 2015 it was submitted to the
XMPP Standards Foundation
for standardisation.
[13]
[14]
Currently, version 4 of the protocol has been designed. It was presented by Sofia Celi and Ola Bini on PETS2018.
[15]
Implementation
[
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]
In addition to providing encryption and authentication ? features also provided by typical public-key cryptography suites, such as
PGP
,
GnuPG
, and
X.509
(
S/MIME
) ? OTR also offers some less common features:
- Forward secrecy
- Messages are only
encrypted
with temporary per-message
AES
keys, negotiated using the
Diffie?Hellman key exchange
protocol. The compromise of any long-lived cryptographic keys does not compromise any previous conversations, even if an attacker is in possession of
ciphertexts
.
- Deniable authentication
- Messages in a conversation do not have
digital signatures
, and after a conversation is complete, anyone is able to forge a message to appear to have come from one of the participants in the conversation, assuring that it is impossible to prove that a specific message came from a specific person. Within the conversation the recipient can be sure that a message is coming from the person they have identified.
Authentication
[
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]
As of OTR 3.1, the protocol supports mutual authentication of users using a shared secret through the
socialist millionaire
protocol. This feature makes it possible for users to verify the identity of the remote party and avoid a
man-in-the-middle attack
without the inconvenience of manually comparing
public key fingerprints
through an outside channel.
Limitations
[
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]
Due to limitations of the protocol, OTR does not support multi-user group chat as of 2009
[update]
[16]
but it may be implemented in the future. As of version 3
[9]
of the protocol specification, an extra symmetric key is derived during authenticated key exchanges that can be used for secure communication (e.g., encrypted
file transfers
) over a different channel. Support for encrypted audio or video is not planned. (
SRTP
with
ZRTP
exists for that purpose.) A project to produce a protocol for multi-party off-the-record messaging (mpOTR) has been organized by
Cryptocat
,
eQualitie
, and other contributors including Ian Goldberg.
[11]
[17]
Since OTR protocol v3 (libotr 4.0.0) the plugin supports multiple OTR conversations with the same buddy who is logged in at multiple locations.
[18]
Client support
[
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]
Native (supported by project developers)
[
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]
These clients support Off-the-Record Messaging out of the box (incomplete list).
Via third-party plug-in
[
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]
Off-The-Record authentication in
Pidgin
using
Socialist millionaires
protocol
The following clients require a plug-in to use Off-the-Record Messaging.
Confusion with Google Talk "off the record"
[
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]
Although Gmail's
Google Talk
uses the term "off the record", the feature has no connection to the Off-the-Record Messaging protocol described in this article, its chats are not encrypted in the way described above?and could be logged internally by Google even if not accessible by end-users.
[31]
[32]
See also
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References
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]
Further reading
[
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]
External links
[
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]