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ProtoXEP otr-info v0.0.1 -- initial draft

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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Current Off-the-Record Messaging Usage</title>
<abstract>
This document outlines the current usage of Off-the-Record messaging in
XMPP, its drawbacks, its strengths, and recommendations for improving the
end user experience.
</abstract>
&LEGALNOTICE;
<number>xxxx</number>
<status>ProtoXEP</status>
<type>Informational</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>NOT_YET_ASSIGNED</shortname>
<author>
<firstname>Sam</firstname>
<surname>Whited</surname>
<email>sam@samwhited.com</email>
<jid>sam@samwhited.com</jid>
</author>
<revision>
<version>0.0.1</version>
<date>2015-07-28</date>
<initials>ssw</initials>
<remark><p>Initial draft.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>
The Off-the-Record messaging protocol (OTR) was originally introduced in
the 2004 paper
<i><link url='https://otr.cypherpunks.ca/otr-wpes.pdf'>
Off-the-Record Communication, or, Why Not To Use PGP
</link></i>
<note>
Nikita Borisov, Ian Goldberg, Eric Brewer (2004-10-28). "Off-the-Record
Communication, or, Why Not To Use PGP"
&lt;<link url='https://otr.cypherpunks.ca/otr-wpes.pdf'>
https://otr.cypherpunks.ca/otr-wpes.pdf
</link>&gt;
</note>
and has since become the de facto standard for performing end-to-end
encryption in XMPP. OTR provides encryption, deniable authentication,
forward secrecy, and malleable encryption.
</p>
<p>
The OTR protocol itself is currently described by the document:
<i><link url='https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html'>
Off-the-Record Messaging Protocol version 3
</link></i>
<note>
"Off-the-Record Messaging Protocol version 3"
&lt;<link url='https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html'>
https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html
</link>&gt;
</note>
and will not be redescribed here. Instead, this document aims to describe
OTR's usage and best practices within XMPP. It is not intended to be a
current standard, or technical specification, as better (albeit, newer and
less well tested) methods of end-to-end encryption exist for XMPP.
</p>
</section1>
<section1 topic='Overview' anchor='overview'>
<p>
Though this document will not focus on the OTR protocol itself, a brief
overview is warranted to better understand the protocols strengths and
weaknesses.
</p>
<p>
OTR uses 128 bit AES symmetric-key encryption and the SHA-1 hash function.
An OTR session can be held only between two parties, meaning that OTR is
incompatible with &xep0045;. It provides deniability in the form of
malleable encryption (a third party may generate fake messages after the
session has ended). This means that if you were not a part of the original
conversation, you cannot prove based on captured messages alone that a
message from the conversation was actually sent by a given party. Unlike
PGP, OTR also provides forward secrecy; even if a session is recorded and
the primary key is compromised at a later date, the OTR messages will not
be able to be decrypted as each was encrypted with an ephemeral key
exchanged with Diffie-Hellman key exchange with a 1536 bit modulus.
</p>
</section1>
<section1 topic='Discovery'>
<p>
Clients that support the OTR protocol do not advertise it in any of the
normal XMPP ways. Instead, OTR provides its own discovery mechanism. If a
client wishes to indicate support for OTR they include a special whitespace
tag in their messages. This tag can appear anywhere in the body of the
message stanza, but it is most often found at the end. The OTR tag
comprises the following bytes:
<example caption='OTR tag'>
\x20\x09\x20\x20\x09\x09\x09\x09 \x20\x09\x20\x09\x20\x09\x20\x20
</example>
and is followed by one or more of the following sequences to indicate the
version of OTR which the client supports:
<example caption='OTR tag version 1'>
\x20\x09\x20\x09\x20\x20\x09\x20
</example>
Note that this version 1 tag must come before other version tags for
compatibility; it is, however, NOT RECOMMENDED to implement version 1 of
the OTR protocol.
<example caption='OTR tag version 2'>
\x20\x20\x09\x09\x20\x20\x09\x20
</example>
<example caption='OTR tag version 3'>
\x20\x20\x09\x09\x20\x20\x09\x09
</example>
</p>
<p>
When a client sees this special string in the body of a message stanza it
may choose to start an OTR session immediately, or merely indicate support
to the user and allow the user to manually start a session. This is done by
sending a message stanza containing an OTR query message in the body which
indicates the supported versions of OTR. In XMPP these are most commonly
version 2 and version 3, which would be indicated by a message stanza which
has a body that starts with the string:
<example caption='OTR query'>
?OTR?v23?
</example>
</p>
<p>
Any message which begins with the afforementioned string (note that the
version number[s] may be different), postfixed with a payload should be
decrypted as an OTR message. The initialization message should not contain
a payload, and should just be the initialization string by itself.
</p>
</section1>
<section1 topic='OTR Messages'>
<section2 topic='Construction and Decoding'>
<p>
When sending a message encrypted with OTR, it is RECOMMENDED to encrypt
only the text node of the &lt;body/&gt; element (the message itself).
However, there are some clients in the wild which will encrypt the entire
contents of the &lt;body/&gt; element, including sub-nodes. Because of
this behavior, it is RECOMMENDED that clients decrypt and expand any OTR
messages inside of the body element before re-processing the element as a
whole. Clients that support OTR MUST tolerate encrypted payloads which
expand to XML, and those which expand to plain text messages.
</p>
</section2>
<section2 topic='Routing'>
<p>
XMPP is designed so that the client needs to know very little about where
and how a message will be routed. Generally, clients are encouraged to
send messages to the bare JID and allow the server to route the messages
as it sees fit. However, OTR requires that messages be sent to a
particular resource. Therefore clients SHOULD send OTR messages to a full
JID, possibly allowing the user to determine which resource they wish to
start an encrypted session with. Furthermore, if a client receives a
request to start an OTR session in a carboned message (due to a server
which does not support the aforementioned "private" directive, or a
client which does not set it), it SHOULD be silently ignored.
</p>
</section2>
<section2 topic='Processing Hints'>
<p>
&xep0334; defines a set of hints for how messages should be handled by
XMPP servers. These hints are not hard and fast rules, but suggestions
which the servers may or may not choose to follow. Best practice is to
include the following hints on all OTR messages:
<code><![CDATA[
<no-copy xmlns="urn:xmpp:hints"/>
<no-permanent-store xmlns="urn:xmpp:hints"/>
]]></code>
</p>
<p>
Similarly the "private" directive from &xep0280; should also be included
to indicate that carbons are not necessary (since no other resource will
be able to read the message):
<code><![CDATA[
<private xmlns="urn:xmpp:carbons:2"/>
]]></code>
All together, an example OTR message might look like this (with the
majority of the body stripped out for readability):
<example caption='OTR message with processing hints'><![CDATA[
<message
from='malvolio@stewardsguild.lit/countesshousehold'
to='olivia@countess.lit/veiled'>
<body>?OTR?v23?...</body>
<no-copy xmlns="urn:xmpp:hints"/>
<no-permanent-store xmlns="urn:xmpp:hints"/>
<private xmlns="urn:xmpp:carbons:2"/>
</message>
]]></example>
</p>
</section2>
</section1>
<section1 topic='Use in XMPP URIs'>
<p>
&rfc5122; defines a Uniform Resource Identifier (URI) and Internationalized
Resource Identifier (IRI) scheme for XMPP entities, and &xep0147; defines
various query components for use with XMPP URI's. When an entity has an
associated OTR fingerprint it's URI is often formed with "otr-fingerprint"
in the query string. Eg.
<example caption='OTR Fingerprint'>
xmpp:feste@allfools.lit?otr-fingerprint=AEA4D503298797D4A4FC823BC1D24524B4C54338
</example>
</p>
<p>
The &REGISTRAR; maintains a registry of queries and key-value pairs for use
in XMPP URIs at &QUERYTYPES;. As of the date this document was authored,
the 'otr-fingerprint' query string has not been formally defined and has
therefore is not officially recognized by the registrar.
</p>
</section1>
<section1 topic='Acknowledgements' anchor='acks'>
<p>
Thanks to Daniel Gultsch for his excellent
<link url='https://github.com/siacs/Conversations/blob/development/docs/observations.md'>
article
</link>
<note>
Daniel Gultsch (Retreived on 2015-07-29). "Observations on Imlementing
XMPP"
&lt;<link url='https://github.com/siacs/Conversations/blob/development/docs/observations.md'>
https://github.com/siacs/Conversations/blob/development/docs/observations.md
</link>&gt;
</note>
on the pitfalls of implementing OTR, and to Georg Lukas for his feedback.
</p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>
While this document describes an existing protocol which is streamed over
XMPP and therefore does not introduce any new security concerns itself, it
is worth mentioning a few security issues with the underlying OTR protocol:
</p>
<p>
Because Diffie-Hellman (D-H) key exchange is unauthenticated, the initial
D-H exchange which sets up the encrypted channel is vulnerable to a
man-in-the-middle attack. No sensitive information should be sent over the
encrypted channel until mutual authentication has been performed
inside the encrypted channel.
</p>
<p>
OTR makes use of the SHA-1 hash algorithm. While no practical attacks have
been observed in SHA-1 at the time of this writing, theoretical attacks
have been constructed, and attacks have been performed on hash functions
that are similar to SHA-1. One cryptographer estimated that the cost
of generating SHA-1 collisions was $2.77 million dollars in 2012, and would
drop to $700,000 by 2015.
<note>
Bruce Schneier (2012-10-05). "When Will We See Collisions for SHA-1?"
&lt;<link url='https://www.schneier.com/blog/archives/2012/10/when_will_we_se.html'>
https://www.schneier.com/blog/archives/2012/10/when_will_we_se.html
</link>&gt;
</note>.
This puts generating SHA-1 collisions well within the reach of governments
and well funded criminal organizations. In this authors opinion, there are
no theoretical vulnerabilities, and SHA-1 should be treated as with extreme
caution.
</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>
This document requires no interaction with the Internet Assigned Numbers
Authority (IANA).
</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>
No namespaces or parameters need to be registered with the XMPP Registrar
as a result of this document.
</p>
</section1>
</xep>