XMPP P. Saint-Andre
Internet-Draft Cisco
Intended status: Informational June 5, 2009
Expires: December 7, 2009
Requirements for End-to-End Encryption in the Extensible Messaging and
Presence Protocol (XMPP)
draft-saintandre-xmpp-e2e-requirements-00
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Abstract
This document describes requirements for end-to-end encryption in the
Extensible Messaging and Presence Protocol (XMPP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Requirements . . . . . . . . . . . . . . . . . . . . . 4
4. Application Requirements . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Informative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
End-to-end or "e2e" encryption of traffic sent over the Extensible
Messaging and Presence Protocol (XMPP) is a desirable goal. Since
1999, the Jabber/XMPP developer community has experimented with
several such technologies, including OpenPGP [XMPP-PGP], S/MIME
[XMPP-SMIME], and encrypted sessions [ESessions]. More recently, the
community has explored the possibility of using Transport Layer
Security [TLS] as the base technology for e2e encryption. In order
to provide a foundation for deciding on a sustainable approach to e2e
encryption, this document specifies a set of requirements that the
ideal technology would meet.
The preferred venue for discussion of this document is the
xmpp@ietf.org mailing list; visit
<https://www.ietf.org/mailman/listinfo/xmpp> for further information.
Much of the text in this document has been copied from [XEP-0210].
2. Scope
There are several different kinds of communications between XMPP
entitites:
1. One-to-one communication sessions between two entities, where
each entity is online and available during the life of the
session so that all of the communications occur in real time.
2. One-to-one messages that are not transferred in real time but
that instead are stored when sent and then forwarded when the
recipient is next online; these are usually called "offline
messages" as described in [OFFLINE].
3. One-to-many information broadcast, such as undirected presence
stanzas sent from one user to many contacts as described in
[XMPP-IM] and data syndication as described in [PUBSUB].
4. Many-to-many communication sessions among more than two entities,
such as a text conference in a chatroom as described in [MUC].
Ideally, any technology for end-to-end encryption in XMPP could be
extended to cover all the scenarios above. However, both one-to-many
broadcast and many-to-many sessions are deemed out-of-scope for this
document, and this document puts more weight on one-to-one
communication sessions (the typical scenario for XMPP) than on
offline messages.
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3. Security Requirements
This document stipulates the following security requirements for end-
to-end encryption of XMPP communications:
Confidentiality: The one-to-one XML stanzas exchanged between two
entities (conventionally, "Alice" and "Bob") must not be
understandable to any other entity that might intercept the
communications. The encrypted stanzas should be understood by an
intermediate server only to the extent required to route them.
Integrity: Alice and Bob must be sure that no other entity can
change the content of the XML stanzas they exchange, or remove or
insert stanzas undetected.
Replay Protection: Alice or Bob must be able to identify and reject
any communications that are copies of their previous
communications resent by another entity.
Perfect Forward Secrecy: The encrypted communication should not be
revealed even if long-lived keys are compromised in the future
(e.g., Steve steals Bob's computer). For long-lived sessions it
must be possible to periodically change the decryption keys.
PKI Independence: The protocol must not force the use of any public
key infrastructure (PKI), certification authority, web of trust,
or any other trust model that is external to the trust established
between Alice and Bob. However, if external authentication or
trust models are available then Alice and Bob should be able to
use such trust models to enhance any trust that exists between
them.
Authentication: Each party to a conversation must know that the
other party is who they want to communicate with (Alice must be
able to know that Bob really is Bob, and vice versa). Note:
Authentication can be as simple as Alice confirming that Bob is
the same Bob that she communicated with yesterday or that she
talked to on the telephone. The reliable association between an
entity and its public keys is "identification" and therefore
beyond the scope of this document.
Identity Protection: No other entity should be able to identify
Alice or Bob. The JabberIDs they use to route their stanzas are
unavoidably vulnerable to interception. Therefore, even if Alice
and Bob protect their identities by using different JabberIDs for
each session, it must be possible for their user agents to
authenticate them transparently, without any other entity
identifying them via an active ("man-in-the-middle") attack, or
even linking them to their previous sessions. If that is not
possible because Alice and Bob choose to authenticate using public
keys instead of retained shared secrets, then the public keys must
not be revealed to other entities using a passive attack. Bob
should also be able to choose between protecting either his public
key or Alice's public key from disclosure through an active
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attack.
Robustness: The protocol should provide more than one difficult
challenge that has to be overcome before an attack can succeed
(for example, by generating encryption keys using as many shared
secrets as possible, such as retained secrets or optional
passwords).
Upgradability: The protocol must be upgradable so that, if a
vulnerability is discovered, a new version can fix it. Alice must
tell Bob which versions of the protocol she is prepared to
support.
4. Application Requirements
In addition to the foregoing security profile, this document also
stipulates the following application-specific requirements:
Generality: The solution must be generally applicable to the full
content of any XML stanza type (<message/>, <presence/>, and
<;iq/>) sent between two entities. It is deemed acceptable if the
solution does not apply to many-to-many stanzas (e.g., groupchat
messages sent within the context of multi-user chat) or one-to-
many stanzas (e.g., presence "broadcasts" and publish-subscribe
notifications); end-to-end encryption of such stanzas might
require separate solutions.
Implementability: The only good security technology is an
implemented security technology. The solution should be one that
XMPP client developers can implement in a relatively
straightforward and interoperable fashion. Ideally the solution
would reuse existing technologies so that client developers can
also reuse existing libraries, as they already do for security
features such as Transport Layer Security [TLS] and the Simple
Authentication and Security Layer [SASL].
Usability: The requirement of usability takes implementability one
step further by stipulating that the solution should be one that
organizations can deploy and humans can use with the ease-of-use
of, say, "https:" URLs. Experience has shown that solutions
requiring a full public key infrastructure do not get widely
deployed and that solutions requiring any user action are not
widely used. If, however, Alice and/or Bob are prepared to verify
the integrity of their copies of each other's keys (thus enabling
them to discover targeted active attacks or even the mass
surveilance of a population), then the actions necessary for them
to achieve that should be minimal (requiring no more effort than a
one-time out-of-band verification of a string of up to 8
alphanumeric characters).
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Efficiency: Cryptographic operations are highly CPU intensive,
particularly public key and Diffie-Hellman operations.
Cryptographic data structures can be relatively large, especially
public keys and certificates. Network round trips can introduce
unacceptable delays, especially over high-latency wireless
connections. The solution must perform efficiently even when CPU
and network bandwidth are constrained. The number of stanzas
required for negotiation of encrypted communication should be
minimized.
Flexibility: The solution must be compatible with a variety of
existing and future cryptographic algorithms and identity
certification schemes, including [X509] and [OpenPGP]. The
protocol must also be able to evolve to correct the weaknesses
that are inevitably discovered once any cryptographic protocol is
in widespread use.
Offline messages: It should be possible to encrypt one-to-one
communications that are stored for later delivery (so-called
"offline messages") and still benefit from Perfect Forward Secrecy
(with a slightly longer period of vulnerability than if both
parties were online simultaneously). However, any vulnerabilities
introduced into the solution in order to enable such offline
communications must not make real-time communications more
vulnerable.
5. Security Considerations
Security issues are discussed throughout this document.
6. IANA Considerations
No actions by the IANA are defined by this document.
7. Informative References
[ESessions]
Paterson, I., Saint-Andre, P., and D. Smith, "Encrypted
Session Negotiation", XSF XEP 0116, May 2007.
[MUC] Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
July 2008.
[OFFLINE] Saint-Andre, P., "Best Practices for Handling Offline
Messages", XSF XEP 0160, January 2006.
[OpenPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
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Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[PUBSUB] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, September 2008.
[SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[X509] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[XEP-0210]
Paterson, I., "Requirements for Encrypted Sessions", XSF
XEP 0210, May 2007.
[XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
draft-ietf-xmpp-3921bis-00 (work in progress), June 2009.
[XMPP-PGP]
Muldowney, T., "Current Jabber OpenPGP Usage", XSF
XEP 0027, November 2006.
[XMPP-SMIME]
Saint-Andre, P., "End-to-End Signing and Object Encryption
for the Extensible Messaging and Presence Protocol
(XMPP)", RFC 3923, October 2004.
Author's Address
Peter Saint-Andre
Cisco
Email: psaintan@cisco.com
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