Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)
draft-ietf-uta-xmpp-01

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                      &yet
Updates: 6120 (if approved)                                  T. Alkemade
Intended status: Standards Track
Expires: March 15, 2015                               September 11, 2014

 Use of Transport Layer Security (TLS) in the Extensible Messaging and
                        Presence Protocol (XMPP)
                         draft-ietf-uta-xmpp-01

Abstract

   This document provides recommendations for the use of Transport Layer
   Security (TLS) in the Extensible Messaging and Presence Protocol
   (XMPP).  This document updates RFC 6120.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Support for TLS . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .   3
     3.3.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .   3
     3.4.  Public Key Length . . . . . . . . . . . . . . . . . . . .   3
     3.5.  Compression . . . . . . . . . . . . . . . . . . . . . . .   3
     3.6.  Session Resumption  . . . . . . . . . . . . . . . . . . .   4
     3.7.  Authenticated Connections . . . . . . . . . . . . . . . .   4
     3.8.  Unauthenticated Connections . . . . . . . . . . . . . . .   4
     3.9.  Server Name Indication  . . . . . . . . . . . . . . . . .   4
     3.10. Human Factors . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Implementation Notes . . . . . . . . . . . . . . . .   7
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Extensible Messaging and Presence Protocol (XMPP) [RFC6120]
   (along with its precursor, the so-called "Jabber protocol") has used
   Transport Layer Security (TLS) [RFC5246] (along with its precursor,
   Secure Sockets Layer or SSL) since 1999.  Both [RFC6120] and its
   predecessor [RFC3920] provided recommendations regarding the use of
   TLS in XMPP.  In order to address the evolving threat model on the
   Internet today, this document provides stronger recommendations based
   on [I-D.ietf-uta-tls-bcp].  This document updates [RFC6120].

2.  Terminology

   Various security-related terms are to be understood in the sense
   defined in [RFC4949].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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3.  Recommendations

3.1.  Support for TLS

   Support for TLS (specifically, the XMPP profile of STARTTLS) is
   mandatory for XMPP implementations, as already specified in [RFC6120]
   and its predecessor [RFC3920].

   If the server to which a client or peer server connects does not
   offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns
   :xmpp-tls'/> (thus indicating that it is an XMPP 1.0 server that
   supports TLS), the initiating entity MUST NOT proceed with the stream
   negotiation and MUST instead abort the connection attempt.  Although
   XMPP servers SHOULD include the <required/> child element to indicate
   that negotiation of TLS is mandatory, clients and peer servers MUST
   NOT depend on receiving the <required/> flag in determining whether
   TLS will be enforced for the stream.

3.2.  Protocol Versions

   Implementations MUST follow the recommendations in
   [I-D.ietf-uta-tls-bcp] as to supporting various TLS versions and
   avoiding fallback to SSL.

3.3.  Cipher Suites

   Implementations MUST follow the recommendations in
   [I-D.ietf-uta-tls-bcp].

3.4.  Public Key Length

   Implementations MUST follow the recommendations in
   [I-D.ietf-uta-tls-bcp].

3.5.  Compression

   Implementations MUST follow the recommendations in
   [I-D.ietf-uta-tls-bcp].

   XMPP supports an application-layer compression technology [XEP-0138],
   which might have slightly stronger security properties than TLS (at
   least because it is enabled after SASL authentication, as described
   in [XEP-0170]).

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3.6.  Session Resumption

   Implementations MUST follow the recommendations in
   [I-D.ietf-uta-tls-bcp].

   Use of session IDs [RFC5246] is RECOMMENDED instead of session
   tickets [RFC5077], since XMPP does not in general use state
   management technologies such as tickets or "cookies" [RFC6265].

   In XMPP, TLS session resumption can be used in concert with the XMPP
   Stream Management extension; see [XEP-0198] for further details.

3.7.  Authenticated Connections

   Both the core XMPP specification [RFC6120] and the "CertID"
   specification [RFC6125] provide recommendations and requirements for
   certificate validation in the context of authenticated connections.
   This document does not supersede those specifications.  Wherever
   possible, it is best to prefer authenticated connections (along with
   SASL [RFC4422]), as already stated in the core XMPP specification
   [RFC6120].  In particular, clients MUST authenticate servers.

3.8.  Unauthenticated Connections

   Given the pervasiveness of passive eavesdropping, even an
   unauthenticated connection might be better than an unencrypted
   connection (this is similar to the "better than nothing security"
   approach for IPsec [RFC5386]).  In particular, because of current
   deployment challenges for authenticated connections between XMPP
   servers (see [I-D.ietf-xmpp-dna] for details), it might be reasonable
   for XMPP server implementations to accept unauthenticated connections
   when the Server Dialback protocol [XEP-0220] is used for weak
   identity verification; this will at least enable encryption of
   server-to-server connections.  Unauthenticated connections include
   connections negotiated using anonymous Diffie-Hellman algorithms or
   using self-signed certificates, among other scenarios.

3.9.  Server Name Indication

   Although there is no harm in supporting the TLS Server Name
   Indication (SNI) extension [RFC6066], this is not necessary since the
   same function is served in XMPP by the 'to' address of the initial
   stream header as explained in Section 4.7.2 of [RFC6120].

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3.10.  Human Factors

   It is RECOMMENDED that XMPP clients provide ways for end users (and
   that XMPP servers provide ways for administrators) to complete the
   following tasks:

   o  Determine if a client-to-server or server-to-server connection is
      encrypted and authenticated.

   o  Determine the version of TLS used for a client-to-server or
      server-to-server connection.

   o  Inspect the certificate offered by an XMPP server.

   o  Determine the cipher suite used to encrypt a connection.

   o  Be warned if the certificate changes for a given server.

4.  IANA Considerations

   This document requests no actions of the IANA.

5.  Security Considerations

   The use of TLS can help limit the information available for
   correlation to the network and transport layer headers as opposed to
   the application layer.  As typically deployed, XMPP technologies do
   not leave application-layer routing data (such as XMPP 'to' and
   'from' addresses) at rest on intermediate systems, since there is
   only one hop between any two given XMPP servers.  As a result,
   encrypting all hops (sending client to sender's server, sender's
   server to recipient's server, recipient's server to recipient's
   client) can help to limit the amount of "metadata" that might leak.

   It is possible that XMPP servers themselves might be compromised.  In
   that case, per-hop encryption would not protect XMPP communications,
   and even end-to-end encryption of (parts of) XMPP stanza payloads
   would leave addressing information and XMPP roster data in the clear.
   By the same token, it is possible that XMPP clients (or the end-user
   devices on which such clients are installed) could also be
   compromised, leaving users utterly at the mercy of an adversary.

   This document and related actions to strengthen the security of the
   XMPP network are based on the assumption that XMPP servers and
   clients have not been subject to widespread compromise.  If this
   assumption is valid, then ubiquitous use of per-hop TLS channel
   encryption and more significant deployment of end-to-end object

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   encryption technologies will serve to protect XMPP communications to
   a measurable degree, compared to the alternatives.

6.  References

6.1.  Normative References

   [I-D.ietf-uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-02 (work in progress), August 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

6.2.  Informative References

   [I-D.ietf-xmpp-dna]
              Saint-Andre, P. and M. Miller, "Domain Name Associations
              (DNA) in the Extensible Messaging and Presence Protocol
              (XMPP)", draft-ietf-xmpp-dna-06 (work in progress), June
              2014.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

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   [RFC5386]  Williams, N. and M. Richardson, "Better-Than-Nothing
              Security: An Unauthenticated Mode of IPsec", RFC 5386,
              November 2008.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [XEP-0138]
              Hildebrand, J. and P. Saint-Andre, "Stream Compression",
              XSF XEP 0138, May 2009.

   [XEP-0170]
              Saint-Andre, P., "Recommended Order of Stream Feature
              Negotiation", XSF XEP 0170, January 2007.

   [XEP-0198]
              Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F.,
              Cridland, D., and M. Wild, "Stream Management", XSF XEP
              0198, June 2011.

   [XEP-0220]
              Miller, J., Saint-Andre, P., and P. Hancke, "Server
              Dialback", XSF XEP 0220, September 2013.

Appendix A.  Implementation Notes

   Some governments enforce legislation prohibiting the export of strong
   cryptographic technologies.  Nothing in this document ought to be
   taken as advice to violate such prohibitions.

Appendix B.  Acknowledgements

   Thanks to the following individuals for their input: Dave Cridland,
   Philipp Hancke, Olle Johansson, Steve Kille, Tobias Markmann, Matt
   Miller, and Rene Treffer.

Authors' Addresses

   Peter Saint-Andre
   &yet
   P.O. Box 787
   Parker, CO  80134
   USA

   Email: peter@andyet.net

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   Thijs Alkemade

   Email: me@thijsalkema.de

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