Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)
draft-ietf-uta-xmpp-05

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Network Working Group                                     P. Saint-Andre
Internet-Draft                                                      &yet
Updates: 6120 (if approved)                                  T. Alkemade
Intended status: Standards Track
Expires: July 27, 2015                                  January 23, 2015

 Use of Transport Layer Security (TLS) in the Extensible Messaging and
                        Presence Protocol (XMPP)
                         draft-ietf-uta-xmpp-05

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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 27, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Support for TLS . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .   3
     3.3.  Session Resumption  . . . . . . . . . . . . . . . . . . .   3
     3.4.  Authenticated Connections . . . . . . . . . . . . . . . .   3
     3.5.  Unauthenticated Connections . . . . . . . . . . . . . . .   4
     3.6.  Server Name Indication  . . . . . . . . . . . . . . . . .   4
     3.7.  Human Factors . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     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.

      NOTE: Unless explicitly noted otherwise, all of the
      recommendations specified in [I-D.ietf-uta-tls-bcp] apply to XMPP.
      In the main, this document merely provides supplementary
      information; those who implement and deploy XMPP technologies are
      expected to follow the recommendations of [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].

   The server (i.e., the XMPP receiving entity) to which a client or
   peer server (i.e., the XMPP initiating entity) connects might not
   offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns
   :xmpp-tls'/>.  Although in general this stream feature indicates that
   the server supports XMPP 1.0 and therefore supports TLS, it is
   possible that this stream feature might be stripped out by an
   attacker (see Section 2.1 of [I-D.ietf-uta-tls-attacks]).  Therefore,
   the initiating entity SHOULD proceed with the stream negotiation even
   if the receiving entity does not advertise support for TLS.
   Similarly, although a receiving entity SHOULD include the <required/>
   child element to indicate that negotiation of TLS is mandatory, an
   initiating entity MUST NOT depend on receiving the <required/> flag
   in determining whether TLS will be enforced for the stream.

3.2.  Compression

   XMPP supports an application-layer compression technology [XEP-0138].
   Although this XMPP extension might have slightly stronger security
   properties than TLS-layer compression (since it is enabled after SASL
   authentication, as described in [XEP-0170]), this document neither
   encourages nor discourages use of XMPP-layer compression.

3.3.  Session Resumption

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

3.4.  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 and
   servers MUST authenticate clients.  This document does not mandate
   that servers need to authenticate peer servers (see next section).

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   This document does not modify the recommendations in [RFC6120]
   regarding the Subject Alternative Names (or other certificate
   details) that need to be supported for authentication of XMPP
   connections using PKIX certificates.

   The Domain Name Associations (DNA) specification [I-D.ietf-xmpp-dna]
   describes a framework for XMPP server authentication methods, which
   include not only PKIX but also DNS-Based Authentication of Named
   Entities (DANE) as defined in [I-D.ietf-dane-srv] and PKIX over
   Secure HTTP (POSH) as defined in [I-D.ietf-xmpp-posh].

3.5.  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]).  Unauthenticated connections include
   connections negotiated using anonymous Diffie-Hellman algorithms or
   using self-signed certificates, among other scenarios.  In
   particular, because of current deployment challenges for
   authenticated connections between XMPP servers (see
   [I-D.ietf-xmpp-dna] and [I-D.ietf-xmpp-posh] for details), it can be
   reasonable for XMPP server implementations to accept unauthenticated
   connections when Server Dialback keys [XEP-0220] are used; although
   such keys on their own provide only weak identity verification (made
   stronger through the use of DNSSEC [RFC4033]), this at least enables
   encryption of server-to-server connections.

3.6.  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].

3.7.  Human Factors

   It is strongly encouraged 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.

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   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 (sender's 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
   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-08 (work in progress), December 2014.

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

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

   [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-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA records
              with SRV and MX records.", draft-ietf-dane-srv-08 (work in
              progress), October 2014.

   [I-D.ietf-uta-tls-attacks]
              Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Current Attacks on TLS and DTLS", draft-ietf-uta-tls-
              attacks-05 (work in progress), October 2014.

   [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-08 (work in progress),
              October 2014.

   [I-D.ietf-xmpp-posh]
              Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
              (POSH)", draft-ietf-xmpp-posh-02 (work in progress),
              October 2014.

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

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [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.

   [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

   The authors would like to thank 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

   Email: peter@andyet.com
   URI:   https://andyet.com/

   Thijs Alkemade

   Email: me@thijsalkema.de

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