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rfc:rfc7590

Internet Engineering Task Force (IETF) P. Saint-Andre Request for Comments: 7590 &yet Updates: 6120 T. Alkemade Category: Standards Track June 2015 ISSN: 2070-1721

            Use of Transport Layer Security (TLS) in the
         Extensible Messaging and Presence Protocol (XMPP)

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

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7590.

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Saint-Andre & Alkemade Standards Track [Page 1] RFC 7590 XMPP TLS June 2015

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 . . . . . . . . . . . . . . . .   4
   3.5.  Server Name Indication  . . . . . . . . . . . . . . . . .   5
   3.6.  Human Factors . . . . . . . . . . . . . . . . . . . . . .   5
 4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
 5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
   5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
 Appendix A.  Implementation Notes . . . . . . . . . . . . . . . .   9
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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.
 In particular, this document updates [RFC6120] by specifying that
 XMPP implementations and deployments MUST follow the best current
 practices documented in the "Recommendations for Secure Use of TLS
 and DTLS" [RFC7525].  This includes stronger recommendations
 regarding SSL/TLS protocol versions, fallback to lower versions,
 TLS-layer compression, TLS session resumption, cipher suites, public
 key lengths, forward secrecy, and other aspects of using TLS with
 XMPP.

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].

Saint-Andre & Alkemade Standards Track [Page 2] RFC 7590 XMPP TLS June 2015

3. Recommendations

 The best current practices documented in the "Recommendations for
 Secure Use of TLS and DTLS" [RFC7525] are included here by reference.
 Instead of repeating those recommendations here, this document mostly
 provides supplementary information regarding secure implementation
 and deployment of XMPP technologies.

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 and offers TLS, this stream feature might be
 stripped out by an attacker (see Section 2.1 of [RFC7457]).
 Similarly, the <required/> child element of the <starttls/> stream
 feature is used to indicate that negotiation of TLS is mandatory;
 however, this could also be stripped out by an attacker.  Therefore,
 the initiating entity MUST NOT be deterred from attempting TLS
 negotiation even if the receiving entity does not advertise support
 for TLS.  Instead, the initiating entity SHOULD (based on local
 policy) proceed with the stream negotiation and attempt to negotiate
 TLS.

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
 Simple Authentication and Security Layer (SASL) authentication, as
 described in [XEP-0170]), this document neither encourages nor
 discourages use of XMPP-layer compression.

3.3. Session Resumption

 To improve the reliability of communications over XMPP, it is common
 practice for clients and servers to implement the stream management
 extension [XEP-0198].  Although that specification includes a method
 for resumption of XMPP streams at the application layer, also using
 session resumption at the TLS layer further optimizes the overall
 process of resuming an XMPP session (see [XEP-0198] for detailed
 information).  Whether or not XEP-0198 is used for application-layer

Saint-Andre & Alkemade Standards Track [Page 3] RFC 7590 XMPP TLS June 2015

 session resumption, implementations MUST follow the recommendations
 provided in [RFC7525] regarding TLS-layer session resumption.

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 (e.g., it 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).
 Wherever possible, it is best to prefer authenticated connections
 (along with SASL [RFC4422]), as already stated in the core XMPP
 specification [RFC6120].  In particular:
 o  Clients MUST authenticate servers.
 o  Servers MUST authenticate clients.
 o  Servers SHOULD authenticate other servers.
 This document does not mandate that servers need to authenticate peer
 servers, although such authentication is strongly preferred.
 Unfortunately, in multi-tenanted environments it can be extremely
 difficult to obtain and deploy PKIX certificates with the proper
 Subject Alternative Names (see [XMPP-DNA] and [PKIX-POSH] for
 details).  To overcome that difficulty, the Domain Name Associations
 (DNAs) specification [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
 [DANE-SRV] and PKIX over Secure HTTP (POSH) as defined in
 [PKIX-POSH].  These methods can provide a basis for server identity
 verification when appropriate PKIX certificates cannot be obtained
 and deployed.
 Given the pervasiveness of eavesdropping [RFC7258], even an encrypted
 but unauthenticated connection might be better than an unencrypted
 connection in these scenarios (this is similar to the "better-than-
 nothing security" approach for IPsec [RFC5386]).  Encrypted but
 unauthenticated connections include connections negotiated using
 anonymous Diffie-Hellman mechanisms or using self-signed
 certificates, among others.  In particular for XMPP server-to-server
 interactions, it can be reasonable for XMPP server implementations to
 accept encrypted but unauthenticated connections when Server Dialback
 keys [XEP-0220] are used; such keys on their own provide only weak

Saint-Andre & Alkemade Standards Track [Page 4] RFC 7590 XMPP TLS June 2015

 identity verification (made stronger through the use of DNSSEC
 [RFC4033]), but this at least enables encryption of server-to-server
 connections.  The DNA prooftypes mentioned above are intended to
 mitigate the residual need for encrypted but unauthenticated
 connections in these scenarios.

3.5. 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].

3.6. 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 given incoming or outgoing XML stream is encrypted
    using TLS.
 o  Determine the version of TLS used for encryption of a given
    stream.
 o  If authenticated encryption is used, determine how the connection
    was authenticated or verified (e.g., via PKI, DANE, POSH, or
    Server Dialback).
 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. Security Considerations

 The use of TLS can help to limit the information available for
 correlation between the XMPP application layer and the underlying
 network and transport layers.  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, and recipient's server to
 recipient's client) can help to limit the amount of metadata that
 might leak.

Saint-Andre & Alkemade Standards Track [Page 5] RFC 7590 XMPP TLS June 2015

 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.
 This document covers only communication over the XMPP network and
 does not take into account gateways to non-XMPP networks.  As an
 example, for security considerations related to gateways between XMPP
 and the Session Initiation Protocol (SIP), see [RFC7247] and
 [RFC7572].

5. References

5.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <http://www.rfc-editor.org/info/rfc2119>.
 [RFC4949]   Shirey, R., "Internet Security Glossary, Version 2",
             FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
             <http://www.rfc-editor.org/info/rfc4949>.
 [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246,
             DOI 10.17487/RFC5246, August 2008,
             <http://www.rfc-editor.org/info/rfc5246>.
 [RFC6120]   Saint-Andre, P., "Extensible Messaging and Presence
             Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
             March 2011, <http://www.rfc-editor.org/info/rfc6120>.

Saint-Andre & Alkemade Standards Track [Page 6] RFC 7590 XMPP TLS June 2015

 [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, DOI 10.17487/RFC6125, March
             2011, <http://www.rfc-editor.org/info/rfc6125>.
 [RFC7525]   Sheffer, Y., Holz, R., and P. Saint-Andre,
             "Recommendations for Secure Use of Transport Layer
             Security (TLS) and Datagram Transport Layer Security
             (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
             2015, <http://www.rfc-editor.org/info/rfc7525>.

5.2. Informative References

 [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.", Work in Progress,
             draft-ietf-dane-srv-14, April 2015.
 [PKIX-POSH] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
             (POSH)", Work in Progress, draft-ietf-xmpp-posh-04,
             February 2015.
 [RFC3920]   Saint-Andre, P., Ed., "Extensible Messaging and Presence
             Protocol (XMPP): Core", RFC 3920, DOI 10.17487/RFC3920,
             October 2004, <http://www.rfc-editor.org/info/rfc3920>.
 [RFC4033]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
             Rose, "DNS Security Introduction and Requirements",
             RFC 4033, DOI 10.17487/RFC4033, March 2005,
             <http://www.rfc-editor.org/info/rfc4033>.
 [RFC4422]   Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
             Authentication and Security Layer (SASL)", RFC 4422,
             DOI 10.17487/RFC4422, June 2006,
             <http://www.rfc-editor.org/info/rfc4422>.
 [RFC5386]   Williams, N. and M. Richardson, "Better-Than-Nothing
             Security: An Unauthenticated Mode of IPsec", RFC 5386,
             DOI 10.17487/RFC5386, November 2008,
             <http://www.rfc-editor.org/info/rfc5386>.
 [RFC6066]   Eastlake 3rd, D., "Transport Layer Security (TLS)
             Extensions: Extension Definitions", RFC 6066,
             DOI 10.17487/RFC6066, January 2011,
             <http://www.rfc-editor.org/info/rfc6066>.

Saint-Andre & Alkemade Standards Track [Page 7] RFC 7590 XMPP TLS June 2015

 [RFC7247]   Saint-Andre, P., Houri, A., and J. Hildebrand,
             "Interworking between the Session Initiation Protocol
             (SIP) and the Extensible Messaging and Presence Protocol
             (XMPP): Architecture, Addresses, and Error Handling",
             RFC 7247, DOI 10.17487/RFC7247, May 2014,
             <http://www.rfc-editor.org/info/rfc7247>.
 [RFC7258]   Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is
             an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
             2014, <http://www.rfc-editor.org/info/rfc7258>.
 [RFC7457]   Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
             Known Attacks on Transport Layer Security (TLS) and
             Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
             February 2015, <http://www.rfc-editor.org/info/rfc7457>.
 [RFC7572]   Saint-Andre, P., Houri, A., and J. Hildebrand,
             "Interworking between the Session Initiation Protocol
             (SIP) and the Extensible Messaging and Presence Protocol
             (XMPP): Instant Messaging", RFC 7572,
             DOI 10.17487/RFC7572, June 2015,
             <http://www.rfc-editor.org/info/rfc7572>.
 [XEP-0138]  Hildebrand, J. and P. Saint-Andre, "Stream Compression",
             XSF XEP 0138, May 2009,
             <http://xmpp.org/extensions/xep-0138.html>.
 [XEP-0170]  Saint-Andre, P., "Recommended Order of Stream Feature
             Negotiation", XSF XEP 0170, January 2007,
             <http://xmpp.org/extensions/xep-0170.html>.
 [XEP-0198]  Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F.,
             Cridland, D., and M. Wild, "Stream Management", XSF XEP
             0198, June 2011,
             <http://xmpp.org/extensions/xep-0198.html>.
 [XEP-0220]  Miller, J., Saint-Andre, P., and P. Hancke, "Server
             Dialback", XSF XEP 0220, August 2014,
             <http://xmpp.org/extensions/xep-0220.html>.
 [XMPP-DNA]  Saint-Andre, P. and M. Miller, "Domain Name Associations
             (DNA) in the Extensible Messaging and Presence Protocol
             (XMPP)", Work in Progress, draft-ietf-xmpp-dna-10, March
             2015.

Saint-Andre & Alkemade Standards Track [Page 8] RFC 7590 XMPP TLS June 2015

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.

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.
 Roni Even caught several important issues in his review on behalf of
 the General Area Review Team.
 Ben Campbell, Spencer Dawkins, and Barry Leiba provided helpful input
 during IESG review.
 Thanks to Leif Johansson and Orit Levin as chairs of the UTA WG, Ben
 Campbell and Joe Hildebrand as chairs of the XMPP WG, and Stephen
 Farrell as the sponsoring Area Director.

Authors' Addresses

 Peter Saint-Andre
 &yet
 EMail: peter@andyet.com
 URI:   https://andyet.com/
 Thijs Alkemade
 EMail: me@thijsalkema.de

Saint-Andre & Alkemade Standards Track [Page 9]

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