Network Working Group P. Saint-Andre
Internet-Draft Cisco Systems, Inc.
Updates: 6120 (if approved) September 12, 2013
Intended status: Informational
Expires: March 16, 2014

Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)
draft-saintandre-xmpp-tls-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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on March 16, 2014.

Copyright Notice

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Table of Contents

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. Given the evolving threat model on the Internet today (see, for example, [I-D.trammell-perpass-ppa]), it is necessary to provide stronger recommendations (see also [I-D.sheffer-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].

3. Discussion Venue

The discussion venue for this document is the mailing list of the XMPP Working Group, for which archives and subscription information can be found at https://www.ietf.org/mailman/listinfo/xmpp.

4. Recommendations

4.1. Support for TLS

Support for TLS (specifically, the XMPP profile of STARTTLS) is mandatory for XMPP implementations. If the server to which an XMPP client or peer server connects does not offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'> as described in [RFC6120], the initiating entity MUST NOT proceed with the stream negotiation and MUST instead abort the connection attempt.

4.2. Protocol Versions

It is important both to stop using old, insecure versions of SSL/TLS and to start using modern, more secure versions. Therefore:

As of the date of this writing, the latest version of TLS is 1.2. When TLS is updated to a newer version, this document will be updated to recommend support for the latest version. If this document is not updated in a timely manner, it can be assumed that support for the latest version of TLS is recommended.

4.3. Ciphersuites

It is important both to stop using old, insecure ciphersuites and to start using modern, more secure ciphersuites. Therefore:

Implementation of the following ciphersuites is RECOMMENDED:

Unfortunately, those ciphersuites are supported only in TLS 1.2. A future version of this document might recommend ciphersuites for earlier versions of TLS.

4.4. Public Key Length

Because Diffie-Hellman keys of 1024 bits are estimated to be roughly equivalent to 80-bit symmetric keys, it is better to use longer keys for the "DH" family of ciphersuites. Unfortunately, some existing software cannot handle (or cannot easily handle) key lengths greater than 1024 bits. The most common workaround for these systems is to prefer the "ECDHE" family of ciphersuites instead of the "DH" family, then use longer keys. Key lengths of at least 2048 bits are RECOMMENDED, since they are estimated to be roughly equivalent to 112-bit symmetric keys and might be sufficient for at least the next ten years..

Note: The foregoing recommendations are preliminary and will likely be corrected and enhanced in a future version of this document.

4.5. Certificate Validation

Both the core XMPP specification [RFC6120] and the "CertID" specification [RFC6125] provide recommendations and requirements for certificate checking. This document does not supersede those specifications.

4.6. Unauthenticated Connections

The core XMPP specification [RFC6120] states a preference for the use of TLS for encryption along with SASL [RFC4422] (specifically the SASL EXTERNAL mechanism) for authentication. In general, it is preferable for a connection to be authenticated, including proper identity checking as defined by the "CertID" specification [RFC6125]. However, 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, given current deployment challenges for authenticated connections between XMPP servers [I-D.ietf-xmpp-dna], it might be reasonable for XMPP 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.

4.7. 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].

4.8. Session Resumption

If TLS session resumption is used (e.g., in concert with the XMPP Stream Management extension [XEP-0198]), care ought to be taken to do so safely. In particular, tickets must be authenticated and encrypted to prevent modification or eavesdropping by an attacker. Details can be found in [RFC5077].

4.9. Compression

XMPP is not generally subject to attacks based on TLS-layer compression (e.g., the "CRIME" attack), since it is not typically used to communicate static strings of the kind communicated over HTTP (e.g., "cookies"). However, because XMPP also supports an application-layer compression technology [XEP-0138], implementers might wish to prefer native XMPP compression over TLS compression.

4.10. Human Factors

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

5. 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.

6. IANA Considerations

This document requests no actions of the IANA.

7. Security Considerations

This entire document discusses security.

8. References

8.1. Normative References

[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.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, March 2011.

8.2. 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)", Internet-Draft draft-ietf-xmpp-dna-03, September 2013.
[I-D.popov-tls-prohibiting-rc4] Popov, A., "Prohibiting RC4 Cipher Suites", Internet-Draft draft-popov-tls-prohibiting-rc4-00, August 2013.
[I-D.sheffer-tls-bcp] Sheffer, Y., "Recommendations for Secure Use of TLS and DTLS", Internet-Draft draft-sheffer-tls-bcp-00, September 2013.
[I-D.trammell-perpass-ppa] Trammell, B., "The Perfect Passive Adversary: A Threat Model for the Evaluation of Protocols under Pervasive Surveillance", Internet-Draft draft-trammell-perpass-ppa-00, September 2013.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.
[RFC3920] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 3920, October 2004.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006.
[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-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, August 2012.

Appendix A. Acknowledgements

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

Author's Address

Peter Saint-Andre Cisco Systems, Inc. 1899 Wynkoop Street, Suite 600 Denver, CO 80202 USA Phone: +1-303-308-3282 EMail: psaintan@cisco.com