Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)
draft-saintandre-xmpp-tls-03

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.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on June 20, 2014.

Copyright Notice

Copyright (c) 2013 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.

Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Discussion Venue
• 4. Recommendations
• 4.1. Support for TLS
• 4.2. Protocol Versions
• 4.3. Cipher Suites
• 4.4. Public Key Length
• 4.5. Certificate Validation
• 4.6. Unauthenticated Connections
• 4.7. Server Name Indication
• 4.8. Session Resumption
• 4.9. Compression
• 4.10. Human Factors
• 5. Implementation Notes
• 6. IANA Considerations
• 7. Security Considerations
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Acknowledgements
• Author's Address

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 (see, for example, [I-D.trammell-perpass-ppa]), this document provides 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. Discussion might also occur on the mailing list of the UTA Working Group, for which archives and subscription information can be found at https://www.ietf.org/mailman/listinfo/uta.

4. Recommendations

4.1. 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 an XMPP 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.

4.2. Protocol Versions

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

• XMPP implementations MUST NOT negotiate SSL version 2.
  
  Rationale: SSLv2 has serious security vulnerabilities [RFC6176].
  
  
• XMPP implementations SHOULD NOT negotiate SSL version 3.
  
  Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and plugged some significant security holes, but did not support strong cipher suites.
  
  
• XMPP implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246].
  
  Rationale: TLS 1.0 (published in 1999) includes a way to downgrade the connection to SSLv3 and does not support more modern, strong cipher suites.
  
  
• XMPP implementations MAY negotiate TLS version 1.1 [RFC4346].
  
  Rationale: TLS 1.1 (published in 2006) prevents downgrade attacks to SSL, but does not support certain stronger cipher suites.
  
  
• XMPP implementations MUST support, and prefer to negotiate, TLS version 1.2 [RFC5246].
  
  Rationale: Several stronger cipher suites are available only with TLS 1.2 (published in 2008).

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. Cipher Suites

NOTE: Currently this document provides its own recommendations regarding TLS cipher suites. However, eventually it will be updated to instead reference [I-D.sheffer-tls-bcp].

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

• XMPP implementations MUST NOT negotiate the NULL cipher suites.
  
  Rationale: The NULL cipher suites offer no encryption whatsoever and thus are completely insecure.
  
  
• XMPP implementations MUST NOT negotiate RC4 cipher suites
  
  Rationale: The RC4 stream cipher has a variety of cryptographic weaknesses, as documented in [I-D.popov-tls-prohibiting-rc4].
  
  
• XMPP implementations MUST NOT negotiate cipher suites offering only so-called "export-level" encryption (including algorithms with 40 bits or 56 bits of security).
  
  Rationale: These cipher suites are deliberately "dumbed down" and are very easy to break.
  
  
• XMPP implementations MUST NOT negotiate cipher suites that use algorithms offering less than 128 bits of security (even if they advertise more bits, such as the 168-bit 3DES cipher suites).
  
  Rationale: Although these cipher suites are not actively subject to breakage, their useful life is short enough that stronger cipher suites are desirable.
  
  
• XMPP implementations SHOULD prefer cipher suites that use algorithms with at least 256 bits of security.
  
  Rationale: The useful life of such cipher suites is probably at least 3-5 years.
  
  
• XMPP implementations MUST support, and SHOULD prefer to negotiate, cipher suites offering authentication, such as the "AES-GCM" family.
  
  Rationale: Authenticated connections are better than unauthenticated connections (although, as explained under Section 4.6, unauthenticated connections are better than nothing).
  
  
• XMPP implementations MUST support, and SHOULD prefer to negotiate, cipher suites offering forward secrecy, such as those in the "EDH", "DHE", and "ECDHE" families.
  
  Rationale: Forward secrecy (sometimes called "perfect forward secrecy") prevents the recovery of information that was encrypted with older session keys, thus limiting the amount of time during which attacks can be successful.

Given the foregoing considerations, implementation of the following cipher suites is RECOMMENDED:

• TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
• TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
• TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384

Unfortunately, those cipher suites are supported only in TLS 1.2. A future version of this document might recommend cipher suites 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 cipher suites. 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 cipher suites 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 10 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] 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 (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.

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, the resumption information (either session IDs [RFC5246] or session tickets [RFC5077]) needs to be authenticated and encrypted to prevent modification or eavesdropping by an attacker.

Use of session IDs [RFC5246] is RECOMMENDED instead of session tickets [RFC5077], since session tickets mandate a relatively small key size and a relatively weak cipher suite (AES_128_CBC_SHA256) that does not support forward secrecy.

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, such as "cookies" [RFC6265]. However, because XMPP also supports an application-layer compression technology [XEP-0138], implementers might wish to prefer XMPP compression over TLS compression in order to avoid any potential security issues with TLS-layer compression. (See [I-D.sheffer-tls-bcp] for related discussion.)

4.10. Human Factors

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

• Determine if a client-to-server or server-to-server connection is encrypted and authenticated.
• Determine the version of TLS used for a client-to-server or server-to-server connection.
• Inspect the certificate offered by an XMPP server.
• Determine the cipher suite used to encrypt a connection.
• Be warned if the certificate changes for a given server.

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

8.2. Informative References

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