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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Discussion Venue . . . . . . . . . . . . . . . . . . . . . . . 3
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Support for TLS . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Protocol Versions . . . . . . . . . . . . . . . . . . . . . 3
4.3. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . . 4
4.4. Public Key Length . . . . . . . . . . . . . . . . . . . . . 6
4.5. Certificate Validation . . . . . . . . . . . . . . . . . . 6
4.6. Unauthenticated Connections . . . . . . . . . . . . . . . . 6
4.7. Server Name Indication . . . . . . . . . . . . . . . . . . 6
4.8. Session Resumption . . . . . . . . . . . . . . . . . . . . 7
4.9. Compression . . . . . . . . . . . . . . . . . . . . . . . . 7
4.10. Human Factors . . . . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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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:
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o XMPP implementations MUST NOT negotiate SSL version 2.
Rationale: SSLv2 has serious security vulnerabilities [RFC6176].
o XMPP implementations MUST NOT negotiate SSL version 3.
Rationale: If SSLv3 is offered, downgrade attacks are made easier.
o XMPP implementations MAY negotiate TLS version 1.0 [RFC2246].
Rationale: TLS 1.0 prevents downgrade attacks to SSL, but does not
support certain stronger ciphersuites.
o XMPP implementations MAY negotiate TLS version 1.1 [RFC4346].
Rationale: TLS 1.1 prevents downgrade attacks to SSL, but does not
support certain stronger ciphersuites.
o XMPP implementations MUST support, and prefer to negotiate, TLS
version 1.2 [RFC5246].
Rationale: Several stronger ciphersuites are available only with
TLS 1.2.
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:
o XMPP implementations MUST NOT negotiate the NULL ciphersuites.
Rationale: The NULL ciphersuites offer no encryption whatsoever
and thus are completely insecure.
o XMPP implementations MUST NOT negotiate RC4 ciphersuites
Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, documented in [I-D.popov-tls-prohibiting-rc4].
o XMPP implementations MUST NOT negotiate ciphersuites that use so-
called "export-level" encryption (including algorithms with 40
bits or 56 bits of security).
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Rationale: These ciphersuites are deliberately "dumbed down" and
are very easy to break.
o XMPP implementations MUST NOT negotiate ciphersuites that use
algorithms that offer less than 128 bits of security (even if they
advertise more bits, such as the 168-bit 3DES ciphersuites).
Rationale: Although these ciphersuites are not actively subject to
breakage, their useful life is short enough that stronger
ciphersuites are desirable.
o XMPP implementations SHOULD prefer ciphersuites that use
algorithms with at least 256 bits of security.
Rationale: The useful life of such ciphersuites is probably at
least 3-5 years.
o XMPP implementations MUST support, and SHOULD prefer to negotiate,
ciphersuites that offer 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).
o XMPP implementations MUST support, and SHOULD prefer to negotiate,
ciphersuites that offer 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 keys, thus limiting the amount of time during which
attack can be successful.
Implementation of the following ciphersuites is RECOMMENDED:
o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
Unfortunately, those ciphersuites are supported only in TLS 1.2. A
future version of this document might recommend ciphersuites for
earlier versions of TLS.
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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
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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:
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 ciphersuite used to encrypt a connection.
o 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.
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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)", draft-ietf-xmpp-dna-03 (work in progress),
September 2013.
[I-D.popov-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites",
draft-popov-tls-prohibiting-rc4-00 (work in progress),
August 2013.
[I-D.sheffer-tls-bcp]
Sheffer, Y., "Recommendations for Secure Use of TLS and
DTLS", draft-sheffer-tls-bcp-00 (work in progress),
September 2013.
[I-D.trammell-perpass-ppa]
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Trammell, B., "The Perfect Passive Adversary: A Threat
Model for the Evaluation of Protocols under Pervasive
Surveillance", draft-trammell-perpass-ppa-00 (work in
progress), September 2013.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC3920] Saint-Andre, P., Ed., "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.
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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
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