Network Working Group P. Saint-Andre
Internet-Draft
Updates: 6120 (if approved) January 23, 2014
Intended status: Standards Track
Expires: July 27, 2014
Use of Transport Layer Security (TLS) in the Extensible Messaging and
Presence Protocol (XMPP)
draft-saintandre-xmpp-tls-04
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
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This Internet-Draft will expire on July 27, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Discussion Venue . . . . . . . . . . . . . . . . . . . . . . 3
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Support for TLS . . . . . . . . . . . . . . . . . . . . . 3
4.2. Protocol Versions . . . . . . . . . . . . . . . . . . . . 3
4.3. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 4
4.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 6
4.5. Certificate Validation . . . . . . . . . . . . . . . . . 6
4.6. Unauthenticated Connections . . . . . . . . . . . . . . . 6
4.7. Server Name Indication . . . . . . . . . . . . . . . . . 7
4.8. Session Resumption . . . . . . . . . . . . . . . . . . . 7
4.9. Compression . . . . . . . . . . . . . . . . . . . . . . . 7
4.10. Human Factors . . . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Notes . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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].
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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 [1]. Discussion might also occur on the mailing list
of the UTA Working Group, for which archives and subscription
information can be found at [2].
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 (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 child element to indicate
that negotiation of TLS is mandatory, clients and peer servers MUST
NOT depend on receiving the 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:
o XMPP implementations MUST NOT negotiate SSL version 2.
Rationale: SSLv2 has serious security vulnerabilities [RFC6176].
o XMPP implementations MAY 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.
o XMPP implementations MAY 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.
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o 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.
o 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:
o XMPP implementations MUST NOT negotiate the NULL cipher suites.
Rationale: The NULL cipher suites offer no encryption whatsoever
and thus are completely insecure.
o 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].
o 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.
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o 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.
o 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.
o 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).
o 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:
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 cipher suites are supported only in TLS 1.2. A
future version of this document might recommend cipher suites 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 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.
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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:
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.
o Determine the cipher suite used to encrypt a connection.
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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
As noted in "A Threat Model for Pervasive Passive Surveillance"
[I-D.trammell-perpass-ppa]), 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 (sending 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, along with actions currently being taken to improve
the security of the XMPP network, do not assume widespread compromise
of XMPP servers and clients or their underlying operating systems or
hardware. Thus it is assumed that 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.
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.
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[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-04 (work in progress),
October 2013.
[I-D.popov-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-popov-
tls-prohibiting-rc4-01 (work in progress), October 2013.
[I-D.sheffer-tls-bcp]
Sheffer, Y., "Recommendations for Secure Use of TLS and
DTLS", draft-sheffer-tls-bcp-01 (work in progress),
September 2013.
[I-D.trammell-perpass-ppa]
Trammell, B., Borkmann, D., and C. Huitema, "A Threat
Model for Pervasive Passive Surveillance", draft-trammell-
perpass-ppa-01 (work in progress), November 2013.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
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[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.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 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, September 2013.
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
Email: ietf@stpeter.im
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