Internet Engineering Task Force (IETF) J. Altman
Request for Comments: 5929 Secure Endpoints
Category: Standards Track N. Williams
ISSN: 2070-1721 Oracle
L. Zhu
Microsoft Corporation
July 2010
Channel Bindings for TLS
Abstract
This document defines three channel binding types for Transport Layer
Security (TLS), tls-unique, tls-server-end-point, and tls-unique-for-
telnet, in accordance with RFC 5056 (On Channel Binding).
Note that based on implementation experience, this document changes
the original definition of 'tls-unique' channel binding type in the
channel binding type IANA registry.
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/rfc5929.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
Altman, et al. Standards Track [Page 1]
RFC 5929 TLS Channel Bindings July 2010
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Table of Contents
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................3
3. The 'tls-unique' Channel Binding Type ...........................3
3.1. Description ................................................3
3.2. Registration ...............................................4
4. The 'tls-server-end-point' Channel Binding Type .................5
4.1. Description ................................................5
4.2. Registration ...............................................6
5. The 'tls-unique-for-telnet' Channel Binding Type ................6
5.1. Description ................................................7
5.2. Registration ...............................................7
6. Applicability of TLS Channel Binding Types ......................7
7. Required Application Programming Interfaces ....................10
8. Description of Backwards-Incompatible Changes Made
Herein to 'tls-unique' .........................................10
9. IANA Considerations ............................................11
10. Security Considerations .......................................11
10.1. Cryptographic Algorithm Agility ..........................12
10.2. On Disclosure of Channel Bindings Data by
Authentication Mechanisms ................................12
11. References ....................................................13
11.1. Normative References .....................................13
11.2. Informative References ...................................14
Altman, et al. Standards Track [Page 2]
RFC 5929 TLS Channel Bindings July 2010
1. Introduction
Subsequent to the publication of "On Channel Bindings" [RFC5056],
three channel binding types for Transport Layer Security (TLS) were
proposed, reviewed, and added to the IANA channel binding type
registry, all in accordance with [RFC5056]. Those channel binding
types are: 'tls-unique', 'tls-server-end-point', and 'tls-unique-for-
telnet'. It has become desirable to have these channel binding types
re-registered through an RFC so as to make it easier to reference
them, and to correct them to describe actual implementations. This
document does just that. The authors of those three channel binding
types have transferred, or have indicated that they will transfer,
"ownership" of those channel binding types to the IESG.
We also provide some advice on the applicability of these channel
binding types, as well as advice on when to use which. Additionally,
we provide an abstract API that TLS implementors should provide, by
which to obtain channel bindings data for a TLS connection.
WARNING: it turns out that the first implementor implemented and
deployed something rather different than what was described in the
IANA registration for 'tls-unique'. Subsequently, it was decided
that we should adopt that form of 'tls-unique'. This means that this
document makes a backwards-incompatible change to 'tls-unique'. See
Section 8 for more details.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The 'tls-unique' Channel Binding Type
IANA updated the registration of the 'tls-unique' channel binding
type to match the description below. There are material and
substantial changes from the original registration, both in the
description as well as registration meta-data (such as registration
ownership).
3.1. Description
Description: The first TLS Finished message sent (note: the Finished
struct, not the TLS record layer message containing it) in the most
recent TLS handshake of the TLS connection being bound to (note: TLS
connection, not session, so that the channel binding is specific to
each connection regardless of whether session resumption is used).
If TLS renegotiation takes place before the channel binding
Altman, et al. Standards Track [Page 3]
RFC 5929 TLS Channel Bindings July 2010
operation, then the first TLS Finished message sent of the latest/
inner-most TLS connection is used. Note that for full TLS
handshakes, the first Finished message is sent by the client, while
for abbreviated TLS handshakes (session resumption), the first
Finished message is sent by the server.
WARNING: The definition, security, and interoperability
considerations of this channel binding type have changed since the
original registration. Implementors should read the document that
last updated this registration for more information.
Interoperability note:
This definition of 'tls-unique' means that a channel's bindings
data may change over time, which in turn creates a synchronization
problem should the channel's bindings data change between the time
that the client initiates authentication with channel binding and
the time that the server begins to process the client's first
authentication message. If that happens, the authentication
attempt will fail spuriously.
Based on the fact that while servers may request TLS
renegotiation, only clients may initiate it, this synchronization
problem can be avoided by clients and servers as follows: server
applications MUST NOT request TLS renegotiation during phases of
the application protocol during which application-layer
authentication occurs. Client applications SHOULD NOT initiate
TLS renegotiation between the start and completion of
authentication.
The rationale for making the server behavior a requirement while
the client behavior is only a recommendation is that there
typically exist TLS APIs for requesting renegotiation on the
server side of a TLS connection, while many client TLS stacks do
not provide fine-grained control over when TLS renegotiation
occurs.
Application protocols SHOULD be designed in such a way that a
server would never need to request TLS renegotiation immediately
before or during application-layer authentication.
3.2. Registration
o Channel binding unique prefix: tls-unique
o Channel binding type: unique
o Channel type: TLS [RFC5246]
Altman, et al. Standards Track [Page 4]
RFC 5929 TLS Channel Bindings July 2010
o Published specification:
o Channel binding is secret: no
o Description:
o Intended usage: COMMON
o Person and email address to contact for further information: Larry
Zhu (larry.zhu@microsoft.com), Nicolas Williams
(Nicolas.Williams@oracle.com).
o Owner/Change controller name and email address: IESG.
o Expert reviewer name and contact information: IETF TLS WG
(tls@ietf.org, failing that, ietf@ietf.org)
o Note: see the published specification for advice on the
applicability of this channel binding type.
4. The 'tls-server-end-point' Channel Binding Type
IANA updated the registration of the 'tls-server-end-point' channel
binding type to match the description below. Note that the only
material changes from the original registration are: the "owner" (now
the IESG), the contacts, the published specification, and a note
indicating that the published specification should be consulted for
applicability advice. References were added to the description. All
other fields of the registration are copied here for the convenience
of readers.
4.1. Description
Description: The hash of the TLS server's certificate [RFC5280] as it
appears, octet for octet, in the server's Certificate message. Note
that the Certificate message contains a certificate_list, in which
the first element is the server's certificate.
The hash function is to be selected as follows:
o if the certificate's signatureAlgorithm uses a single hash
function, and that hash function is either MD5 [RFC1321] or SHA-1
[RFC3174], then use SHA-256 [FIPS-180-3];
o if the certificate's signatureAlgorithm uses a single hash
function and that hash function neither MD5 nor SHA-1, then use
the hash function associated with the certificate's
signatureAlgorithm;
Altman, et al. Standards Track [Page 5]
RFC 5929 TLS Channel Bindings July 2010
o if the certificate's signatureAlgorithm uses no hash functions or
uses multiple hash functions, then this channel binding type's
channel bindings are undefined at this time (updates to is channel
binding type may occur to address this issue if it ever arises).
The reason for using a hash of the certificate is that some
implementations need to track the channel binding of a TLS session in
kernel-mode memory, which is often at a premium.
4.2. Registration
o Channel binding unique prefix: tls-server-end-point
o Channel binding type: end-point
o Channel type: TLS [RFC5246]
o Published specification:
o Channel binding is secret: no
o Description:
o Intended usage: COMMON
o Person and email address to contact for further information: Larry
Zhu (larry.zhu@microsoft.com), Nicolas Williams
(Nicolas.Williams@oracle.com).
o Owner/Change controller name and email address: IESG.
o Expert reviewer name and contact information: IETF TLS WG
(tls@ietf.org, failing that, ietf@ietf.org)
o Note: see the published specification for advice on the
applicability of this channel binding type.
5. The 'tls-unique-for-telnet' Channel Binding Type
IANA updated the registration of the 'tls-unique-for-telnet' channel
binding type to match the description below. Note that the only
material changes from the original registration are: the "owner" (now
the IESG), the contacts, the published specification, and a note
indicating that the published specification should be consulted for
applicability advice. The description is also clarified. We also
moved the security considerations notes to the security
considerations section of this document. All other fields of the
registration are copied here for the convenience of readers.
Altman, et al. Standards Track [Page 6]
RFC 5929 TLS Channel Bindings July 2010
5.1. Description
Description: There is a proposal for adding a "StartTLS" extension to
TELNET, and a channel binding extension for the various TELNET AUTH
mechanisms whereby each side sends the other a "checksum" (MAC --
message authentication code) of their view of the channel's bindings.
The client uses the TLS Finished messages (note: the Finished struct)
sent by the client and server, each concatenated in that order and in
their clear text form, of the first TLS handshake to which the
connection is being bound. The server does the same but in the
opposite concatenation order (server, then client).
5.2. Registration
o Channel binding unique prefix: tls-unique-for-telnet
o Channel binding type: unique
o Channel type: TLS [RFC5246]
o Published specification:
o Channel binding is secret: no
o Description:
o Intended usage: COMMON
o Person and email address to contact for further information: Jeff
Altman (jaltman@secure-endpoints.com), Nicolas Williams
(Nicolas.Williams@oracle.com).
o Owner/Change controller name and email address: IESG.
o Expert reviewer name and contact information: IETF TLS WG
(tls@ietf.org, failing that, ietf@ietf.org)
o Note: see the published specification for advice on the
applicability of this channel binding type.
6. Applicability of TLS Channel Binding Types
The 'tls-unique-for-telnet' channel binding type is only applicable
to TELNET [RFC0854] and is available for all TLS connections.
The 'tls-unique' channel binding type is available for all TLS
connections, while 'tls-server-end-point' is only available when TLS
cipher suites with server certificates are used, specifically: cipher
Altman, et al. Standards Track [Page 7]
RFC 5929 TLS Channel Bindings July 2010
suites that use the Certificate handshake message, which typically
involve the use of PKIX [RFC5280]. For example, 'tls-server-end-
point' is available when using TLS ciphers suites such as (this is
not an exhaustive list):
o TLS_DHE_DSS_WITH_*
o TLS_DHE_RSA_WITH_*
o TLS_DH_DSS_WITH_*
o TLS_DH_RSA_WITH_*
o TLS_ECDHE_ECDSA_WITH_*
o TLS_ECDHE_RSA_WITH_*
o TLS_ECDH_ECDSA_WITH_*
o TLS_ECDH_RSA_WITH_*
o TLS_RSA_PSK_WITH_*
o TLS_RSA_WITH_*
o TLS_SRP_SHA_DSS_WITH_*
o TLS_SRP_SHA_RSA_WITH_*
but is not available when using TLS cipher suites such as (this is
not an exhaustive list):
o TLS_DHE_PSK_WITH_*
o TLS_DH_anon_WITH_*
o TLS_ECDHE_PSK_WITH_*
o TLS_ECDH_anon_WITH_*
o TLS_KRB5_WITH_*
o TLS_PSK_WITH_*
o TLS_SRP_SHA_WITH_*
Altman, et al. Standards Track [Page 8]
RFC 5929 TLS Channel Bindings July 2010
'tls-server-end-point' is also not applicable for use with OpenPGP
server certificates [RFC5081] [RFC4880] (since these don't use the
Certificate handshake message).
Therefore, 'tls-unique' is applicable to more contexts than 'tls-
server-end-point'. However, 'tls-server-end-point' may be used with
existing TLS server-side proxies ("concentrators") without
modification to the proxies, whereas 'tls-unique' may require
firmware or software updates to server-side proxies. Therefore there
may be cases where 'tls-server-end-point' may interoperate but where
'tls-unique' may not.
Also, authentication mechanisms may arise that depend on channel
bindings to contribute entropy, in which case unique channel bindings
would always have to be used in preference to end-point channel
bindings. At this time there are no such mechanisms, though one such
SASL mechanism has been proposed. Whether such mechanisms should be
allowed is out of scope for this document.
For many applications, there may be two or more potentially
applicable TLS channel binding types. Existing security frameworks
(such as the GSS-API [RFC2743] or the SASL [RFC4422] GS2 framework
[RFC5801]) and security mechanisms generally do not support
negotiation of channel binding types. Therefore, application peers
need to agree a priori as to what channel binding type to use (or
agree to rules for deciding what channel binding type to use).
The specifics of whether and how to negotiate channel binding types
are beyond the scope of this document. However, it is RECOMMENDED
that application protocols making use of TLS channel bindings, use
'tls-unique' exclusively, except, perhaps, where server-side proxies
are common in deployments of an application protocol. In the latter
case an application protocol MAY specify that 'tls-server-end-point'
channel bindings must be used when available, with 'tls-unique' being
used when 'tls-server-end-point' channel bindings are not available.
Alternatively, the application may negotiate which channel binding
type to use, or may make the choice of channel binding type
configurable.
Specifically, application protocol specifications MUST indicate at
least one mandatory to implement channel binding type, MAY specify a
negotiation protocol, MAY allow for out-of-band negotiation or
configuration, and SHOULD have a preference for 'tls-unique' over
'tls-server-end-point'.
Altman, et al. Standards Track [Page 9]
RFC 5929 TLS Channel Bindings July 2010
7. Required Application Programming Interfaces
TLS implementations supporting the use of 'tls-unique' and/or 'tls-
unique-for-telnet' channel binding types MUST provide application
programming interfaces by which applications (clients and servers
both) may obtain the channel bindings for a TLS connection. Such
interfaces may be expressed in terms of extracting the channel
bindings data for a given connection and channel binding type.
Alternatively, the implementor may provide interfaces by which to
obtain the initial client Finished message, the initial server
Finished message, and/or the server certificate (in a form that
matches the description of the 'tls-server-end-point' channel binding
type). In the latter case, the application has to have knowledge of
the channel binding type descriptions from this document. This
document takes no position on which form these application
programming interfaces must take.
TLS implementations supporting TLS renegotiation SHOULD provide APIs
that allow applications to control when renegotiation can take place.
For example, a TLS client implementation may provide a "callback"
interface to indicate that the server requested renegotiation, but
may not start renegotiation until the application calls a function to
indicate that now is a good time to renegotiate.
8. Description of Backwards-Incompatible Changes Made Herein to
'tls-unique'
The original description of 'tls-unique' read as follows:
|OLD| Description: The client's TLS Finished message (note: the
|OLD| Finished struct) from the first handshake of the connection
|OLD| (note: connection, not session, so that the channel binding
|OLD| is specific to each connection regardless of whether session
|OLD| resumption is used).
Original 'tls-unique' description
In other words: the client's Finished message from the first
handshake of a connection, regardless of whether that handshake was a
full or abbreviated handshake, and regardless of how many subsequent
handshakes (renegotiations) might have followed.
As explained in Section 1, this is no longer the description of 'tls-
unique', and the new description is not backwards compatible with the
original except in the case of TLS connections where: a) only one
handshake has taken place before application-layer authentication,
and b) that one handshake was a full handshake.
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RFC 5929 TLS Channel Bindings July 2010
This change has a number of implications:
o Backwards-incompatibility. It is possible that some
implementations of the original 'tls-unique' channel binding type
have been deployed. We know of at least one TLS implementation
that exports 'tls-unique' channel bindings with the original
semantics, but we know of no deployed application using the same.
Implementations of the original and new 'tls-unique' channel
binding type will only interoperate when: a) full TLS handshakes
are used, and b) TLS renegotiation is not used.
o Security considerations -- see Section 10.
o Interoperability considerations. As described in Section 3, the
new definition of the 'tls-unique' channel binding type has an
interoperability problem that may result in spurious
authentication failures unless the application implements one or
both of the techniques described in that section.
9. IANA Considerations
IANA updated three existing channel binding type registrations. See
the rest of this document.
10. Security Considerations
The Security Considerations sections of [RFC5056], [RFC5246], and
[RFC5746] apply to this document.
The TLS Finished messages (see Section 7.4.9 of [RFC5246]) are known
to both endpoints of a TLS connection and are cryptographically bound
to it. For implementations of TLS that correctly handle
renegotiation [RFC5746], each handshake on a TLS connection is bound
to the preceding handshake, if any. Therefore, the TLS Finished
messages can be safely used as a channel binding provided that the
authentication mechanism doing the channel binding conforms to the
requirements in [RFC5056]. Applications utilizing 'tls-unique'
channel binding with TLS implementations without support for secure
renegotiation [RFC5746] MUST ensure that ChangeCipherSpec has been
used in any and all renegotiations prior to application-layer
authentication, and MUST discard any knowledge learned from the
server prior to the completion of application-layer authentication.
The server certificate, when present, is also cryptographically bound
to the TLS connection through its use in key transport and/or
authentication of the server (either by dint of its use in key
transport, by its use in signing key agreement, or by its use in key
Altman, et al. Standards Track [Page 11]
RFC 5929 TLS Channel Bindings July 2010
agreement). Therefore, the server certificate is suitable as an end-
point channel binding as described in [RFC5056].
10.1. Cryptographic Algorithm Agility
The 'tls-unique' and 'tls-unique-for-telnet' channel binding types do
not add any use of cryptography beyond that used by TLS itself.
Therefore, these two channel binding types add no considerations with
respect to cryptographic algorithm agility.
The 'tls-server-end-point' channel binding type consists of a hash of
a server certificate. The reason for this is to produce manageably
small channel binding data, as some implementations will be using
kernel-mode memory (which is typically scarce) to store these. This
use of a hash algorithm is above and beyond TLS's use of
cryptography, therefore the 'tls-server-end-point' channel binding
type has a security consideration with respect to hash algorithm
agility. The algorithm to be used, however, is derived from the
server certificate's signature algorithm as described in Section 4.1;
to recap: use SHA-256 if the certificate signature algorithm uses MD5
or SHA-1, else use whatever hash function the certificate uses
(unless the signature algorithm uses no hash functions or more than
one hash function, in which case 'tls-server-end-point' is
undefined). The construction of 'tls-server-end-point' channel
bindings is not directly hash-agile (since no negotiation of hash
function is provided for), but it is hash-agile nonetheless. The
hash agility of 'tls-server-end-point' channel bindings derives from
PKIX and TLS.
Current proposals for randomized signatures algorithms [RHASH]
[NIST-SP.800-106.2009] use hash functions in their construction -- a
single hash function in each algorithm. Therefore, the 'tls-server-
end-point' channel binding type should be available even in cases
where new signatures algorithms are used that are based on current
randomized hashing proposals (but we cannot guarantee this, of
course).
10.2. On Disclosure of Channel Bindings Data by Authentication
Mechanisms
When these channel binding types were first considered, one issue
that some commenters were concerned about was the possible impact on
the security of the TLS channel, of disclosure of the channel
bindings data by authentication mechanisms. This can happen, for
example, when an authentication mechanism transports the channel
bindings data, with no confidentiality protection, over other
transports (for example, in communicating with a trusted third
party), or when the TLS channel provides no confidentiality
Altman, et al. Standards Track [Page 12]
RFC 5929 TLS Channel Bindings July 2010
protection and the authentication mechanism does not protect the
confidentiality of the channel bindings data. This section considers
that concern.
When the TLS connection uses a cipher suite that does not provide
confidentiality protection, the TLS Finished messages will be visible
to eavesdroppers, regardless of what the authentication mechanism
does. The same is true of the server certificate which, in any case,
is generally visible to eavesdroppers. Therefore we must consider
our choices of TLS channel bindings here to be safe to disclose by
definition -- if that were not the case, then TLS with cipher suites
that don't provide confidentiality protection would be unsafe.
Furthermore, the TLS Finished message construction depends on the
security of the TLS PRF, which in turn needs to be resistant to key
recovery attacks, and we think that it is, as it is based on HMAC,
and the master secret is, well, secret (and the result of key
exchange).
Note too that in the case of an attempted active man-in-the-middle
attack, the attacker will already possess knowledge of the TLS
Finished messages for both inbound and outbound TLS channels (which
will differ, given that the attacker cannot force them to be the
same). No additional information is obtained by the attacker from
the authentication mechanism's disclosure of channel bindings data --
the attacker already has it, even when cipher suites providing
confidentiality protection are provided.
None of the channel binding types defined herein produce channel
bindings data that must be kept secret. Moreover, none of the
channel binding types defined herein can be expected to be private
(known only to the end-points of the channel), except that the unique
TLS channel binding types can be expected to be private when a cipher
suite that provides confidentiality protection is used to protect the
Finished message exchanges and the application data records
containing application-layer authentication messages.
11. References
11.1. Normative References
[FIPS-180-3] United States of America, National Institute
of Standards and Technology, "Secure Hash
Standard", Federal Information Processing
Standard (FIPS) 180-3, October 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
Altman, et al. Standards Track [Page 13]
RFC 5929 TLS Channel Bindings July 2010
[RFC5056] Williams, N., "On the Use of Channel Bindings
to Secure Channels", RFC 5056, November 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport
Layer Security (TLS) Protocol Version 1.2",
RFC 5246, August 2008.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N.
Oskov, "Transport Layer Security (TLS)
Renegotiation Indication Extension",
RFC 5746, February 2010.
11.2. Informative References
[NIST-SP.800-106.2009] National Institute of Standards and
Technology, "NIST Special Publication 800-
106: Randomized Hashing for Digital
Signatures", February 2009.
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC1321] Rivest, R., "The MD5 Message-Digest
Algorithm", RFC 1321, April 1992.
[RFC2743] Linn, J., "Generic Security Service
Application Program Interface Version 2,
Update 1", RFC 2743, January 2000.
[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash
Algorithm 1 (SHA1)", RFC 3174,
September 2001.
[RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed.,
"Simple Authentication and Security Layer
(SASL)", RFC 4422, June 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H.,
Shaw, D., and R. Thayer, "OpenPGP Message
Format", RFC 4880, November 2007.
[RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys
for Transport Layer Security (TLS)
Authentication", RFC 5081, November 2007.
Altman, et al. Standards Track [Page 14]
RFC 5929 TLS Channel Bindings July 2010
[RFC5280] Cooper, D., Santesson, S., Farrell, S.,
Boeyen, S., Housley, R., and W. Polk,
"Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic
Security Service Application Program
Interface (GSS-API) Mechanisms in Simple
Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
[RHASH] Halevi, S. and H. Krawczyk, "Strengthening
Digital Signatures via Randomized Hashing",
Work in Progress, October 2007.
Authors' Addresses
Jeff Altman
Secure Endpoints
255 W 94TH ST PHB
New York, NY 10025
US
EMail: jaltman@secure-endpoints.com
Nicolas Williams
Oracle
5300 Riata Trace Ct
Austin, TX 78727
US
EMail: Nicolas.Williams@oracle.com
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: larry.zhu@microsoft.com
Altman, et al. Standards Track [Page 15]
RFC, FYI, BCP