NETWORK WORKING GROUP J. Altman
Internet-Draft Secure Endpoints
Intended status: Standards Track N. Williams
Expires: December 13, 2009 Sun
June 11, 2009
Channel Bindings for TLS
draft-altman-tls-channel-bindings-04.txt
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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).
Table of Contents
1. Conventions used in this document . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The 'tls-unique' Channel Binding Type . . . . . . . . . . . 5
4. The 'tls-server-end-point' Channel Binding Type . . . . . . 6
5. The 'tls-unique-for-telnet' Channel Binding Type . . . . . . 8
6. Applicability of TLS Channel Binding Types . . . . . . . . . 9
7. Required Application Programming Interfaces . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . 12
9.1. Cryptographic Algorithm Agility . . . . . . . . . . . . . . 12
9.2. On Disclosure of Channel Bindings Data by Authentication
Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . . . 14
10.2. Normative References for 'tls-server-end-point' . . . . . . 14
10.3. Informative References . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
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1. 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].
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2. Introduction
Subsequent to the publication of "On Channel Bindings" [RFC5246],
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 [RFC5246]. 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. This document does just that. The authors of those three
channel binding types have, or have indicated that they will,
transferred "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. And we
provide an abstract API that TLS implementors should provide, by
which to obtain channel bindings data for a TLS connection.
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3. The 'tls-unique' Channel Binding Type
IANA is hereby directed to update the registration of the 'tls-
unique' channel binding type to match the following. Note that the
only material changes from the original registration should be: the
"owner" (now the IESG), contacts, the published specfication, and a
clarification to the description by the addition of a parenthetical
note (that is, the first such note in the descritption is a new
addition). We also added a note indicating that this specification
contains applicability advice, and we moved 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.
o Channel binding unique prefix: tls-unique
o Channel binding type: unique
o Channel type: TLS [RFC5246]
o Published specification: <this document>
o Channel binding is secret: no
o Description: The client's TLS Finished message (note: the Finished
struct) from the first handshake of the connection (note:
connection, not session, so that the channel binding is specific
to each connection regardless of whether session resumption is
used).
o Intended usage: COMMON
o Person and email address to contact for further information: Larry
Zhu (lzhu@microsoft.com), Nicolas Williams
(Nicolas.Williams@sun.com).
o Owner/Change controller name and email address: IESG.
o Expert reviewer name and contact information: IETF (ietf@ietf.org)
o Note: see the published specification for advice on the
applicability of this channel binding type.
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4. The 'tls-server-end-point' Channel Binding Type
IANA is hereby directed to update the registration of the 'tls-
server-end-point' channel binding type to match the following. Note
that the only material changes from the original registration should
be: the "owner" (now the IESG), the contacts, the published
specfication, 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.
o Channel binding unique prefix: tls-server-end-point
o Channel binding type: end-point
o Channel type: TLS [RFC5246]
o Published specification: <this document>
o Channel binding is secret: no
o Description: The hash of the TLS server's end entity certificate
[RFC5280] as it appears, octet for octet, in the server's
Certificate message (note that the Certificate message contains a
certificate_list, the first element of which is the server's end
entity certificate.) The hash function to be selected is as
follows: if the certificate's signature hash algorithm is either
MD5 [RFC1321] or SHA-1 [RFC3174], then use SHA-256 [FIPS-180-2],
otherwise use the certificate's signature hash algorithm.
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.
o Intended usage: COMMON
o Person and email address to contact for further information: Larry
Zhu (lzhu@microsoft.com), Nicolas Williams
(Nicolas.Williams@sun.com).
o Owner/Change controller name and email address: IESG.
o Expert reviewer name and contact information: IETF (ietf@ietf.org)
o Note: This channel binding is only suitable for use with PKIX
server certificates [RFC5280], not OpenPGP certificates [RFC5081]
[RFC4880].
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o Note: see the published specification for advice on the
applicability of this channel binding type.
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5. The 'tls-unique-for-telnet' Channel Binding Type
IANA is hereby directed to update the registration of the 'tls-
unique-for-telnet' channel binding type to match the following. Note
that the only material changes from the original registration should
be: the "owner" (now the IESG), the contacts, the published
specfication, and a note indicating that the published specification
should be consulted for applicability advice. The description is
also clarified. We also moved 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.
o Channel binding unique prefix: tls-unique-for-telnet
o Channel binding type: unique
o Channel type: TLS [RFC5246]
o Published specification: <this document>
o Channel binding is secret: no
o 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) of their view of the channel's bindings. The client uses
the first TLS Finished messages (note: the Finished struct) from
the client and server, each concatenated in that order and in
their clear text form. The server does the same but in the
opposite concatenation order (server, then client).
o Intended usage: COMMON
o Person and email address to contact for further information: Jeff
Altman (jaltman@secure-endpoints.com), Nicolas Williams
(Nicolas.Williams@sun.com).
o Owner/Change controller name and email address: IESG.
o Expert reviewer name and contact information: IETF (ietf@ietf.org)
o Note: see the published specification for advice on the
applicability of this channel binding type.
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6. Applicability of TLS Channel Binding Types
The 'tls-unique-for-telnet' channel binding type is only applicable
to TELNET [RFC0854].
The 'tls-unique' channel binding type is always available for TLS
connections, while 'tls-server-end-point' is only available when TLS
cipher suites with server certificates are used. Therefore 'tls-
unique' is generally better 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 are cases where 'tls-server-
end-point' may interoperate but where 'tls-unique' may not.
In other words, for many applications there may be two potentially
applicable TLS channel binding types. Channel binding is all or
nothing for the GSS-API [RFC2743], and likely other frameworks.
Therefore agreement on the use of channel binding, and a particular
channel binding type is necessary. Such agreement can be a priori or
negotiated.
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 prefer 'tls-unique' over 'tls-server-end-
point'.
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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 may obtain the channel
bindings for a TLS connection. An implementation MAY provide
interfaces for obtaining the initial Finished messages of a
connection separately, letting TELNET [RFC0854] construct 'tls-
unique-for-telnet' channel bindings from those, or the implementation
MAY provide an interface specifically for extracting channel bindings
data from a connection, and for a given channel binding type.
TLS implementations supporting the use of 'tls-server-end-point'
channel bindings MUST provide application programming interfaces to
obtain this channel binding. Such an interface SHOULD produce the
'tls-server-end-point' channel bindings data directly, but MAY
produce the certificate of the server for the connection instead, as
it appears, octet for octet, in the server's Certificate message.
When a connection results from TLS session resumption, the
implementation may need to have cached the server certificate from
the original connection, but MAY return an error instead of the
channel binding or server certificate. Applications wishing to use
'tls-server-end-point' channel bindings and TLS session resumption
MUST be prepared to handle the unavailability of 'tls-server-end-
point' channel bindings in the case of TLS session resumption.
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8. IANA Considerations
The IANA is hereby directed to update three existing channel binding
type registrations. See the rest of this document.
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9. Security Considerations
The Security Considerations section of [RFC5056] applies 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 cryptographycally
bound to it. 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].
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
agreement). Therefore the server certificate is suitable as an end-
point channel binding as described in [RFC5056].
9.1. 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 consist of a hash of
a server certificate. 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 certificate itself: use SHA-256 if the certificate
uses MD5 or SHA-1, else use whatever hash function the certificate
uses. This construction automatically makes 'tls-server-end-point'
hash algorithm agile.
9.2. 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
protection and the authentication mechanism does not protect the
confidentiality of the channel bindings data. This section considers
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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.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
10.2. Normative References for 'tls-server-end-point'
[FIPS-180-2]
United States of America, National Institute of Standards
and Technology, "Secure Hash Standard (Federal Information
Processing Standard (FIPS) 180-2".
[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.
10.3. Informative References
[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.
[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.
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Authors' Addresses
Jeff Altman
Secure Endpoints
255 W 94TH ST PHB
New York, NY 10025
US
Email: jaltman@secure-endpoints.com
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
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