[CHANNEL-BINDING] Updates to draft-altman-tls-channel-bindings (PLEASE REVIEW)

[Nicolas Williams <[e-mail]>]

Now that TLS re-negotiation security (RFC5746) is done we need to finish
draft-altman-tls-channel-bindings.

It's become clear that MSFT implemented and deployed something slightly
different than the 'tls-unique' binding description that had been
registered.  They seem to be the only implementors to date; their
implementation is secure, therefore I propose that we simply modify the
description of 'tls-unique' and be done.

The differences are:

 - it's not always the client's Finished message, but the _first_
   Finished message in the relevant handshake (in a session resumption
   handshake the server sends its Finished message first);

 - the Finished message is taken from the latest/inner-most TLS
   handshake, rather than the outer-most one

For the most common case, where there's no TLS re-negotiation and no TLS
session resumption, there is no difference.

It's past the I-D submission cut-off, so I can't submit an updated I-D.
Below are the full diffs from -07 to the -08 to-be.

Please review,

Nico


--- altman-tls-channel-bindings-07.unpg	Wed Mar 17 13:32:15 2010
+++ altman-tls-channel-bindings-08.unpg	Wed Mar 17 13:32:51 2010
@@ -1,594 +1,603 @@
 
 
 
 NETWORK WORKING GROUP                                          J. Altman
 Internet-Draft                                          Secure Endpoints
 Intended status: Standards Track                             N. Williams
-Expires: April 4, 2010                                               Sun
+Expires: September 18, 2010                                       Oracle
                                                                   L. Zhu
                                                    Microsoft Corporation
-                                                            October 2009
+                                                          March 17, 2010
 
 
                         Channel Bindings for TLS
-                draft-altman-tls-channel-bindings-07.txt
+                draft-altman-tls-channel-bindings-08.txt
 
 Status of this Memo
 
    This Internet-Draft is submitted to IETF in full conformance with the
    provisions of BCP 78 and BCP 79.  This document may contain material
    from IETF Documents or IETF Contributions published or made publicly
    available before November 10, 2008.  The person(s) controlling the
    copyright in some of this material may not have granted the IETF
    Trust the right to allow modifications of such material outside the
    IETF Standards Process.  Without obtaining an adequate license from
    the person(s) controlling the copyright in such materials, this
    document may not be modified outside the IETF Standards Process, and
    derivative works of it may not be created outside the IETF Standards
    Process, except to format it for publication as an RFC or to
    translate it into languages other than English.
 
    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as Internet-
    Drafts.
 
    Internet-Drafts are draft documents valid for a maximum of six months
    and may be updated, replaced, or obsoleted by other documents at any
    time.  It is inappropriate to use Internet-Drafts as reference
    material or to cite them other than as "work in progress."
 
    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt.
 
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.
 
-   This Internet-Draft will expire on April 4, 2010.
+   This Internet-Draft will expire on September 18, 2010.
 
 Copyright Notice
 
-   Copyright (c) 2009 IETF Trust and the persons identified as the
+   Copyright (c) 2010 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 in effect on the date of
    publication of this document (http://trustee.ietf.org/license-info).
    Please review these documents carefully, as they describe your rights
    and restrictions with respect to this document.
 
 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
    2.    Introduction
    3.    The 'tls-unique' Channel Binding Type
    3.1.  Description
    3.2.  Registration
    4.    The 'tls-server-end-point' Channel Binding Type
    4.1.  Description
    4.2.  Registration
    5.    The 'tls-unique-for-telnet' Channel Binding Type
    5.1.  Description
    5.2.  Registration
    6.    Applicability of TLS Channel Binding Types
    7.    Required Application Programming Interfaces
    8.    IANA Considerations
    9.    Security Considerations
    9.1.  Cryptographic Algorithm Agility
    9.2.  On Disclosure of Channel Bindings Data by Authentication
          Mechanisms
    10.   References
    10.1. Normative References
    10.2. Normative References for 'tls-server-end-point'
    10.3. Informative References
          Authors' Addresses
 
 
 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].
 
 
 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.
+   them, and to correct them to describe actual implementations.  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.
 
 
 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.
+   unique' channel binding type to match the following.  There are
+   material changes from the original registration, both in the
+   description as well as registration meta-data (such as registration
+   ownership).
 
 3.1.  Description
 
-   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).
+   Description: The first TLS Finished message sent (note: the Finished
+   struct) 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 re-negotiation takes place before the
+   channel binding 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 the first Finished message is
+   sent by the server.
 
 3.2.  Registration
 
    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: <See specification>
 
    o  Intended usage: COMMON
 
    o  Person and email address to contact for further information: Larry
       Zhu ([e-mail]), Nicolas Williams
       ([e-mail]).
 
    o  Owner/Change controller name and email address: IESG.
 
    o  Expert reviewer name and contact information: IETF TLS WG
       ([e-mail], failing that, [e-mail])
 
    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 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.
 
 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, the first
    element of which 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-180-2];
 
    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;
 
    o  if the certificate's signatureAlgorithm uses no hash functions or
       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: <this document>
 
    o  Channel binding is secret: no
 
    o  Description: <See specification>
 
    o  Intended usage: COMMON
 
    o  Person and email address to contact for further information: Larry
       Zhu ([e-mail]), Nicolas Williams
       ([e-mail]).
 
    o  Owner/Change controller name and email address: IESG.
 
    o  Expert reviewer name and contact information: IETF TLS WG
       ([e-mail], failing that, [e-mail])
 
    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 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.
 
 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) 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).
+   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 of the connection being bound to.  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: <this document>
 
    o  Channel binding is secret: no
 
    o  Description: <See specification>
 
    o  Intended usage: COMMON
 
    o  Person and email address to contact for further information: Jeff
       Altman ([e-mail]), Nicolas Williams
       ([e-mail]).
 
    o  Owner/Change controller name and email address: IESG.
 
    o  Expert reviewer name and contact information: IETF TLS WG
       ([e-mail], failing that, [e-mail])
 
    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
    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_*
 
    Nor is this channel binding type available for use with OpenPGP
    server certificates [RFC5081] [RFC4880] (since these don't use the
    Certificate handshake message).
 
    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 may be cases where
    'tls-server-end-point' may interoperate but where 'tls-unique' may
    not.
 
    Also, authentications mechanisms may arise which depend on channel
    bindings to contribute entropy, in which case unique channel bindings
    would have to always 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.
 
    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 obtained a
    priori, by convention, 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 have a preference for 'tls-unique' over
    'tls-server-end-point'.
 
 
 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.
 
 
 8.  IANA Considerations
 
    The IANA is hereby directed to update three existing channel binding
    type registrations.  See the rest of this document.
 
 
 9.  Security Considerations
 
-   The Security Considerations section of [RFC5056] applies to this
-   document.
+   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 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].
+   bound to it.  For implementations of TLS that correctly handle re-
+   negotiation [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].
 
    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.  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).  This construction automatically makes 'tls-server-end-
    point' hash algorithm agile, with a dependency on PKIX and TLS for
    hash agility.
 
    Current proposals for randomized signatures algorithms
    [I-D.irtf-cfrg-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).
 
 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
    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.
 
 
 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.
 
+   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
+              "Transport Layer Security (TLS) Renegotiation Indication
+              Extension", RFC 5746, February 2010.
+
 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".
 
 10.3.  Informative References
 
    [I-D.irtf-cfrg-rhash]
               Halevi, S. and H. Krawczyk, "Strengthening Digital
               Signatures via Randomized Hashing",
               draft-irtf-cfrg-rhash-01 (work in progress), October 2007.
 
    [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.
 
    [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.
 
    [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.
 
 
 Authors' Addresses
 
    Jeff Altman
    Secure Endpoints
    255 W 94TH ST PHB
    New York, NY  10025
    US
 
    Email: [e-mail]
 
 
    Nicolas Williams
-   Sun Microsystems
+   Oracle
    5300 Riata Trace Ct
    Austin, TX  78727
    US
 
-   Email: [e-mail]
+   Email: [e-mail]
 
 
    Larry Zhu
    Microsoft Corporation
    One Microsoft Way
    Redmond, WA  98052
    US
 
    Email: [e-mail]