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Versions: (draft-williams-kitten-channel-bound-flag) 00

KITTEN                                                       N. Williams
Internet-Draft                                              Cryptonector
Updates: 2743, 2744 (if approved)                           July 7, 2013
Intended status: Standards Track
Expires: January 8, 2014


Channel Binding Signalling for the Generic Security Services Application
                         Programming Interface
                draft-ietf-kitten-channel-bound-flag-00

Abstract

   Channel binding is a technique that allows applications to use a
   secure channel at a lower layer without having to use authentication
   at that lower layer.  The concept of channel binding comes from the
   Generic Security Services Application Programming Interface (GSS-
   API).  It turns out that the semantics commonly implemented are
   different that those specified in the base GSS-API RFC (RFC2743), and
   that that specification has a serious bug.  This document addresses
   both, the inconsistency as-implemented and the specification bug.

   This Internet-Draft proposes the addition of a "channel bound" return
   flag for the GSS_Init_sec_context() and GSS_Accept_sec_context()
   functions.  Two behaviors are specified: a default, safe behavior
   reflecting existing implementation deployments, and a behavior that
   is only safe when the application specifically tells the GSS-API that
   it (the application) supports the new behavior.  Additional API
   elements related to this are also added, including a new security
   context establishment API.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on January 8, 2014.




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Copyright Notice

   Copyright (c) 2013 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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.


Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.    Error in RFC2743 . . . . . . . . . . . . . . . . . . . . .  3
   1.2.    Design . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.3.    Alternative Design . . . . . . . . . . . . . . . . . . . .  4
   1.4.    Future Directions  . . . . . . . . . . . . . . . . . . . .  4
   1.5.    Conventions used in this document  . . . . . . . . . . . .  4
   2.      Channel Binding State Extension  . . . . . . . . . . . . .  5
   2.1.    GSS_Create_sec_context() . . . . . . . . . . . . . . . . .  5
   2.1.1.  C-Bindings . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.2.    GSS_Set_context_flags()  . . . . . . . . . . . . . . . . .  6
   2.2.1.  C-Bindings . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.3.    Return Flag for Channel Binding State Signalling . . . . .  6
   2.3.1.  C-Bindings . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.4.    New Mechanism Attributes . . . . . . . . . . . . . . . . .  7
   2.5.    Request Flag for Acceptor Confirmation of Channel
           Binding  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.5.1.  C-Bindings . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.6.    GSS_Delete_sec_context() Behavior When Applied to
           Empty Security Contexts  . . . . . . . . . . . . . . . . .  8
   3.      Modified Channel Binding Semantics . . . . . . . . . . . .  9
   4.      Security Considerations  . . . . . . . . . . . . . . . . . 10
   5.      IANA Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.      References . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.1.    Normative References . . . . . . . . . . . . . . . . . . . 12
   6.2.    Informative References . . . . . . . . . . . . . . . . . . 12
           Author's Address . . . . . . . . . . . . . . . . . . . . . 13







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1.  Introduction

   The GSS-API [RFC2743] supports "channel binding" [RFC5056], a
   technique for detection of man-in-the-middle (MITM) attacks in secure
   channels at lower network layers.  This facility is meant to be all-
   or-nothing: either both the initiator and acceptor use it and it
   succeeds, or both must not use it.  This has created a negotiation
   problem when retrofitting the use of channel binding into existing
   application protocols.

   Many implementations of the Kerberos V5 GSS-API mechanism [RFC4121]
   cause the acceptor to succeed when the initiator used channel binding
   but the acceptor application did not.  This has helped deployment of
   channel binding in existing applications: first fix all the
   initiators, then fix all the acceptors.  But even this technique is
   insufficient when there are many clients to fix, such that fixing
   them all will take a long time.

   This document proposes a new method for deployment of channel binding
   that allows the feature to be enabled on the acceptor side before
   fixing all initiators.  If the GSS-API had always had a return flag
   by which to indicate channel binding state then we could have had a
   simpler method of deploying channel binding: applications check that
   return flag and act accordingly (e.g., fail when channel binding is
   required).  We cannot safely introduce this behavior now without an
   indication of support by the application.

   It is worth noting that at least one implementor of GSS-API
   mechanisms (but not of the GSS-API itself) has similar semantics in
   its API to those proposed herein.  [XXX add references to the
   relevant SSPI docs? -Nico]

   Additionally, there may be applications where it is important for
   initiators to know that acceptors did use channel binding, and even
   to know whether a mechanism is capable of indicating as much.  We add
   a request flag and two mechanism attributes for such applications.

1.1.  Error in RFC2743

   The GSS-APIv2u1 [RFC2743] seems to indicate that mechanisms must
   ignore channel bindings when one party provided none.  In practice
   some mechanisms ignore channel bindings when the acceptor provides
   none, but not when the initiator provides none.  Note that it would
   be useless to allow security context establishment to succeed when
   the initiator does not provide channel bindings but the acceptor
   does, at least as long as there's no outward indication of whether
   channel binding was used!  And indeed, the GSS-APIv2u1 does not
   provide any such indication.  We correct this flaw in this document.



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1.2.  Design

   After some discussion on the mailing list of various designs for
   signalling application support for the new flag we've settled on
   copying an aspect of the Java Bindings of the GSS-API [RFC5653],
   specifically the notion of creating an "empty" SECURITY CONTEXT
   handle that can then be passed to GSS_Init_sec_context() and
   GSS_Accept_sec_context() where they normally expect a NULL handle.
   This empty security context handle can then be used to set options
   relating to security context token establishment.

   In [I-D.williams-williams-kitten-ctx-simple-async] we explore and
   extend this design to produce a more usable GSS-API (as well as
   support for asynchronous operation).

1.3.  Alternative Design

   An earlier design was based on an existing, non-standard extension
   for carrying security context establishment options in CREDENTIAL
   HANDLEs.  A notion of CREDENTIAL HANDLE options might still be useful
   for options that are really specific to credentials rather than
   security context tokens, but for the time being we have no use for
   such a thing.

1.4.  Future Directions

   We're likely to introduce additional mutator functions of empty
   contexts, with mutators corresponding to many of the existing input
   arguments of GSS_Init_sec_context() and GSS_Accept_sec_context(), as
   well as a few additional security context inquiry functions.  We're
   also likely to then introduce new variants of GSS_Init_sec_context()
   and GSS_Accept_sec_context() with all of those input and output
   parameters removed that could be set or retrieved with the other new
   functions.  The only inputs that the new GSS_Init/
   Accept_sec_context() must have are: a security context handle (never
   NULL), and an input context token, and the only outputs should be the
   status indicators and an output token.  In fact, we may want to have
   just one new function called, perhaps, GSS_Step_sec_context(), with
   the role of initiator or acceptor set as a context option.

   See [I-D.williams-williams-kitten-ctx-simple-async].

1.5.  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.  Channel Binding State Extension

   We propose a new return flag for GSS_Init_sec_context() and
   GSS_Accept_sec_context(), as well as a pair of functions for a)
   creating "empty" security context handles, b) setting req_flags and
   indicating which ret_flags the application understands.  We also add
   new mechanism attributes describing mechanism capabilities.

   C bindings of these extensions are provided along the lines of
   [RFC2744] and [RFC5587].

   In the future we might move more of the many input (and output)
   arguments to GSS_Init_sec_context() and GSS_Accept_sec_context() into
   mutators on empty security context handles.

2.1.  GSS_Create_sec_context()

   Inputs:

   o  <none>

   Outputs:

   o  major_status INTEGER

   o  minor_status INTEGER -- note: mostly useless, but we should keep
      it

   o  context SECURITY CONTEXT -- "empty" security context

   Return major status codes:

   o  GSS_S_COMPLETE indicates success.

   o  GSS_S_UNAVAILABLE indicates that memory is not available, for
      example.

   o  GSS_S_FAILURE indicates a general failure.

   This function creates an "empty" security context handle that can be
   passed to GSS_Init_sec_context() or GSS_Accept_sec_context() where
   they expect a NULL context.

2.1.1.  C-Bindings

               OM_uint32
               gss_create_sec_context(OM_uint32 *minor_status,
                                      gss_ctx_id_t *context);



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2.2.  GSS_Set_context_flags()

   Inputs:

   context CONTEXT HANDLE

   req_flags FLAGS  Requested flags.  Applicable to acceptors and
      initiators.

   ret_flags_understood FLAGS  The set of return flags understood by the
      caller.

   Outputs:

   o  major_status INTEGER

   o  minor_status INTEGER

   Return major status codes:

   o  GSS_S_COMPLETE indicates success.

   o  GSS_S_FAILURE indicates a general failure.

   This function tells the mechanism (when one is eventually chosen and
   invoked) that the application requests the given req_flags and
   undestands the given ret_flags.  Initiators can override the
   req_flags in their GSS_Init_sec_context() call, but if no flags are
   requested there then the req_flags set on the empty context will be
   used.

   NOTE: The abstract GSS-API [RFC2743] uses individual elements -one
   per-flag- instead of a "FLAGS" type.  This is unwieldy, therefore we
   introduce an abstract type named "FLAGS" to act as a set of all the
   request/return flags defined for the abstract GSS-API.

2.2.1.  C-Bindings

               OM_uint32
               gss_set_context_flags(OM_uint32 *minor_status,
                                     gss_ctx_id_t context,
                                     uint64_t req_flags,
                                     uint64_t ret_flags);

2.3.  Return Flag for Channel Binding State Signalling

   Whenever both the initiator and the acceptor provide matching channel
   bindings to GSS_Init_sec_context() and GSS_Accept_sec_context(),



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   respectively, then the mechanism SHALL indicate that the context is
   channel bound via an output flag, ret_channel_bound_flag, for the
   established context.  Note that some mechanisms have no way for the
   acceptor to signal CB success to the initiator, in which case
   GSS_Init_sec_context() MUST NOT output the ret_channel_bound_flag.

2.3.1.  C-Bindings

            #define GSS_C_CHANNEL_BOUND_FLAG 2048 /* 0x00000800 */

2.4.  New Mechanism Attributes

   o  We add a new mechanism attribute, GSS_C_MA_CBINDING_CONFIRM, to
      indicate that the initiator can and always does learn whether the
      acceptor application supplied channel bindings.  Mechanisms
      advertising this attribute MUST always indicate acceptor channel
      bound state to the initiator.

   o  We add a new mechanism attribute, GSS_C_MA_CBINDING_MAY_CONFIRM,
      to indicate that the initiator may learn whether the acceptor
      application supplied channel bindings, but only when the acceptor
      implementation of the mechanism has been suitably updated.
      Mechanisms MUST advertise this attribute when the local initiator
      functionality for acceptor channel bound state indication exists
      but the acceptor may lack the same functionality (because, for
      example, the mechanism predates this specification).

   OID assignments TBD.

2.5.  Request Flag for Acceptor Confirmation of Channel Binding

   We add a new request flag for GSS_Init_sec_context(),
   req_cb_confirmation_flag, to be used by initiators that insist on
   acceptors providing channel bindings.  This flag is only of use to
   mechanism-negotiation pseudo-mechanisms (e.g., SPNEGO [RFC4178]): if
   set the pseudo-mechanism MUST NOT negotiate any mechanisms that lack
   the GSS_C_MA_CBINDING_CONFIRM or GSS_C_MA_CBINDING_MAY_CONFIRM
   mechanism attributes, and SHOULD NOT negotiate mechanisms that lack
   the GSS_C_MA_CBINDING_CONFIRM mechanism attribute (except if allowed
   by local configuration).

2.5.1.  C-Bindings

   Because GSS_C_CHANNEL_BOUND_FLAG is a return flag only, and this flag
   is a request flag only, and to save on precious flag bits, we use the
   same flag bit assignment for both flags:

             #define GSS_C_CB_CONFIRM_FLAG 2048 /* 0x00000800 */



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2.6.  GSS_Delete_sec_context() Behavior When Applied to Empty Security
      Contexts

   GSS_Delete_sec_context() MUST NOT output a context deletion token
   when applied to empty security contexts.














































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3.  Modified Channel Binding Semantics

   The channel binding semantics of the base GSS-API are modified as
   follows:

   o  Whenever both, the initiator and acceptor shall have provided
      input_channel_bindings to GSS_Init/Accept_sec_context() and the
      channel bindings do not match, then the mechanism MUST fail to
      establish a security context token.  This is a restatement of an
      existing requirement in the base specification, restated for the
      reader's convenience.

   o  Whenever the acceptor application shall have a) provided channel
      bindings to GSS_Accept_sec_context(), and b) not indicated support
      for the ret_channel_bound_flag flag, then the mechanism MUST fail
      to establish a security context if the initiator did not provide
      channel bindings data.  This requirement is critical for security
      purposes, to make applications predating this document secure, and
      this requirement reflects actual implementations as deployed.

   o  Whenever the initiator application shall have a) provided channel
      bindings to GSS_Init_sec_context(), and b) not indicated support
      for the ret_channel_bound_flag flag, then the mechanism SHOULD NOT
      fail to establish a security context just because the acceptor
      failed to provide channel bindings data.  This recommendation is
      for interoperability purposes, and reflects actual implementations
      that have been deployed.  It is possible that not all security
      mechanism protocols can implement this requirement easily.

   o  Whenever the application shall have a) provided channel bindings
      to GSS_Init_sec_context() or GSS_Accept_sec_context(), and b)
      indicated support for the ret_channel_bound_flag flag, then the
      mechanism MUST NOT fail to establish a security context just
      because the peer did not provide channel bindings data.  The
      mechanism MUST output the ret_channel_bound_flag if both peers
      provided the same input_channel_bindings to GSS_Init_sec_context()
      and GSS_Accept_sec_context.  The mechanism MUST NOT output the
      ret_channel_bound_flag if either (or both) peer did not provide
      input_channel_bindings to GSS_Init/Accept_sec_context().  This
      requirement restores the original base GSS-API specified behavior,
      with the addition of the ret_channel_bound_flag flag.










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4.  Security Considerations

   This document deals with security.  There are no security
   considerations that should be documented separately in this section.
   To recap, this document fixes a significant flaw in the base GSS-API
   [RFC2743] specification that fortunately has not been implemented,
   and it adds a feature (that should have been in the base
   specification) for improved negotiation of use of channel binding
   [RFC5056].










































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5.  IANA Considerations

   Two GSS-API mechanism attributes are to be added to the "SMI Security
   for Mechanism gsscma Codes" registry established by RFC5587
   [RFC5587].  See Section 2.4.














































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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, January 2000.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5587]  Williams, N., "Extended Generic Security Service Mechanism
              Inquiry APIs", RFC 5587, July 2009.

6.2.  Informative References

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.

   [RFC4178]  Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
              Simple and Protected Generic Security Service Application
              Program Interface (GSS-API) Negotiation Mechanism",
              RFC 4178, October 2005.

   [RFC5653]  Upadhyay, M. and S. Malkani, "Generic Security Service API
              Version 2: Java Bindings Update", RFC 5653, August 2009.

   [I-D.williams-williams-kitten-ctx-simple-async]
              Williams, N., "Simplified and Asynchronous Security
              Context Interfaces for the Generic Security Services
              Application Programming Interface",
              draft-williams-williams-kitten-ctx-simple-async-00 (work
              in progress), February 2013.











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Author's Address

   Nicolas Williams
   Cryptonector, LLC

   Email: nico@cryptonector.com













































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