Structure of the GSS Negotiation Loop
draft-kaduk-kitten-gss-loop-00

Network Working Group                                           B. Kaduk
Internet-Draft                                                       MIT
Intended status: Standards Track                        October 21, 2013
Expires: April 24, 2014

                 Structure of the GSS Negotiation Loop
                     draft-kaduk-kitten-gss-loop-00

Abstract

   This document specifies the generic structure of the negotiation loop
   to establish a GSS security context between initiator and acceptor.
   The control flow of the loop is indicated for both parties, including
   error conditions, and indications are given for where application-
   specific behavior must be specified.

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1.  Introduction

   The Generic Security Service Application Program Intervace version 2
   [RFC2743] provides a generic interface for security services, in the
   form of an abstraction layer over the underlying security mechanisms
   that an application may use.  A GSS initiator and acceptor exchange
   messages, called tokens, until a security context is established.
   Such a security context allows for mutual authentication of the two
   parties, the passing of confidential or integrity-protected messages
   between the initiator and acceptor, the generation of identical
   pseudo-random bit strings by both participants [RFC4401], and more.
   The number of tokens which must be exchanged between initiator and
   acceptor in order to establish the security context is dependent on
   the underlying mechanism as well as the desired properties of the
   security context, and is in general not known to the application.
   Accordingly, the application's control flow must include a loop
   within which GSS security context tokens are exchanged, which
   terminates upon successful establishment of a security context (or an
   error condition).

   The GSS-API C bindings [RFC2744] provide some example code for such a
   negotiation loop, but this code does not specify the application's
   behavior on unexpected or error conditions.  As such, individual
   application protocol specifications have had to specify the structure
   of their GSS negotiation loops, including error handling, on a per-
   protocol basis.  [RFC4462], [RFC3645], [RFC5801], [RFC4752],
   [RFC2203] This represents a substantial duplication of effort, and
   the various specifications go into different levels of detail and
   describe different possible error conditions.  It is therefore
   preferable to have the structure of the GSS negotiation loop,
   including error conditions and token passing, described in a single
   specification, which can then be referred to from other documents in
   lieu of repeating the structure of the loop each time.  This document
   will perform that role.

1.1.  Requirements Language

   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.  Loop Structure

   The loop is begun by the appropriately named initiator, which calls
   GSS_Init_sec_context() with an empty (zero-length) input_token and a
   fixed set of input flags containing the desired attributes for the
   security context.  The initiator MUST NOT change any of the input
   parameters to GSS_Init_sec_context() between calls to it during the

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   loop, with the exception of the input_token parameter, which will
   contain a message from the acceptor after the initial call, and the
   input_context_handle, which MUST be the result returned in the
   output_context_handle of the previous call to GSS_Init_sec_context()
   (or GSS_C_NO_CONTEXT for the first call).

   The following subsections will describe the various steps of the
   loop, without special consideration to whether a call to
   GSS_Init_sec_context() or GSS_Accept_sec_context() is the first such
   call in the loop.  For the first call to each routine in the loop,
   the major status code from the previous call to
   GSS_Init_sec_context() or GSS_Accept_sec_context() should be taken as
   GSS_S_CONTINUE_NEEDED.

2.1.  Anonymous Initiators

   If the initiator is requesting anonymity by setting the anon_req_flag
   input to GSS_Init_sec_context(), then on non-error returns from
   GSS_Init_sec_context() (that is, the major status is GSS_S_COMPLETE
   or GSS_S_CONTINUE_NEEDED), the initiator MUST verify that the output
   value of anon_state from GSS_Init_sec_context() is true before
   sending the security context token to the acceptor.  Failing to
   perform this check could cause the initiator to lose anonymity.

2.2.  GSS_Init_sec_context

   The initiator calls GSS_Init_sec_context(), using the
   input_context_handle for the current proto-security-context and its
   fixed set of input parameters, and the input_token received from the
   acceptor (if not the first iteration of the loop).  The presence of a
   nonempty output_token and the value of the major status code are the
   indicators for how to proceed:

      If the major status code is GSS_S_COMPLETE and the output_token is
      empty, then the context negotiation is fully complete and ready
      for use by the initiator with no further actions.

      If the major status code is GSS_S_COMPLETE and the output_token is
      nonempty, then the initiator's portion of the security context
      negotiation is complete but the acceptor's is not.  The initiator
      MUST send the output_token to the acceptor so that the acceptor
      can establish its half of the security context.

      If the major status code is GSS_S_CONTINUE_NEEDED and the
      output_token is nonempty, the context negotiation is incomplete.
      The initiator MUST send the output_token to the acceptor and await
      another input_token from the acceptor.

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      If the major status code is GSS_S_CONTINUE_NEEDED and the
      output_token is empty, the mechanism has produced an inconsistent
      output and the security context negotiation has failed.  The
      initiator SHOULD indicate the failure to the acceptor if an
      appropriate channel to do so is available.

      If the major status code is any other value, the context
      negotiation has failed.  If the output_token is nonempty, it is an
      error token, and the initiator SHOULD send it to the acceptor.  If
      the output_token is empty, then the initiator SHOULD indicate the
      failure to the acceptor if an appropriate channel to do so is
      available.

2.3.  Sending from Initiator to Acceptor

   The establishment of a GSS security context between initiator and
   acceptor requires some communication channel by which to exchange the
   context negotiation tokens.  The nature of this channel is not
   specified by the GSS specification -- it could be a synchronous TCP
   channel, a UDP-based RPC protocol, or any other sort of channel.  In
   many cases, the channel will be multiplexed with non-GSS application
   data; the application protocol must provide some means by which the
   GSS context tokens can be identified and passed through to the
   mechanism accordingly.  It is in such cases where the application
   protocol has a means to indicate error conditions that the initiator
   could indicate a failure to the acceptor, as mentioned in some of the
   above cases conditional on "an appropriate channel to do so".

   However, even the presence of a communication channel does not
   necessarily indicate that it is appropriate for the initiator to
   indicate such errors.  For example, if the acceptor is a stateless or
   near-stateless UDP server, there is probably no need for the
   initiator to explicitly indicate its failure to the acceptor.
   Conditions such as this can be treated in individual application
   protocol specifications.

   If a regular security context output_token is produced by the call to
   GSS_Init_sec_context(), the initiator MUST transmit this token to the
   acceptor over the application's communication channel.  If the call
   to GSS_Init_sec_context() returns an error token as output_token, it
   is RECOMMENDED that the intiator transmit this token to the acceptor
   over the application's communication channel.

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2.4.  Acceptor Sanity Checking

   The acceptor's half of the negotiation loop is triggered by the
   receipt of a context token from the initiator.  Before calling
   GSS_Accept_sec_context(), the acceptor may find it useful to perform
   some sanity checks on the state of the negotiation loop.

   If the acceptor receives a context token but was not expecting such a
   token (for example, if the acceptor's previous call to
   GSS_Accept_sec_context() returned GSS_S_COMPLETE), this is an error
   condition indicating that the initiator's state is invalid.  It is
   likely appropriate for the acceptor to report this error condition to
   the acceptor via the application's communication channel.

   If the acceptor is expecting a context token (e.g., if the previous
   call to GSS_Accept_sec_context() returned GSS_S_CONTINUE_NEEDED), but
   does not receive such a token within a reasonable amount of time
   after transmitting the previous output_token to the initiator, the
   acceptor should assume that the initiator's state is invalid and fail
   the GSS negotiation.  Again, it is likely appropriate for the
   acceptor to report this error condition to the initiator via the
   application's communication channel.

   [Are there other checks to perform here?]

2.5.  GSS_Accept_sec_context

   The GSS acceptor responds to the actions of an initiator; as such,
   there should always be a nonempty input_token to calls to
   GSS_Accept_sec_context().  The input_context_handle parameter will
   always be given as the output_context_handle from the previous call
   to GSS_Accept_sec_context() in a given negotiation loop (or
   GSS_C_NO_CONTEXT on the first call), but the acceptor_cred_handle and
   chan_bindings arguments MUST remain fixed over the course of a given
   GSS negotiation loop.

   The GSS acceptor calls GSS_Accept_sec_context(), using the
   input_context_handle for the current proto-security-context and the
   input_token received from the initiator.  The presence of a nonempty
   output_token and the value of the major status code are the
   indicators for how to proceed:

      If the major status code is GSS_S_COMPLETE and the output_token is
      empty, then the context negotiation is fully complete and ready
      for use by the acceptor with no further actions.

      If the major status code is GSS_S_COMPLETE and the output_token is
      nonempty, then the acceptor's portion of the security context

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      negotiation is complete but the initiator's is not.  The acceptor
      MUST send the output_token to the initiator so that the initiator
      can establish its half of the security context.

      If the major status code is GSS_S_CONTINUE_NEEDED and the
      output_token is nonempty, the context negotiation is incomplete.
      The acceptor MUST send the output_token to the initiator and await
      another input_token from the initiator.

      If the major status code is GSS_S_CONTINUE_NEEDED and the
      output_token is empty, the mechanism has produced an inconsistent
      output and the security context negotiation has failed.  The
      acceptor SHOULD indicate the failure to the initiator if an
      appropriate channel to do so is available.

      If the major status code is any other value, the context
      negotiation has failed.  If the output_token is nonempty, it is an
      error token, and the acceptor SHOULD send it to the initiator.  If
      the output_token is empty, then the acceptor SHOULD indicate the
      failure to the initiator if an appropriate channel to do so is
      available.

2.6.  Sending from Acceptor to Initiator

   The mechanism for sending the context token from acceptor to
   initiator will depend on the nature of the communication channel
   between the two parties.  For a synchronous bidirectional channel, it
   can be just another piece of data sent over the link, but for a
   stateless UDP RPC acceptor, the token will probably end up being sent
   as an RPC output parameter.  Application protocol specifications will
   need to specify the nature of this behavior.

   If the application protocol has the initiator driving the
   application's control flow (with the acceptor just responding to
   actions from the initiator), it is particularly helpful for the
   acceptor to indicate a failure to the initiator, as mentioned in some
   of the above cases conditional on "an appropriate channel to do so".

   If a regular security context output_token is produced by the call to
   GSS_Accept_sec_context(), the acceptor MUST transmit this token to
   the initiator over the application's communication channel.  If the
   call to GSS_Accept_sec_context() returns an error token as
   output_token, it is RECOMMENDED that the acceptor transmit this token
   to the initiator over the application's communication channel.

2.7.  Initiator input validation

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   The initiator's half of the negotiation loop is triggered (after the
   first call) by receipt of a context token from the acceptor.  Before
   calling GSS_Init_sec_context(), the initiator may find it useful to
   perform some sanity checks on the state of the negotiation loop.

   If the initiator receives a context token but was not expecting such
   a token (for example, if the initiator's previous call to
   GSS_Init_sec_context() returned GSS_S_COMPLETE), this is an error
   condition indicating that the acceptor's state is invalid.  It may be
   appropriate for the initiator to report this error condition to the
   acceptor via the application's communication channel.

   If the initiator is expecting a context token (that is, the previous
   call to GSS_Init_sec_context() returned GSS_S_CONTINUE_NEEDED), but
   does not receive such a token within a reasonable amount of time
   after transmitting the previous output_token to the acceptor, the
   initiator should assume that the acceptor's state is invalid and fail
   the GSS negotiation.  Again, it may be appropriate for the initiator
   to report this error condition to the acceptor via the application's
   communication channel.

   [Are there other checks to perform here?]

2.8.  Continue the Loop

   If the loop is in neither a success or failure condition, then the
   loop must continue.  Control flow returns to Section 2.2.

3.  After Security Context Negotiation

   Once a party has completed its half of the security context and
   fulfilled its obligations to the other party, the context is
   complete, but it is not necessarily ready and appropriate for use.
   (In some cases the context may be ready for use earlier than this,
   see Section 3.1.)  In particular, the security context flags may not
   be appropriate for the given application's use.

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   The initiator specifies as part of its fixed set of inputs to
   GSS_Init_sec_context() values for the following booleans:
   deleg_req_flag, mutual_req_flag, replay_det_req_flag,
   sequence_req_flag, conf_req_flag, and integ_req_flag.  Upon
   completion of security context negotiation, the initiator MUST verify
   the values of the deleg_state, mutual_state, replay_det_state,
   sequence_state, conf_avail, and integ_avail flags from the last call
   to GSS_Init_sec_context() corresponding to the requested flags.  If a
   flag was requested but is not available, and that feature is
   necessary for the appplication protocol, the initiator MUST destroy
   the security context and not use the security context for application
   traffic.

   Application protocol specifications citing this document MUST
   indicate which context flags are required for the application
   protocol.

   The acceptor receives as output the following booleans: deleg_state,
   mutual_state, replay_det_state, sequence_state, anon_state,
   trans_state, conf_avail, and integ_avail.  The acceptor MUST verify
   that any flags necessary for the application protocol are set.  If a
   necessary flag is not set, the acceptor MUST destroy the security
   context and not use the security context for application traffic.

3.1.  Using Partially Complete Security Contexts

   For mechanism/flag combinations that require multiple token
   exchanges, an application protocol may find it desirable to begin
   sending application data protected with GSS per-message operations
   while continuing to exchange security context tokens to complete the
   security context negotiation.  The prot_ready_state output value from
   GSS_Init_sec_context() and GSS_Accept_sec_context() indicates when
   per-message operations are avaialble.

   Applications requiring confidentiality and/or integrity protection
   from such messages MUST check the value of the conf_avail and/or
   integ_avail output flags from GSS_Init_sec_context()/
   GSS_Accept_sec_context() as well as the conf_state output of
   GSS_Wrap() (if GSS_Wrap() is used).

4.  References

4.1.  Normative References

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

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   [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.

   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401, February 2006.

4.2.  Informational References

   [RFC4462]  Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
              "Generic Security Service Application Program Interface
              (GSS-API) Authentication and Key Exchange for the Secure
              Shell (SSH) Protocol", RFC 4462, May 2006.

   [RFC3645]  Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
              and R. Hall, "Generic Security Service Algorithm for
              Secret Key Transaction Authentication for DNS (GSS-TSIG)",
              RFC 3645, October 2003.

   [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.

   [RFC4752]  Melnikov, A., "The Kerberos V5 ("GSSAPI") Simple
              Authentication and Security Layer (SASL) Mechanism", RFC
              4752, November 2006.

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, September 1997.

Appendix A.  Acknowledgements

   Acknowledgements.

Author's Address

   Benjamin Kaduk
   MIT Kerberos Consortium

   Email: kaduk@mit.edu

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