The Simple and Protected Generic Security Service Application Program Interface (GSS-API) Negotiation Mechanism
RFC 4178

Network Working Group                                             L. Zhu
Request for Comments: 4178                                      P. Leach
Obsoletes: 2478                                            K. Jaganathan
Category: Standards Track                          Microsoft Corporation
                                                            W. Ingersoll
                                                        Sun Microsystems
                                                            October 2005

                       The Simple and Protected
    Generic Security Service Application Program Interface (GSS-API)
                         Negotiation Mechanism

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies a negotiation mechanism for the Generic
   Security Service Application Program Interface (GSS-API), which is
   described in RFC 2743.  GSS-API peers can use this negotiation
   mechanism to choose from a common set of security mechanisms.  If
   per-message integrity services are available on the established
   mechanism context, then the negotiation is protected against an
   attacker that forces the selection of a mechanism not desired by the
   peers.

   This mechanism replaces RFC 2478 in order to fix defects in that
   specification and to describe how to inter-operate with
   implementations of that specification that are commonly deployed on
   the Internet.

Zhu, et al.                 Standards Track                     [Page 1]
RFC 4178           The GSS-API Negotiation Mechanism        October 2005

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. Negotiation Protocol ............................................3
      3.1. Negotiation Description ....................................4
      3.2. Negotiation Procedure ......................................5
   4. Token Definitions ...............................................7
      4.1. Mechanism Types ............................................7
      4.2. Negotiation Tokens .........................................7
           4.2.1. negTokenInit ........................................8
           4.2.2. negTokenResp ........................................9
   5. Processing of mechListMIC ......................................10
   6. Extensibility ..................................................13
   7. Security Considerations ........................................13
   8. Acknowledgments ................................................14
   9. References .....................................................14
      9.1. Normative References ......................................14
      9.2. Informative References ....................................15
   Appendix A.  SPNEGO ASN.1 Module ..................................16
   Appendix B.  GSS-API Negotiation Support API ......................17
      B.1.  GSS_Set_neg_mechs Call ...................................17
      B.2.  GSS_Get_neg_mechs Call ...................................18
   Appendix C.  Changes since RFC 2478 ...............................18
   Appendix D.  mechListMIC Computation Example ......................20

1.  Introduction

   The GSS-API [RFC2743] provides a generic interface that can be
   layered atop different security mechanisms such that, if
   communicating peers acquire GSS-API credentials for the same security
   mechanism, then a security context may be established between them
   (subject to policy).  However, GSS-API does not prescribe the method
   by which GSS-API peers can establish whether they have a common
   security mechanism.

   The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism
   defined here is a pseudo security mechanism that enables GSS-API
   peers to determine in-band whether their credentials support a common
   set of one or more GSS-API security mechanisms; if so, it invokes the
   normal security context establishment for a selected common security
   mechanism.  This is most useful for applications that depend on GSS-
   API implementations and share multiple mechanisms between the peers.

   The SPNEGO mechanism negotiation is based on the following model: the
   initiator proposes a list of security mechanism(s), in decreasing
   preference order (favorite choice first), the acceptor (also known as