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Versions: (draft-howlett-eap-gss) 00 01 02 03 04 05 06 07 08 09 RFC 7055

Network Working Group                                    S. Hartman, Ed.
Internet-Draft                                         Painless Security
Intended status: Standards Track                              J. Howlett
Expires: January 12, 2012                                      JANET(UK)
                                                           July 11, 2011

     A GSS-API Mechanism for the Extensible Authentication Protocol


   This document defines protocols, procedures, and conventions to be
   employed by peers implementing the Generic Security Service
   Application Program Interface (GSS-API) when using the EAP mechanism.
   Through the GS2 family of mechanisms, these protocols also define how
   Simple Authentication and Security Layer (SASL, RFC 4422)
   applications use the Extensible Authentication Protocol.

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 12, 2012.

Copyright Notice

   Copyright (c) 2011 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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Authentication . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Secure Association Protocol  . . . . . . . . . . . . . . .  6
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  7
   3.  EAP Channel Binding and Naming . . . . . . . . . . . . . . . .  8
     3.1.  Mechanism Name Format  . . . . . . . . . . . . . . . . . .  8
     3.2.  Exported Mechanism Names . . . . . . . . . . . . . . . . . 10
     3.3.  Acceptor Name RADIUS AVP . . . . . . . . . . . . . . . . . 11
     3.4.  Proxy Verification of Acceptor Name  . . . . . . . . . . . 12
   4.  Selection of EAP Method  . . . . . . . . . . . . . . . . . . . 13
   5.  Context Tokens . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Mechanisms and Encryption Types  . . . . . . . . . . . . . 15
     5.2.  Processing received tokens . . . . . . . . . . . . . . . . 15
     5.3.  Error Subtokens  . . . . . . . . . . . . . . . . . . . . . 16
     5.4.  Initial State  . . . . . . . . . . . . . . . . . . . . . . 16
       5.4.1.  Vendor Subtoken  . . . . . . . . . . . . . . . . . . . 16
       5.4.2.  Acceptor Name Request  . . . . . . . . . . . . . . . . 17
       5.4.3.  Acceptor Name Response . . . . . . . . . . . . . . . . 17
     5.5.  Authenticate State . . . . . . . . . . . . . . . . . . . . 18
       5.5.1.  EAP Request Subtoken . . . . . . . . . . . . . . . . . 18
       5.5.2.  EAP Response Subtoken  . . . . . . . . . . . . . . . . 19
       5.5.3.  Example Token  . . . . . . . . . . . . . . . . . . . . 19
     5.6.  Extension State  . . . . . . . . . . . . . . . . . . . . . 19
       5.6.1.  Flags Subtoken . . . . . . . . . . . . . . . . . . . . 19
       5.6.2.  Channel Bindings Subtoken  . . . . . . . . . . . . . . 20
       5.6.3.  MIC Subtoken . . . . . . . . . . . . . . . . . . . . . 20
     5.7.  Context Options  . . . . . . . . . . . . . . . . . . . . . 21
   6.  Acceptor Services  . . . . . . . . . . . . . . . . . . . . . . 23
     6.1.  GSS-API Channel Binding  . . . . . . . . . . . . . . . . . 23
     6.2.  Per-message security . . . . . . . . . . . . . . . . . . . 23
     6.3.  Pseudo Random Function . . . . . . . . . . . . . . . . . . 24
   7.  Applicability Considerations . . . . . . . . . . . . . . . . . 25
   8.  Iana Considerations  . . . . . . . . . . . . . . . . . . . . . 26
     8.1.  RFC 4121 Token Identifiers . . . . . . . . . . . . . . . . 26
     8.2.  GSS EAP Subtoken Types . . . . . . . . . . . . . . . . . . 26
     8.3.  RADIUS Attribute Assignments . . . . . . . . . . . . . . . 26
     8.4.  GSS EAP Errors . . . . . . . . . . . . . . . . . . . . . . 26
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 30

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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32

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1.  Introduction

   The ABFAB architecture [I-D.lear-abfab-arch] describes an
   architecture for providing federated access management to
   applications using the Generic Security Services Application
   Programming Interface (GSS-API) [RFC2743] and Simple Authentication
   and Security Layers (SASL) [RFC4422].  This specification provides
   the core mechanism for bringing federated authentication to these

   The Extensible Authentication Protocol (EAP) [RFC3748] defines a
   framework for authenticating a network access client and server in
   order to gain access to a network.  A variety of different EAP
   methods are in wide use; one of EAP's strengths is that for most
   types of credentials in common use, there is an EAP method that
   permits the credential to be used.

   EAP is often used in conjunction with a backend authentication server
   via RADIUS [RFC3579] or Diameter [RFC4072].  In this mode, the NAS
   simply tunnels EAP packets over the backend authentication protocol
   to a home EAP/AAA server for the client.  After EAP succeeds, the
   backend authentication protocol is used to communicate key material
   to the NAS.  In this mode, the NAS need not be aware of or have any
   specific support for the EAP method used between the client and the
   home EAP server.  The client and EAP server share a credential that
   depends on the EAP method; the NAS and AAA server share a credential
   based on the backend authentication protocol in use.  The backend
   authentication server acts as a trusted third party enabling network
   access even though the client and NAS may not actually share any
   common authentication methods.  As described in the architecture
   document, using AAA proxies, this mode can be extended beyond one
   organization to provide federated authentication for network access.

   The GSS-API provides a generic framework for applications to use
   security services including authentication and per-message data
   security.  Between protocols that support GSS-API directly or
   protocols that support SASL [RFC4422], many application protocols can
   use GSS-API for security services.  However, with the exception of
   Kerberos [RFC4121], few GSS-API mechanisms are in wide use on the
   Internet.  While GSS-API permits an application to be written
   independent of the specific GSS-API mechanism in use, there is no
   facility to separate the server from the implementation of the
   mechanism as there is with EAP and backend authentication servers.

   The goal of this specification is to combine GSS-API's support for
   application protocols with EAP/AAA's support for common credential
   types and for authenticating to a server without requiring that
   server to specifically support the authentication method in use.  In

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   addition, this specification supports thearchitecture goal of
   transporting attributes about subjects to relying parties.  Together
   this combination will provide federated authentication and
   authorisation for GSS-API applications.

   This mechanism is a GSS-API mechanism that encapsulates an EAP
   conversation.  From the perspective of RFC 3748, this specification
   defines a new lower-layer protocol for EAP.  From the prospective of
   the application, this specification defines a new GSS-API mechanism.

   Section 1.3 of [RFC5247] outlines the typical conversation between
   EAP peers where an EAP key is derived:

   o  Phase 0: Discovery

   o  Phase 1: Authentication

   o  1a: EAP authentication

   o  1b: AAA Key Transport (optional)

   o  Phase 2: Secure Association Protocol

   o  2a: Unicast Secure Association

   o  2b: Multicast Secure Association (optional)

1.1.  Discovery

   GSS-API peers discover each other and discover support for GSS-API in
   an application-dependent mechanism.  SASL [RFC4422] describes how
   discovery of a particular SASL mechanism such as a GSS-API mechanism
   is conducted.  The Simple and Protected Negotiation mechanism
   (SPNEGO) [RFC4178] provides another approach for discovering what
   GSS-API mechanisms are available.  The specific approach used for
   discovery is out of scope for this mechanism.

1.2.  Authentication

   GSS-API authenticates a party called the GSS-API initiator to the
   GSS-API acceptor, optionally providing authentication of the acceptor
   to the initiator.  Authentication starts with a mechanism-specific
   message called a context token sent from the initiator to the
   acceptor.  The acceptor may respond, followed by the initiator, and
   so on until authentication succeeds or fails.  GSS-API context tokens
   are reliably delivered by the application using GSS-API.  The
   application is responsible for in-order delivery and retransmission.

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   EAP authentication can be started by either the peer or the
   authenticator, although the first EAP message travels from the
   authenticator to the peer.  The EAP peer maps onto the GSS-API
   initiator.  The role of the GSS-API acceptor is split between the EAP
   authenticator and the EAP server.  When these two entities are
   combined, the division resembles GSS-API acceptors in other
   mechanisms.  When a more typical deployment is used and there is a
   passthrough authenticator, most context establishment takes place on
   the EAP server and per-message operations take place on the
   authenticator.  EAP messages from the peer to the authenticator are
   called responses; messages from the authenticator to the peer are
   called requests.

   This specification permits a GSS-API peer to hand-off the processing
   of the EAP packets to a remote EAP server by using AAA protocols such
   as RADIUS, RadSec or Diameter.  In this case, the GSS-API peer acts
   as an EAP pass-through authenticator.  If EAP authentication is
   successful, and where the chosen EAP method supports key derivation,
   EAP keying material may also be derived.  If an AAA protocol is used,
   this can also be used to replicate the EAP Key from the EAP server to
   the EAP authenticator.

   See Section 5 for details of the authentication exchange.

1.3.  Secure Association Protocol

   After authentication succeeds, GSS-API provides a number of per-
   message security services that can be used:

      GSS_Wrap() provides integrity and optional confidentiality for a

      GSS_GetMIC() provides integrity protection for data sent
      independently of the GSS-API

      GSS_Pseudo_random [RFC4401] provides key derivation functionality.

   These services perform a function similar to security association
   protocols in network access.  Like security association protocols,
   these services need to be performed near the authenticator/acceptor
   even when a AAA protocol is used to separate the authenticator from
   the EAP server.  The key used for these per-message services is
   derived from the EAP key; the EAP peer and authenticator derive this
   key as a result of a successful EAP authentication.  In the case that
   the EAP authenticator is acting as a pass-through it obtains it via
   the AAA protocol.  See Section 6 for details.

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2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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3.  EAP Channel Binding and Naming

   EAP authenticates a realm.  The peer knows that it has exchanged
   authentication with an EAP server in a given realm.  Today, the peer
   does not typically know which NAS it is talking to securely.  That is
   often fine for network access.  However privileges to delegate to a
   chat server seem very different than privileges for a file server or
   trading site.  Also, an EAP peer knows the identity of the home
   realm, but perhaps not even the visited realm.

   In contrast, GSS-API takes a name for both the initiator and acceptor
   as inputs to the authentication process.  When mutual authentication
   is used, both parties are authenticated.  The granularity of these
   names is somewhat mechanism dependent.  In the case of the Kerberos
   mechanism, the acceptor name typically identifies both the protocol
   in use (such as IMAP) and the specific instance of the service being
   connected to.  The acceptor name almost always identifies the
   administrative domain providing service.

   An EAP GSS-API mechanism needs to provide GSS-API naming semantics in
   order to work with existing GSS-API applications.  EAP channel
   binding [I-D.ietf-emu-chbind] is used to provide GSS-API naming
   semantics.  Channel binding sends a set of attributes from the peer
   to the EAP server either as part of the EAP conversation or as part
   of a secure association protocol.  In addition, attributes are sent
   in the backend authentication protocol from the authenticator to the
   EAP server.  The EAP server confirms the consistency of these
   attributes.  Confirming attribute consistency also involves checking
   consistency against a local policy database as discussed below.  In
   particular, the peer sends the name of the acceptor it is
   authenticating to as part of channel binding.  The acceptor sends its
   full name as part of the backend authentication protocol.  The EAP
   server confirms consistency of the names.

   EAP channel binding is easily confused with a facility in GSS-API
   also called channel binding.  GSS-API channel binding provides
   protection against man-in-the-middle attacks when GSS-API is used as
   authentication inside some tunnel; it is similar to a facility called
   cryptographic binding in EAP.  See [RFC5056] for a discussion of the
   differences between these two facilities and Section 6.1 for how GSS-
   API channel binding is handled in this mechanism.

3.1.  Mechanism Name Format

   Before discussing how the initiator and acceptor names are validated
   in the AAA infrastructure, it is necessary to discuss what composes a
   name for an EAP GSS-API mechanism.  GSS-API permits several types of
   generic names to be imported using GSS_Import_name().  Once a

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   mechanism is chosen, these names are converted into a mechanism name
   form.  This section first discusses the mechanism name form and then
   discusses what name forms are supported.

   The string representation of the GSS-EAP mechanism name has the
   following ABNF [RFC5234] representation:

          ; Define name-string to handle escaping and prevent / and @
          empty =
          user-or-service = name-string
          host = empty/name-string
          realm = name-string
          service-specific = name-string
          service-specifics = service-specific 0*('/' service-specifics)
          name = user-or-service '/' host [ '/' service-specifics] [ '@'
                  realm ]

   The user-or-service component is the portion of a network access
   identifier (NAI) before the '@' symbol for initiator names and the
   service name from the registry of GSS-API host-based services in the
   case of acceptor names [GSS-IANA].  The host portion is empty for
   initiators and typically contains the domain name of the system on
   which an acceptor service is running.  Some services MAY require
   additional parameters to distinguish the entity being authenticated
   against.  Such parameters are encoded in the service-specifics
   portion of the name.  The EAP server MUST reject authentication of
   any acceptor name that has a non-empty service-specifics component
   unless the EAP server understands the service-specifics and
   authenticates them.  The interpretation of the service-specifics is
   scoped by the user-or-service portion.  The realm is the realm
   portion of a NAI for initiator names.  The realm is the
   administrative realm of a service for an acceptor name.

   The string representation of this name form is designed to be
   generally compatible with the string representation of Kerberos names
   defined in [RFC1964].

   The GSS_C_NT_USER_NAME form represents the name of an individual
   user.  From the standpoint of this mechanism it may take the form
   either of an undecorated user name or a network access identifier
   (NAI) [RFC4282].  The name is split into the part proceeding the
   realm which is the user-or-service portion of the mechanism name and
   the realm portion which is the realm portion of the mechanism name.

   The GSS_C_NT_HOSTBASED_SERVICE name form represents a service running
   on a host; it is textually represented as "HOST@SERVICE".  This name
   form is required by most SASL profiles and is used by many existing
   applications that use the Kerberos GSS-API mechanism.  While support

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   for this name form is critical, it presents an interesting challenge
   in terms of EAP channel binding.  Consider a case where the server
   communicates with a "server proxy," or a AAA server near the server.
   That server proxy communicates with the EAP server.  The EAP server
   and server proxy are in different administrative realms.  The server
   proxy is in a position to verify that the request comes from the
   indicated host.  However the EAP server cannot make this
   determination directly.  So, the EAP server needs to determine
   whether to trust the server proxy to verify the host portion of the
   acceptor name.  This trust decision depends both on the host name and
   the realm of the server proxy.  In effect, the EAP server decides
   whether to trust that the realm of the server proxy is the right
   realm for the given hostname and then makes a trust decision about
   the server proxy itself.  The same problem appears in Kerberos:
   there, clients decide what Kerberos realm to trust for a given
   hostname.  The service portion of this name is imported into the
   user-or-service portion of the mechanism name; the host portion is
   imported into the host portion of the mechanism name.  The realm
   portion is empty.  However, authentication will typically fail unless
   some AAA component indicates the realm to the EAP server.  If the
   application server knows its realm, then it should be indicated in
   the outgoing AAA request.  Otherwise, a proxy SHOULD add the realm.
   An alternate form of this name type MAY be used on acceptors; in this
   case the name form is "service" with no host component.  This is
   imported with the service as user-or-service and an empty host and
   realm portion.  This form is useful when a service is unsure which
   name an initiator knows it by.

   Sometimes, the client may know what AAA realm a particular host
   should belong to.  In this case it would be desirable to use a name
   form that included a service, host and realm.  Syntactically, this
   appears the same as the domain-based name discussed in [RFC5178], but
   the semantics are not similar enough semantics to use the same name

   If the null name type or the GSS_EAP_NT_EAP_NAME (oid XXX) is
   imported, then the string representation above should be directly
   imported.  Mechanisms MAY support the GSS_KRB5_NT_KRB5_PRINCIPAL_NAME
   name form with the OID {iso(1) member-body(2) United States(840)
   mit(113554) infosys(1) gssapi(2) krb5(2) krb5_name(1)}.

3.2.  Exported Mechanism Names

   GSS-API provides the GSS_Export_name call.  This call can be used to
   export the binary representation of a name.  This name form can be
   stored on access control lists for binary comparison.

   The exported name token MUST use the format described in section 3.2

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   of RFC 2743.  The mechanism specific portion of this name token is
   the string format of the mechanism name described in Section 3.1.

   RFC 2744 [RFC2744] places the requirement that the result of
   importing a name, canonicalizing it to a mechanism and then exporting
   it needs to be the same as importing that name, obtaining credentials
   for that principal, initiating a context with those credentials and
   exporting the name on the acceptor.  In practice, GSS mechanisms
   often, but not always meet this requirement.  For names expected to
   be used as initiator names, this requirement is met.  However,
   permitting empty host and realm components when importing hostbased
   services may make it possible for an imported name to differ from the
   exported name actually used.  Other mechanisms such as Kerberos have
   similar situations where imported and exported names may differ.

3.3.  Acceptor Name RADIUS AVP

   Currently, GSS-EAP uses a RADIUS vendor-specific attribute for
   carrying the acceptor name.  The VSA with enterprise ID 25622 is
   formatted as a VSA according to the recommendation in the RADIUS
   specification.  The following sub-attributes are defined:

   | Name                          | Attribute | Description           |
   | GSS-Acceptor-Service-Name     | 128       | user-or-service       |
   |                               |           | portion of name       |
   |                               |           |                       |
   | GSS-Acceptor-Host-Name        | 129       | host portion of name  |
   |                               |           |                       |
   | GSS-Acceptor-Service-specific | 130       | service-specifics     |
   |                               |           | portion of name       |
   |                               |           |                       |
   | GSS-Acceptor-Realm-Name       | 131       | Realm portion of name |

   All these items are strings.  See Section 3.1 for details of the
   values in a name.

   If RADIUS is used as an AAA transport, the acceptor MUST send the
   acceptor name in the VSA.

   The initiator MUST require that the EAP method in use support channel
   binding and MUST send the acceptor name as part of the channel
   binding data.  The client MUST NOT indicate mutual authentication
   unless all name elements that the client supplied are in a successful
   channel binding response.  For example, if the client supplied a
   hostname in channel binding data, the hostname MUST be in a

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   successful channel binding response.

   If an empty target name is supplied to GSS_Init_Sec_Context, the
   initiator MUST fail context establishment unless the acceptor
   supplies the acceptor name response Section 5.4.3.  If a null target
   name is supplied, the initiator MUST use this response to populate
   EAP channel bindings.

3.4.  Proxy Verification of Acceptor Name

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4.  Selection of EAP Method

   The specification currently describes a single GSS-API mechanism.
   The peer and authenticator exchange EAP messages.  The GSS-API
   mechanism specifies no constraints about what EAP method types are
   used; text in the specification says that negotiation of which EAP
   method to use happens at the EAP layer.

   EAP does not provide a facility for an EAP server to advertise what
   methods are available to a peer.  Instead, a server starts with its
   preferred method selection.  If the peer does not accept that method,
   the peer sends a NAK response containing the list of methods
   supported by the client.

   Providing multiple facilities to negotiate which security mechanism
   to use is undesirable.  Section 7.3 of [RFC4462]describes the problem
   referencing the SSH key exchange negotiation and the SPNEGO GSS-API
   mechanism.  If a client preferred an EAP method A, a non-EAP
   authentication mechanism B, and then an EAP method C, then the client
   would have to commit to using EAP before learning whether A is
   actually supported.  Such a client might end up using C when B is

   The standard solution to this problem is to perform all the
   negotiation at one layer.  In this case, rather than defining a
   single GSS-API mechanism, a family of mechanisms should be defined.
   Each mechanism corresponds to an EAP method.  The EAP method type
   should be part of the GSS-API OID.  Then, a GSS-API rather than EAP
   facility can be used for negotiation.

   Unfortunately, using a family of mechanisms has a number of problems.
   First, GSS-API assumes that both the initiator and acceptor know the
   entire set of mechanisms that are available.  Some negotiation
   mechanisms are driven by the client; others are driven by the server.
   With EAP GSS-API, the acceptor does not know what methods the EAP
   server implements.  The EAP server that is used depends on the
   identity of the client.  The best solution so far is to accept the
   disadvantages of multi-layer negotiation and commit to using EAP GSS-
   API before a specific EAP method.  This has two main disadvantages.
   First, authentication may fail when other methods might allow
   authentication to succeed.  Second, a non-optimal security mechanism
   may be chosen.

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5.  Context Tokens

   All context establishment tokens emitted by the EAP mechanism SHALL
   have the framing described in section 3.1 of [RFC2743], as
   illustrated by the following pseudo-ASN.1 structures:


            MechType ::= OBJECT IDENTIFIER
            -- representing EAP mechanism
            GSSAPI-Token ::=
            -- option indication (delegation, etc.) indicated within
            -- mechanism-specific token
                    thisMech MechType,
                    innerToken ANY DEFINED BY thisMech
                       -- contents mechanism-specific
                       -- ASN.1 structure not required

   The innerToken field starts with a 16-bit network byte order token
   type.  The remainder of the innerToken field is a set of type-length-
   value subtokens.  The following figure describes the structure of the
   inner token:

               | Position       | Description              |
               | 0..1           | token iD                 |
               |                |                          |
               | 2..5           | inner token type         |
               |                |                          |
               | 6..9           | length of first subtoken |
               |                |                          |
               | 10..10+n       | first subtoken body      |
               |                |                          |
               | 10+n+1..10+n+4 | second inner type        |

   The inner token continues with length, second body, and so forth.  If
          a type is present, its length and body must be present.

                         Structure of Inner Token

   The length does not include the length of the type field or the
   length field; the length only covers the value.

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   Tokens from the initiator to acceptor use an outer token type of 06
   01; tokens from acceptor to initiator use an outer token type of 06
   02.  These token types are registered in the registry of RFC 4121
   token types; see Section 8.1.

   See Section 5.5.3 for the encoding of a complete token.  The
   following sections discuss how mechanism OIDs are chosen and the
   state machine that defines what subtokens are permitted at each point
   in the context establishment process.

5.1.  Mechanisms and Encryption Types

   This mechanism family uses the security services of the Kerberos
   cryptographic framework [RFC3961].  As such, a particular encryption
   type needs to be chosen.  By convention, there is a single object
   identifier arc for the EAP family of GSS-API mechanisms.  A specific
   mechanism is chosen by adding the numeric Kerberos encryption type
   number to the root of this arc.  However, in order to register the
   SASL name, the specific usage with a given encryption type needs to
   be registered.  This document defines the eap-aes128-cts-hmac-sha1-96
   GSS-API mechanism.  XXX define an OID for that and use the right
   language to get that into the appropriate SASL registry.

5.2.  Processing received tokens

   Whenever a context token is received, the receiver performs the
   following checks.  First the receiver confirms the object identifier
   is that of the mechanism being used.  The receiver confirms that the
   token type field corresponds to the role of the peer: acceptors will
   only process initiator tokens and initiators will only process
   acceptor tokens.

   Implementations of this mechanism maintain a state machine for the
   context establishment process.  Both the initiator and acceptor start
   out in the initial state; see Section 5.4 for a description of this
   state.  Associated with each state are a set of subtoken types that
   are processed in that state and rules for processing these subtoken
   types.  The reciever examines the subtokens in order, processing any
   that are appropriate for the current state.

   A state may have a set of required subtoken types.  If a subtoken
   type is required by the current state but no subtoken of that type is
   present, then the context establishment MUST fail.

   The most-significant bit (0x80000000) in a subtoken type is the
   critical bit.  If a subtoken with this bit set in the type is
   received, the receiver MUST fail context establishment unless the
   subtoken is understood and processed for the current state.

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   The subtoken type MUST be unique within a given token.

5.3.  Error Subtokens

   The acceptor may always end the exchange by generating an error
   subtoken.  The error subtoken has the following format:

   | Pos    | Description                                              |
   | 0..3   | 0x80 00 00 01                                            |
   |        |                                                          |
   | 4..7   | length of error token                                    |
   |        |                                                          |
   | 8..11  | major status from RFC 2744 as 32-bit network byte order  |
   |        |                                                          |
   | 12..15 | GSS EAP error code as 32-bit network byte order; see     |
   |        | Section 8.4                                              |

   Initiators MUST ignore tokens of length greater than 8 for future
   extensibility.  As indicated, the error token is always marked

5.4.  Initial State

   Both the acceptor and initiator start the context establishment
   process in the initial state.

   The initiator sends a token to the acceptor.  It MAY be empty; no
   subtokens are required in this state.  Alternatively the initiator
   MAY include a vendor ID subtoken or an acceptor name subtoken.

   The acceptor responds to this message.  It MAY include an acceptor
   name info subtoken.  It MUST include a first eap request; this is an
   EAP request/identity message.

   The initiator and acceptor then transition to authenticate state.

5.4.1.  Vendor Subtoken

   The vendor ID token has type 0x0000000B and the following structure:

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                 | Pos         | Description            |
                 | 0..3        | 0x0000000B             |
                 |             |                        |
                 | 4..7        | length of vendor token |
                 |             |                        |
                 | 8..8+length | Vendor ID string       |

   The vendor ID string is an ASCII string describing the vendor of this
   implementation.  This string is unstructured and for debugging
   purposes only.

5.4.2.  Acceptor Name Request

   The acceptor name request token is sent from the initiator to the
   acceptor indicating that the initiator wishes a particular acceptor
   name.  This is similar to TLS Server Name Indication.  The structure
   is as follows:

                  | Pos  | Description                  |
                  | 0..3 | 0x00000002                   |
                  |      |                              |
                  | 4..7 | Length of subtoken           |
                  |      |                              |
                  | 8..n | string form of acceptor name |

5.4.3.  Acceptor Name Response

   The acceptor name response subtoken indicates what acceptor name is
   used.  This is useful for example if the initiator supplied no target
   name to context initialization.  This allows the initiator to learn
   the acceptor name.  EAP channel bindings will provide confirmation
   that the acceptor is accurately naming itself.

   this token is sent from the acceptor to initiator.  Typically this
   token would only be send if the acceptor name request is absent.

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                  | Pos  | Description                  |
                  | 0..3 | 0x00000003                   |
                  |      |                              |
                  | 4..7 | Length of subtoken           |
                  |      |                              |
                  | 8..n | string form of acceptor name |

5.5.  Authenticate State

   In this state, the acceptor sends EAP requests to the initiator and
   the initiator generates EAP responses.  The goal of the state is to
   perform a successful EAP authentication.  Since the acceptor sends an
   identity request at the end of the initial state, the first half-
   round-trip in this state is a response to that request from the

   The EAP conversation can end in a number of ways:

   o  If the EAP state machine generates an EAP success message, then
      EAP believes the authentication is successful.  The ACCEPTOR MUST
      confirm that a key has been derived.  The acceptor MUST confirm
      that this success indication is consistent with any protected
      result indication.  If any of these checks fail, the acceptor MUST
      send an error subtoken and fail the context establishment.  If
      these checks succeed the acceptor sends the success message using
      the EAP Request subtoken type and transitions to Extensions state.
      If the initiator receievs an EAP Success message, it confirms that
      a key has been derived and that the EAP success is consistent with
      any protected result indication.  If so, it transitions to
      Extensions state.  Otherwise, it returns an error.

   o  If the acceptor receives an EAP failure, then the acceptor sends
      this in the Eap Request subtoken type.  If the initiator receives
      an EAP Failure, it returns GSS failure.

   o  If there is some other error, the acceptor MAY return an error

5.5.1.  EAP Request Subtoken

   The EAP Request subtoken is sent from the acceptor to the initiator.
   This subtoken is always critical and is required in the
   authentication state.

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                  | Pos         | Description           |
                  | 0..3        | 0x8000005             |
                  |             |                       |
                  | 4..7        | Length of EAP message |
                  |             |                       |
                  | 8..8+length | EAP message           |

5.5.2.  EAP Response Subtoken

   This subtoken is required in authentication state messages from the
   initiator to the acceptor.  It is always critical.

                  | Pos         | Description           |
                  | 0..3        | 0x8000004             |
                  |             |                       |
                  | 4..7        | Length of EAP message |
                  |             |                       |
                  | 8..8+length | EAP message           |

5.5.3.  Example Token

   XXX fill in binary encoding of an example token

5.6.  Extension State

   After EAP success, the initiator sends a token to the acceptor
   including additional subtokens that negotiate optional features or
   provide channel binding.  The acceptor then responds with a token to
   the initiator.  When the acceptor produces its final token it returns
   GSS_S_COMPLETE; when the initiator consumes this token it returns
   GSS_S_COMPLETE if no errors are detected.

   Both the initiator and acceptor MUST include and verify a MIC
   subtoken to protect the extensions exchange.

5.6.1.  Flags Subtoken

   This token is sent to convey initiator flags to the acceptor.  The
   flags are sent as a 32-bit integer in network byte order.  The only
   flag defined so faris GSS_C_MUTUAL_FLAG, indicating that the acceptor
   successfully performed mutual authentication.  This flag has the
   value 0x2.

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                     | Pos   | Description           |
                     | 0..3  | 0x0000000C            |
                     |       |                       |
                     | 4..7  | length of flags token |
                     |       |                       |
                     | 8..11 | flags                 |

   Initiators MUST send 4 octets of flags.  Acceptors MUST ignore flag
   octets beyond the first 4 and MUST ignore flag bits other than

5.6.2.  Channel Bindings Subtoken

   This token is required and always critical.  It is sent from the
   initiator to the acceptor.  The contents of this token are an RFC
   4121 GSS wrap token containing the application data from the GSS
   channel bindings.

   | Pos         | Description                                         |
   | 0..3        | 0x80000006                                          |
   |             |                                                     |
   | 4..7        | length of wrap token                                |
   |             |                                                     |
   | 8..8+length | Wrap token containing channel binding application   |
   |             | data                                                |

   Again, only the application data is sent in the channel binding.  The
   initiator and acceptor addresses are ignored.

5.6.3.  MIC Subtoken

   This token MUST be the last subtoken in the tokens sent in Extensions
   state.  This token is sent both by the initiator and acceptor.

    | Pos         | Description                                      |
    | 0..3        | 0x8000000D for initiator 0x8000000E for acceptor |
    |             |                                                  |
    | 4..7        | Length of RFC 4121 MIC token                     |
    |             |                                                  |

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    | 8..8+length | RFC 4121 result of GSS_GetMIC                    |

   The input to GSS_GetMIC is as follows:

   1.  The DER-encoded object identifier of the mechanism in use; this
       value starts with 0x06 (the tag for object identifier).  When
       encoded in an RFC 2743 context token, the object identifier is
       preceeded by the tag and length for [Application 0] SEQUENCE.
       This tag and the length of the overall token is not inclded; only
       the tag, length and value of the object identifier itself.

   2.  A 16-bit token type in network byte order of the RFC 4121 token
       identifier (0x0601 for initiator, 0x0602 for acceptor).

   3.  For each subtoken other than the MIC subtoken itself:

       1.  A four octet subtoken type in network byte order

       2.  A four byte length in network byte order

       3.  Length octets of value from that subtoken

5.7.  Context Options

   GSS-API provides a number of optional per-context services requested
   by flags on the call to GSS_Init_sec_context and indicated as outputs
   from both GSS_Init_sec_context and GSS_Accept_sec_context.  This
   section describes how these services are handled.  Which services the
   client selects in the call to GSS_Init_sec_context controls what EAP
   methods MAY be used by the client.  Section 7.2 of RFC 3748 describes
   a set of security claims for EAP.  As described below, the selected
   GSS options place requirements on security claims that MUST be met.

   This GSS mechanism MUST only be used with EAP methods that provide
   dictionary attack resistance.

   The EAP method MUST support key derivation.  Integrity,
   confidentiality, sequencing and replay detection MUST be indicated in
   the output of GSS_Init_Sec_Context and GSS_Accept_Sec_context
   regardless of which services are requested.

   The PROT_READY service is never available with this mechanism.
   Implementations MUST NOT offer this flag or permit per-message
   security services to be used before context establishment.

   The EAP method MUST support mutual authentication and channel
   binding.  See Section 3.3 for details on what is required for

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   successful mutual authentication.  Regardless of whether mutual
   authentication is requested, the implementation MUST include channel
   bindings in the EAP authentication.  If mutual authentication is
   requested and successful mutual authentication takes place as defined
   in Section 3.3, the initiator MUST send a flags subtoken
   Section 5.6.1 in Extensions state.

   Open issue: handling of lifetime parameters.

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6.  Acceptor Services

   The context establishment process may be passed through to a EAP
   server via a backend authentication protocol.  However after the EAP
   authentication succeeds, security services are provided directly by
   the acceptor.

   This mechanism uses an RFC 3961 cryptographic key called the context
   root key (CRK).  The CRK is derived from the GMSK (GSS-API MSK).  The
   GMSK is the result of the random-to-key [RFC3961] operation consuming
   the appropriate number of bits from the EAP master session key.  For
   example for aes128-cts-hmac-sha1-96, the random-to-key operation
   consumes 16 octets of key material; thus the first 16 bytes of the
   master session key are input to random-to-key to form the GMSK.

   The CRK is derived from the GMSK using the following procedur

   Tn = pseudo-random(KMSK, n || "rfc4121-gss-eap")
   CRK = truncate(L, T1 || T2 || .. || Tn)
   L = output RFC 3961 key size

6.1.  GSS-API Channel Binding

   GSS-API channel binding [RFC5554] is a protected facility for
   exchanging a cryptographic name for an enclosing channel between the
   initiator and acceptor.  The initiator sends channel binding data and
   the acceptor confirms that channel binding data has been checked.

   The acceptor SHOULD accept any channel binding providing by the
   initiator if null channel bindings are passed into
   gss_accept_sec_context.  Protocols such as HTTP Negotiate depend on
   this behavior of some Kerberos implementations.  It is reasonable for
   the protocol to distinguish an acceptor ignoring channel bindings
   from an acceptor successfully validating them.  No facility is
   currently provided for an initiator implementation to expose this
   distinction to the initiator code.

   In this mechanism an extension option of type 0 with the critical bit
   set is sent from the initiator to the acceptor.  This option contains
   a GSS_Wrap token of the channel binding data passed into

6.2.  Per-message security

   The per-message tokens of section 4 of RFC 4121 are used.  The CRK
   SHALL be treated as the initiator sub-session key, the acceptor sub-
   session key and the ticket session key.

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6.3.  Pseudo Random Function

   The pseudo random function defined in [RFC4402] is used.

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7.  Applicability Considerations

   Section 1.3 of RFC 3748 provides the applicability statement for EAP.
   Among other constraints, EAP is scoped for use in network access.
   This specification anticipates using EAP beyond its current scope.
   The assumption is that some other document will discuss the issues
   surrounding the use of EAP for application authentication and expand
   EAP's applicability.  That document will likely enumerate
   considerations that a specific use of EAP for application
   authentication needs to handle.  Examples of such considerations
   might include the multi-layer negotiation issue, deciding when EAP or
   some other mechanism should be used, and so forth.  This section
   serves as a placeholder to discuss any such issues with regard to the
   use of EAP and GSS-API.

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8.  Iana Considerations

   This specification creates a number of IANA registries.

8.1.  RFC 4121 Token Identifiers

   A new top level registry titled "Kerberos GSS-API Mechanism
   Parameters," should be created.  This registry should be separate
   from the existing "Kerberos Parameters" registry.

   In this registry is a subregistry called "Kerberos GSS-API Token
   Identifiers"; the overall reference for this subregistry is section
   4.1 of RFC 4121.

8.2.  GSS EAP Subtoken Types

   This document creates a top level registry called "The Extensible
   Authentication Protocol Mechanism for the Generic Security Services
   Application Programming Interface (GSS-EAP) Parameters".  In any
   short form of that name, including any URI for this registry, it is
   important that the string GSS come before the string EAP; this will
   help to distinguish registries if EAP methods for performing GSS-API
   authenitication are ever defined.

   In this registry is a subregistry of subtoken types; identifiers are
   32-bit integers; the upper bit (0x80000000) is reserved as a critical
   flag and should not be indicated in the registration.  Assignments of
   GSS EAP subtoken types are made by expert review.  The expert is
   expected to require a public specification of the subtoken similar in
   detail to registrations given in this document.  The security of GSS-
   EAP depends on making sure that subtoken information has adequate
   protection and that the overall mechanism continues to be secure.
   Examining the security and architectural consistency of the proposed
   registration is the primary responsibility of the expert.

8.3.  RADIUS Attribute Assignments

   XXX register RADIUS attributes.

8.4.  GSS EAP Errors

   A new subregistry is created in the GSS EAP parameters registry
   titled "Error Codes".  XXX fill in minor statuses.

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9.  Security Considerations

   RFC 3748 discusses security issues surrounding EAP.  RFC 5247
   discusses the security and requirements surrounding key management
   that leverages the AAA infrastructure.  These documents are critical
   to the security analysis of this mechanism.

   RFC 2743 discusses generic security considerations for the GSS-API.
   RFC 4121 discusses security issues surrounding the specific per-
   message services used in this mechanism.

   As discussed in Section 4, this mechanism may introduce multiple
   layers of security negotiation into application protocols.  Multiple
   layer negotiations are vulnerable to a bid-down attack when a
   mechanism negotiated at the outer layer is preferred to some but not
   all mechanisms negotiated at the inner layer; see section 7.3 of
   [RFC4462] for an example.  One possible approach to mitigate this
   attack is to construct security policy such that the preference for
   all mechanisms negotiated in the inner layer falls between
   preferences for two outer layer mechanisms or falls at one end of the
   overall ranked preferences including both the inner and outer layer.
   Another approach is to only use this mechanism when it has
   specifically been selected for a given service.  The second approach
   is likely to be common in practice because one common deployment will
   involved an EAP supplicant interacting with a user to select a given
   identity.  Only when an identity is successfully chosen by the user
   will this mechanism be attempted.

   The security of this mechanism depends on the use and verification of
   EAP channel binding.  Today EAP channel binding is in very limited
   deployment.  If EAP channel binding is not used, then the system may
   be vulnerable to phishing attacks where a user is diverted from one
   service to another.  These attacks are possible with EAP today
   although not typically with common GSS-API mechanisms.  For this
   reason, implementations are required to implement and use EAP channel
   binding; see Section 3 for details.

   Every proxy in the AAA chain from the authenticator to the EAP server
   needs to be trusted to help verify channel bindings and to protect
   the integrity of key material.  GSS-API applications may be built to
   assume a trust model where the acceptor is directly responsible for
   authentication.  However, GSS-API is definitely used with trusted-
   third-party mechanisms such as Kerberos.

   RADIUS does provide a weak form of hop-by-hop confidentiality of key
   material based on using MD5 as a stream cipher.  Diameter can use TLS
   or IPsec but has no mandatory-to-implement confidentiality mechanism.
   Operationally, protecting key material as it is transported between

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   the IDP and RP is critical to per-message security and verification
   of GSS-API channel binding [RFC5056].  Mechanisms such as RADIUS over
   TLS [I-D.ietf-radext-radsec] provide significantly better protection
   of key material than the base RADIUS specification.

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10.  References

10.1.  Normative References

              IANA, "GSS-API Service Name Registry", <http://

              Hartman, S., Clancy, C., and K. Hoeper, "Channel Binding
              Support for EAP Methods", draft-ietf-emu-chbind-07 (work
              in progress), February 2011.

   [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.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [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.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

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

   [RFC4402]  Williams, N., "A Pseudo-Random Function (PRF) for the
              Kerberos V Generic Security Service Application Program
              Interface (GSS-API) Mechanism", RFC 4402, February 2006.

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

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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5554]  Williams, N., "Clarifications and Extensions to the
              Generic Security Service Application Program Interface
              (GSS-API) for the Use of Channel Bindings", RFC 5554,
              May 2009.

10.2.  Informative References

              Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
              "TLS encryption for RADIUS", draft-ietf-radext-radsec-09
              (work in progress), July 2011.

              Howlett, J., Hartman, S., Tschofenig, H., and E. Lear,
              "Application Bridging for Federated Access Beyond Web
              (ABFAB) Architecture", draft-lear-abfab-arch-02 (work in
              progress), March 2011.

   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
              RFC 1964, June 1996.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 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.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [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.

   [RFC5178]  Williams, N. and A. Melnikov, "Generic Security Service
              Application Program Interface (GSS-API)
              Internationalization and Domain-Based Service Names and

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              Name Type", RFC 5178, May 2008.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

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Authors' Addresses

   Sam Hartman (editor)
   Painless Security

   Email: hartmans-ietf@mit.edu

   Josh Howlett

   Email: josh.howlett@ja.net

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