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Versions: (draft-eisler-nfsv4-ccm) 00 01 02 03

Network Working Group                                          M. Eisler
Internet-Draft                                   Network Appliance, Inc.
                                                             N. Williams
                                                  Sun Microsystems, Inc.
                                                                May 2003


               The Channel Conjunction Mechanism (CCM) for GSS
                        draft-ietf-nfsv4-ccm-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-
   Drafts as reference material or to cite them other than as
   "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

ABSTRACT

   This document describes a suite of new mechanisms under the GSS
   [RFC2743].  Some protocols, such as RPCSEC_GSS [RFC2203], use GSS to
   authenticate every message transfer, thereby incurring significant
   overhead due to the costs of cryptographic computation.  While
   hardware-based cryptographic accelerators can mitigate such overhead,
   it is more likely that acceleration will be available for lower layer
   protocols, such as IPsec [RFC2401] than for upper layer protocols
   like RPCSEC_GSS.  CCM can be used as a way to allow GSS mechanism-
   independent upper layer protocols to leverage the data stream
   protections of lower layer protocols, without the inconvenience of
   modifying the upper layer protocol to do so.

TABLE OF CONTENTS

   1.  Conventions Used in this Document . . . . . . . . . . . . . . . 3



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   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.1.  Example Application of CCM  . . . . . . . . . . . . . . . . . 4
   3.2.  A Suite of CCM Mechanisms . . . . . . . . . . . . . . . . . . 4
   3.3.  QOPs  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Token Formats . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.1.  Mechanism Object Identifier . . . . . . . . . . . . . . . . . 6
   4.2.  Tokens for the CCM-BIND mechanisms  . . . . . . . . . . . . . 6
   4.3.  Context Establishment Tokens for CCM-BIND Mechanisms  . . . . 6
   4.3.1.  Initial Context Token for CCM-BIND  . . . . . . . . . . . . 7
   4.3.2.  Subsequent Context Tokens for CCM-BIND  . . . . . . . . . . 7
   4.3.2.1.  Subsequent Initiator Context Initialization Token for
             CCM-BIND  . . . . . . . . . . . . . . . . . . . . . . . . 7
   4.3.2.2.  Response Token for CCM-BIND . . . . . . . . . . . . . . . 7
   4.4.  MIC Token for CCM-BIND  . . . . . . . . . . . . . . . . . . . 7
   4.5.  Wrap Token for CCM-BIND . . . . . . . . . . . . . . . . . . . 7
   4.6.  Other Tokens for CCM-BIND . . . . . . . . . . . . . . . . . . 8
   4.7.  Tokens for CCM-MIC  . . . . . . . . . . . . . . . . . . . . . 8
   4.8.  Context Establishment Tokens for CCM-MIC  . . . . . . . . . . 8
   4.8.1.  Initial Context Token for CCM-MIC . . . . . . . . . . . . . 8
   4.8.2.  Subsequent Context Tokens for CCM-MIC . . . . . . . . . . . 9
   4.8.2.1.  Subsequent Initiator Context Initialization Token for
             CCM-MIC . . . . . . . . . . . . . . . . . . . . . . . . . 9
   4.8.2.2.  Response Token for CCM-MIC  . . . . . . . . . . . . . .  10
   4.9.  MIC Token for CCM-MIC . . . . . . . . . . . . . . . . . . .  12
   4.10.  Wrap Token for CCM-MIC . . . . . . . . . . . . . . . . . .  12
   4.11.  Context Deletion Token . . . . . . . . . . . . . . . . . .  12
   4.12.  Exported Context Token . . . . . . . . . . . . . . . . . .  12
   4.13.  Other Tokens for CCM-MIC . . . . . . . . . . . . . . . . .  12
   5.  GSS Channel Bindings for Common Secure Channel Protocols  . .  12
   5.1.  GSS Channel Bindings for IKEv1  . . . . . . . . . . . . . .  13
   5.2.  GSS Channel Bindings for IKEv2  . . . . . . . . . . . . . .  13
   5.3.  GSS Channel Bindings for SSHv2  . . . . . . . . . . . . . .  13
   5.4.  GSS Channel Bindings for TLS  . . . . . . . . . . . . . . .  13
   6.  Use of Channel Bindings with CCM-BIND and SPKM  . . . . . . .  13
   7.  CCM-KEY and Anonymous IPsec . . . . . . . . . . . . . . . . .  14
   8.  Other Protocol Issues for CCM . . . . . . . . . . . . . . . .  14
   9.  Implementation Issues . . . . . . . . . . . . . . . . . . . .  15
   9.1.  Management of gss_targ_ctx  . . . . . . . . . . . . . . . .  15
   9.2.  CCM-BIND Versus CCM-MIC . . . . . . . . . . . . . . . . . .  15
   9.3.  Initiating CCM-MIC Contexts . . . . . . . . . . . . . . . .  16
   9.4.  Accepting CCM-MIC Contexts  . . . . . . . . . . . . . . . .  17
   9.5.  Non-Token Generating GSS-API Routines . . . . . . . . . . .  17
   9.6.  CCM-MIC and GSS_Delete_sec_context()  . . . . . . . . . . .  17
   9.7.  GSS Status Codes  . . . . . . . . . . . . . . . . . . . . .  18
   9.7.1.  Status Codes for CCM-BIND . . . . . . . . . . . . . . . .  18
   9.7.2.  Status Codes for CCM-MIC  . . . . . . . . . . . . . . . .  18
   9.7.2.1.  CCM-MIC: GSS_Accept_sec_context() status codes  . . . .  18
   9.7.2.2.  CCM-MIC: GSS_Init_sec_context() status codes  . . . . .  19


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   9.8.  Channel Bindings on the Target  . . . . . . . . . . . . . .  20
   10.  Advice for NFSv4 Implementors  . . . . . . . . . . . . . . .  21
   11.  Man in the Middle Attacks without CCM-KEY  . . . . . . . . .  21
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  22
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  25
   14.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  26
   15.  Normative References . . . . . . . . . . . . . . . . . . . .  27
   16.  Informative References . . . . . . . . . . . . . . . . . . .  28
   17.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  28
   18.  IPR Notices  . . . . . . . . . . . . . . . . . . . . . . . .  29
   19.  Copyright Notice . . . . . . . . . . . . . . . . . . . . . .  29

1.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Introduction

   The GSS framework provides a general means for authenticating clients
   and servers, as well as providing a general means for encrypting and
   integrity protecting data exchanged during a session.  GSS specifies
   formats for a set of tokens for authentication, integrity, and
   privacy.  The formats consist of a mechanism independent form, and a
   mechanism dependent form.  An example of a set of mechanism dependent
   forms is the Kerberos V5 mechanism definition [RFC1964].

   It is possible for a protocol to use GSS for one time authentication,
   or for per message authentication.  An example of the former is DAFS
   [DAFS].  An example of the latter is RPCSEC_GSS.  Obviously, it is
   more secure to authenticate each message.  On the other hand, it is
   also more expensive.  However, suppose the data stream of the upper
   layer protocol (the layer using GSS) is protected at a lower layer
   protocol from tampering, such as via a cryptographic checksum.  If
   so, it may not be necessary to additionally authenticate each message
   of the upper layer protocol.  Instead, it may suffice to use GSS to
   authenticate at the beginning of the upper layer protocol's session.

   To take advantage of one time authentication, existing consumers of
   GSS that authenticate exclusively on each message have to change.
   One way to change is to modify the protocol that is using GSS.  This
   has disadvantages including, introducing a protocol incompatibility,
   and effectively introducing another authentication paradigm.  Another
   way to change, is the basis of the proposal in this document:  the
   Channel Conjunction Mechanism (CCM).  CCM allows a GSS initiator and
   target to conjunct (bind) a secure session (or channel) at one
   protocol layer with (e.g.  IPsec) a security context of a non-CCM GSS
   mechanism.  Since CCM is yet another mechanism under the GSS, the


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   effect is that there are no modifications to the protocol the GSS
   consumer is using.

3.  Overview

   CCM is a "wrapper" mechanism over the set of all other GSS
   mechanisms.  When CCM creates a context, it invokes an underlying
   mechanism to create a child context.  CCM determines the underlying
   mechanism by examining the mechanism object identifier (OID) that it
   is called with.  The prefix will always be the OID of CCM, and the
   suffix will be the OID of the underlying mechanism.  The context
   initiation and acceptance entry points of CCM wrap the resulting the
   context tokens with a CCM header.

3.1.  Example Application of CCM

   Let us use RPCSEC_GSS and NFSv4 [RFC3530] as our example.  Basic
   understanding of the RPCSEC_GSS protocol is assumed.  If an NFSv4
   client uses the wrong security mechanism, the server returns the
   NFS4ERR_WRONGSEC error.  The client can then use NFSv4's SECINFO
   operation to ask the server which GSS mechanism to use.

   Let us say the client and server are using Kerberos V5 [RFC1964] to
   secure the traffic.  Suppose the TCP connection NFSv4 uses is secured
   and encrypted with IPsec.  It is therefore not necessary for
   NFSv4/RPCSEC_GSS to use integrity or privacy.  Fortunately,
   RPCSEC_GSS has an authentication mode, whereby only the header of
   each remote procedure call and response is integrity protected.  So,
   this minimizes the overhead somewhat, but there is still the cost of
   the headers being checksummed.  Since IPsec is protecting the
   connection, incurring even that minimal per remote procedure call
   overhead may not be necessary.

   Enter CCM.  The server detects that the connection is protected with
   IPsec.  Via SECINFO, the client is informed that it should use
   CCM/Kerberos V5.  Via the RPCSEC_GSS protocol, the server
   authenticates the end-user on the client with Kerberos V5.  The
   context tokens exchanged over RPCSEC_GSS are wrapped inside CCM
   tokens.

3.2.  A Suite of CCM Mechanisms


   CCM consists of a suite of GSS mechanisms.  CCM-NULL, CCM-ADDR, and
   CCM-KEY bind a GSS mechanism context to a secure channel via GSS
   channel bindings (see section 1.1.6 of RFC2743).  As noted in
   RFC2743, the purpose of channel bindings are to limit the scope
   within which an intercepted GSS context token can be used by an
   attacker.  CCM-KEY requires the use of channel bindings that are


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   derived from the secure channel's encryption keys.  CCM-ADDR requires
   the use of channel bindings that are derived from network addresses
   associated with the secure channel.  For environments where it is not
   feasible to use key-based channel bindings (e.g., the programming
   interfaces to get them are not available) or address-based channel
   bindings (e.g., the secure channel may be constructed over a path
   that requires the use of Network Address Translation), CCM-NULL is
   also defined.  CCM-NULL requires the use of null channel bindings.

   As discussed later in this document CCM-MIC exists for the purpose of
   optimizing the use of CCM.

   Implementations that claim compliance with this document are REQUIRED
   to implement CCM-KEY and CCM-MIC.  CCM-NULL and CCM-ADDR
   implementation are OPTIONAL.  Specifications that make normative
   references to CCM are free to mandate any subset of the suite CCM
   mechanisms.

   Because the GSS channel bindings to IPsec [RFC2401, RFC2409, IKEv2]
   have not been previously defined, and to ensure the usefulness of
   CCM, they are defined in this document.

   Also, the SPKM (1, 2 and 3) [RFC2025, RFC2847] mechanism is not clear
   on how channel bindings work with SPKM; a simple clarification is
   provided.

   CCM-MIC is intended to reduce the instances of full GSS context
   establishment to a per- {initiator principal, target} tuple.  CCM-MIC
   is used to establish a new context by proving that the initiator and
   target both have a previously established, unexpired GSS context; the
   proof is accomplished by exchanging MICs made with the previously
   established GSS context.  The CCM-MIC context creation entry points
   utilize the CCM_REAL_QOP (discussed later Overview section) in the
   value to generate and verify the MICs.  The type of channel bindings
   used when initiating CCM-MIC contexts MUST match that used when
   creating the previously established context.

3.3.  QOPs

   The CCM mechanisms provide two QOPs: the default QOP (0) that amounts
   to no protection, and a QOP (CCM_REAL_QOP, defined as value 1) that
   maps to the default QOP of the underlying GSS mechanism. The MIC
   tokens for CCM are zero length values.  When qop_req is 0, the wrap
   output tokens for CCM are equal to the input tokens.

        [ XXX - We assume that applications can cope with zero length
        MICs.  We propose that implementations try and find out.  We may
        revisit this by requiring a small (8-32 bits) MIC token.
        However, given that the C bindings of GSS allocates the MIC on


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        the heap, this could introduce an unnecessary and expensive
        allocation, we suggest applications be fixed to deal with zero
        length tokens.  ]

4.  Token Formats

   This section discusses the protocol visible tokens that GSS consumers
   exchange when using CCM.

4.1.  Mechanism Object Identifier

   There are two classes of Mechanism object identifiers (OIDs) for CCM.
   The first class consists of the channel binding specific OIDs, and
   will be referred to as the CCM-BIND mechanisms:

        {iso(1)identified-organization(3)dod(6)internet(1)security(5)
        mechanisms(5)ccm-family(TBD1)ccm-bind(1)ccm-null(1)}

        {iso(1)identified-organization(3)dod(6)internet(1)security(5)
        mechanisms(5)ccm-family(TBD1)ccm-bind(1)ccm-addr(2)}

        {iso(1)identified-organization(3)dod(6)internet(1)security(5)
        mechanisms(5)ccm-family(TBD1)ccm-bind(1)ccm-key(3)}

   The above three object identifiers are not complete mechanism OIDs.
   Complete CCM mechanism OIDs MUST consist of one of the above OIDs as
   prefix, followed by a real mechanism OID, such as that of Kerberos V5
   as defined in [RFC1964].  The second class consists of a single OID
   for the CCM-MIC mechanism.

        {iso(1)identified-organization(3)dod(6)internet(1)security(5)
        mechanisms(5)ccm-family(TBD1)ccm-mic(2)}

   The CCM-MIC OID is a complete mechanism OIDs, and is not a prefix.

   GSS defines the generic part of a token in ASN.1 encoding.  GSS does
   not require ASN.1 for the mechanism specific part of a token.

4.2.  Tokens for the CCM-BIND mechanisms


4.3.  Context Establishment Tokens for CCM-BIND Mechanisms

   The CCM-BIND context establishment tokens are simple wrappers around
   a real GSS mechanism's tokens.  The CCM-BIND mechanisms use the same
   number context token exchanges as required by they underlying real
   mechanism.




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4.3.1.  Initial Context Token for CCM-BIND

   GSS requires that the initial context token from the initiator to the
   target use the format as described in section 3.1 of RFC2743.  The
   format consists of a mechanism independent prefix, and a mechanism
   dependent suffix.  The mechanism independent token includes the
   MechType field.  The MechType MUST be equal to the OID of CCM-NULL,
   CCM-ADDR, or CCM-KEY.  The mechanism dependent portion of the Initial
   Context Token is always equal to the full InitialContextToken as
   returned by the underlying real mechanism.  This will include yet
   another MechType, which will have the underlying mechanism's OID.

4.3.2.  Subsequent Context Tokens for CCM-BIND

   A subsequent context token can be any subsequent context token from
   the initiator context initialization entry point, or any response
   context from the target's context acceptance entry point.  The GSS
   specification [RFC2743] does not prescribe any format.

4.3.2.1.  Subsequent Initiator Context Initialization Token for CCM-BIND

   A SubsequentContextToken for a CCM-BIND mechanism is equal to that
   returned by the initiator's context initialization routine of the
   underlying real mechanism.

4.3.2.2.  Response Token for CCM-BIND

   The response token for a CCM-BIND mechanism is equal to that returned
   by the target's context acceptance routine of the underlying real
   mechanism.

4.4.  MIC Token for CCM-BIND

   This token corresponds to the PerMsgToken type as defined in section
   3.1 of RFC2743.  When the qop_req is the default QOP (0), then the
   PerMsgToken is a quantity zero bits in length.  A programming API
   that calls GSS_GetMIC() with the default QOP will thus produce an
   octet string of zero length.

   When the qop_req is CCM_REAL_QOP (1), then PerMsgToken is whatever
   the underlying real mechanism returns from GSS_GetMIC() when passed
   the default QOP value (0).

4.5.  Wrap Token for CCM-BIND

   This token corresponds to the SealedMessage type as defined in
   section 3.1 of RFC2743.  When the qop_req is the default QOP (0),
   then the SealedMessage token is equal to the unmodified input to
   GSS_Wrap().


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   When the qop_req is CCM_REAL_QOP (1), then SealedMessage is whatever
   the underlying real mechanism returns from GSS_Wrap(), when passed
   the default QOP value (0).

4.6.  Other Tokens for CCM-BIND

   All other tokens are what the real underlying mechanism returns as a
   token.

4.7.  Tokens for CCM-MIC


4.8.  Context Establishment Tokens for CCM-MIC


4.8.1.  Initial Context Token for CCM-MIC

   The initial context token from the initiator to the target uses the
   format as described in section 3.1 of RFC2743.  The format consists
   of a mechanism independent prefix, and a mechanism dependent suffix.
   The mechanism independent token includes the MechType field.  The
   MechType MUST be equal to the OID of CCM-MIC.  RFC2743 refers to the
   mechanism dependent token as the innerContextToken.  This is the
   CCM-MIC specific token and is XDR [RFC1832] encoded as follows, using
   XDR description language:

      typedef struct {
              unsigned int ctx_sh_number;
              unsigned int rand;
      } CCM_nonce_t;

      typedef struct {
              CCM_nonce_t nonce;
              opaque gss_targ_ctx[20];
              opaque chan_bindings<>;
      } CCM_MIC_unwrapped_init_token_t;

      /*
       * The result of CCM_MIC_unwrapped_init_token_t after
       * Invoking GSS_GetMIC() on it.  qop_req is CCM_REAL_QOP, and
       * conf_flag is FALSE.
       */
      typedef opaque CCM_MIC_wrapped_init_token_t<>;


   Once an initiator has established an initial CCM context with a
   target via a CCM-BIND mechanism, the additional contexts can be
   established via the CCM-MIC mechanism.  The disadvantage of re-
   establishing additional contexts via the CCM-BIND route is that the


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   underlying mechanism context set up must be repeated, which can be
   expensive.  Whereas, the CCM-MIC mechanism route merely requires that
   the first CCM context's underlying mechanism context be available to
   produce an integrity checksum.  The initial context token for CCM-MIC
   is computed as follows.

   *    The gss_targ_ctx is computed as the SHA-1 checksum of the
        concatenation of SHA-1 [FIPS] checksums of the context tokens
        exchanged by the CCM-BIND mechanism in the order in which they
        were processed. For example, the context handle identifier for a
        CCM-KEY context exchange over a Kerberos V5 context exchange
        would be:  SHA-1( { SHA-1(CCM-KEY's initiator's token), SHA-
        1(CCM-KEY's target's token)) }.  Since the SHA-1 standard
        mandates a 160 bit output, (20 octets), gss_targ_ctx is a fixed
        length, 20 octet string.

   *    The subfield nonce.rand is set a random or pseudo random value.
        It is provided so as to ensure more variability of the the mic
        that GSS will calculate when CCM_MIC_unwrapped_init_token_t is
        GSS_Wrap()ed into CCM_MIC_wrapped_init_token_t.

   *    The subfield nonce.ctx_sh_number is the identifier of the CCM-
        MIC context relative to the CCM-BIND context (as identified by
        gss_targ_ctx) that the initiator is assigning.  The value for
        ctx_sh_number is selected by the initiator such that it is
        larger than any previous ctx_sh_number for the given
        gss_targ_ctx.  This way, the target need only keep track of the
        largest ctx_sh_number received.  Once ctx_sh_number has reached
        the maximum value for an unsigned 32 bit integer, the given
        gss_targ_ctx can no longer be used.

   *    Once the above fields are calculated, GSS_Wrap() is performed on
        the CCM_MIC_unwrapped_init_token_t value, to produce a
        CCM_MIC_wrapped_init_token_t value that becomes the initial
        context token to send to the target.

4.8.2.  Subsequent Context Tokens for CCM-MIC

   A subsequent context token can be any subsequent context token from
   the initiator context initialization entry point, or any response
   context from the target's context acceptance entry point.  The GSS
   specification [RFC2743] does not prescribe any format.

4.8.2.1.  Subsequent Initiator Context Initialization Token for CCM-MIC

   As CCM-MIC has only one round trip for context token exchange, there
   are no subsequent initiator context tokens.




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4.8.2.2.  Response Token for CCM-MIC

   The CCM response token, in XDR encoding is:

      typedef enum {
              CCM_OK = 0,

              /*
               * gss_targ_ctx was malformed.
               */
              CCM_ERR_HANDLE_MALFORMED = 1,

              /*
               * GSS context corresponding to gss_targ_ctx expired.
               */

              CCM_ERR_HANDLE_EXPIRED = 2,

              /*
               * gss_targ_ctx was not found.
               */
              CCM_ERR_HANDLE_NOT_FOUND = 3,

              /*
               * The ctx_sh_number has already been received
               * by the target.  Or the maximum ctx_sh_number has
               * been previously received.
               */
              CCM_ERR_TKN_REPLAY = 4,

              /*
               * Channel binding type mismatch between CCM-BIND context
               * and the CCM-MIC initial context.
               */
              CCM_ERR_CHAN_MISMATCH = 5,

              /*
               * The GSS_Unwrap() failed on initial context token
               */
              CCM_ERR_TKN_UNWRAP = 6,

              /*
               * The GSS_GetMIC() called failed on the target().
               */

              CCM_ERR_TKN_GET_MIC = 7,

              /*
               * The GSS_Wrap() failed on the initiator.  Not reported


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               * by target.
               */

              CCM_ERR_TKN_WRAP = 8,

              /*
               * The GSS_VerifyMIC() failed on the initiator.  Not
               * reported by target.
               */

              CCM_ERR_TKN_VER_MIC = 9

      } CCM_MIC_status_t;

      /*
       * GSS errors returned by the underlying mechanism
       */
      typedef struct {
              unsigned int gss_major;
              unsigned int gss_minor;
      } CCM_MIC_real_gss_err_t;

      /*
       * The response context token for CCM-MIC.
       */
      typedef union switch (CCM_MIC_status status) {
              case CCM_OK:
                      opaque mic_init_tkn<>;
              case CCM_ERR_TKN_UNWRAP:
              case CCM_ERR_TKN_GET_MIC:
                      CCM_real_gss_err_t gss_err;
              default:
                      void;
      } CCM_MIC_resp_t;


   If a value of the status field is CCM_OK, then the CCM-MIC context
   has been established on the target.  The field mic_init_tkn is equal
   to the output of GSS_GetMIC() (qop_req is CCM_REAL_QOP (1)) on the
   entire and original token that came from the initiator.  In other
   words, the input_token value to GSS_Accept_sec_context().  This is
   necessary because the inner token from the initiator is wrapped with
   GSS_Wrap(), and thus contains a MIC.  If we performed GSS_GetMIC() on
   the unwrapped inner token, then for some underlying mechanisms, we
   would end up with a mic_init_tkn in the response token equal to what
   was embedded in the request token.

   If the status field is CCM_ERR_TKN_UNWRAP or CCM_ERR_TKN_GET_MIC,
   then gss_err.gss_major and gss_err.minor are set to the major and


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   minor GSS statuses as returned by GSS_Unwrap() or GSS_GetMIC().  The
   values for the gss_major field are as defined in [RFC2744].  The
   values for the gss_minor field are both mechanism dependent and
   mechanism implemented dependent.  They are nonetheless potentially
   useful as debugging aids.

4.9.  MIC Token for CCM-MIC

   The MIC token for CCM-MIC is the same as the MIC token for CCM-BIND.

4.10.  Wrap Token for CCM-MIC

   The wrap token for CCM-MIC is the same as the wrap token for CCM-
   BIND.

4.11.  Context Deletion Token

   The context deletion token for CCM-MIC is a zero length token.

4.12.  Exported Context Token

   The Exported context token for CCM-MIC is implementation defined.

4.13.  Other Tokens for CCM-MIC

   All other tokens are the same as corresponding tokens for CCM-BIND.

5.  GSS Channel Bindings for Common Secure Channel Protocols

   For CCM-KEY to be useful and secure, CCM-KEY MUST be used in
   conjunction with channel bindings to bind GSS authentication at the
   application layer to a lower layer in the network that provides
   cryptographic session protection.

   To date only network address type channel bindings have been defined
   for GSS [RFC2743].  But the GSS also allows for channel bindings of
   "transformations of encryption keys" [RFC2743].  The actual generic
   representation of channel bindings is defined in the C-Bindings of
   the GSS-API [RFC2744].

   Modern secure transports generally define some quantity or quantities
   which are either derived from the session keys (or from key exchange
   material) or which are securely exchanged in such a way that both
   peers of any one connection or association can arrive at the same
   derived quantities, while a man-in-the-middle cannot make these
   quantities match for both peers.  Signatures of these quantities can
   be exchanged to prove that there is no man-in-the-middle (because a
   man-in-the-middle cannot cause them to be the same for both peers).
   These quantities correspond to what the GSS terms "transformations of


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   encryption keys" that are referred to in [RFC2743].

   Where a secure transport clearly defines a session identifier
   securely derived from session keys or key exchange material, that
   identifier MUST be used as the GSS channel bindings data when CCM-
   BIND is used to bind GSS to that transport.

   This section defines four forms of "transformations of encryption
   keys," one for IKEv1, one for IKEv2, one for SSHv2 and one for TLS.
   All four forms are to be used as the value of the "application_data"
   field of the gss_channel_bindings_struct type defined in [RFC2744].

5.1.  GSS Channel Bindings for IKEv1

   IKEv1 does not define a single value which can be used -- by both the
   IPsec initiator and responder of an IPsec SA -- to identify a given
   SA.  IKEv1 does, however, define public values derived from the IKEv1
   key exchange: 'HASH_I' and 'HASH_R'.

   For IKEv1, the GSS channel bindings data to use with CCM-KEY consists
   of the concatenation of HASH_I and HASH_R octet string values, in
   that order, from the underlying IPsec session being bound to [IKEv1].

5.2.  GSS Channel Bindings for IKEv2

   IKEv2 peers assign and exchange 8-octet "Security Parameters Index"
   (SPI) values, such that a pair of SPIs suffices to uniquely identify
   a given IPsec security association.

   For IKEv2 the GSS channel bindings data to use with CCM-KEY is simply
   the concatenation of the SPIi and SPIr values, in that order, which
   identify the IPsec SA being bound to.

5.3.  GSS Channel Bindings for SSHv2

   SSHv2 defines a session ID derived from the initial key exchange of
   an SSHv2 connection; this value is not secret and is the same for
   both the client and the server for any given connection.

   For SSHv2 the GSS channel bindings data for use with CCM-KEY consists
   of the SSHv2 session ID.

5.4.  GSS Channel Bindings for TLS

   XXX - This section is To Be Defined.

6.  Use of Channel Bindings with CCM-BIND and SPKM

   Whereas the Kerberos V5 mechanism specification [RFC1964] is quite


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   detailed with respect to the use of GSS channel bindings, the same is
   not true for SPKM, which merely provides a field named "channelId"
   for passing channel bindings data, as octet strings, from initiators
   to acceptors.  No interpretation is given in RFC2025 for the value of
   the channelId field.  Therefore SPKM requires some clarification to
   be usable with channel bindings and CCM-KEY: The channelId field of
   SPKM Context-Data ASN.1 structure MUST be set to the checksum of the
   channel bindings data that is defined for the Kerberos V5 mechanism
   [RFC1964], using SHA-1 instead of MD5 as the hash algorithm.

   [Note:  This checksum can be computed independently of the GSS
   language bindings used by the application, even though RFC1964
   references the C-Bindings of the GSS-API [RFC2744] in the
   construction of this checksum (read the RFC1964 text carefully).]

7.  CCM-KEY and Anonymous IPsec

   For sites that do not use IPsec, but use Kerberos V5, SPKM, or
   LIPKEY, deploying IPsec, a PKI infrastructure and certificates for
   use with IKE may prove quite difficult to deploy just for secure
   application (e.g., NFS) performance improvements.  Such sites could
   avoid the need to deploy a PKI and certificates to all clients and
   server by using "anonymous IPsec" for the application (e.g., NFS
   with/ RPCSEC_GSS) and CCM-KEY.

   Though there is no such thing as "anonymous IPsec," the effect can be
   achieved by using self-signed certificates.

   By using anonymous IPsec with the application and CCM-KEY, the full
   benefit of offloading session cryptography from upper layer protocol
   layer to the IP layer can be had without having to deploy an
   authentication infrastructure for IPsec.

8.  Other Protocol Issues for CCM

   CCM-BIND is a trivial mechanism, and normally will return the same
   major status code as the underlying real mechanism, including
   GSS_S_COMPLETE as returned by GSS_Init_sec_context().  However, the
   first time GSS_Init_sec_context is called on a CCM-BIND mechanism, if
   the underlying real mechanism returns GSS_S_COMPLETE, CCM-BIND's
   GSS_Init_sec_context() entry point MUST return GSS_S_CONTINUE_NEEDED
   to the caller.  This way, the initiator will receive another context
   token from the target, even if the underlying real mechanism context
   set up is done.  The CCM-BIND initiator will need to record state
   that indicates that the underlying mechanism has reached a completely
   established state (and so is uninterested in any token the target
   returns).  This way, the initiator can process every token produced
   by the target's GSS_Accept_sec_context() routine and so calculate
   gss_targ_ctx  value that matches that of the target.


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9.  Implementation Issues

   The "over the wire" aspects of CCM have been completely specified.
   However, GSS is usually implemented as an Application Programming
   Interface (the GSS-API), and security mechanisms are often
   implemented as modules that are plugged into the GSS-API.  It is
   useful to discuss implementation issues and workable resolutions.
   The reader is cautioned that the authors have not implemented CCM, so
   what follows is at best a series of educated guesses.

9.1.  Management of gss_targ_ctx

   The gss_targ_ctx value is computed by the initiator and target based
   on SHA-1 computations of the CCM-BIND context tokens.  There is a
   space/time trade off between the initiator and target storing the
   sequence of context tokens until needed by CCM-BIND, versus computing
   the SHA-1 checksums and then disposing of the context tokens when
   CCM-BIND no longer needs them.  If it is likely there will be CCM-MIC
   contexts created for the CCM-BIND context, and if the sequence of
   context tokens requires more space than a 20 octet SHA-1 value, then
   the tradeoff is obvious.

   Since the bit space of all possible sequences of CCM-BIND context
   tokens is larger than the 160 bit space of possible SHA-1 checksums,
   in theory two or more different CCM-BIND contexts will produce
   produce the same SHA-1 context, and thus for CCM-MIC context
   initiation, there will be ambiguity as to which CCM-BIND context the
   initiator is binding to.  The target can resolve this ambiguity by
   attempting to unwrap the inner context token from the CCM-MIC
   initiator for each matching CCM-BIND context.  In theory no more than
   one GSS_Unwrap() attempt for each matching CCM-BIND context will
   succeed.  If multiple succeed, then clearly the underlying mechanism
   is doing poor job at generating "unique" session keys.  CCM
   implementations that detect this SHOULD log it so that the problem in
   the underlying mechanism can be discovered and fixed.

9.2.  CCM-BIND Versus CCM-MIC

   The first time a CCM context is needed between an principal on the
   initiator and a principal on the target, the initiator has no choice
   but to create an underlying mechanism context via a CCM-BIND context
   token exchange.  Once that is done, subsequent CCM contexts between
   the initiator and target can be created via CCM-MIC.  CCM-MIC context
   establishment is better because no more than one round trip is
   necessary to establish a CCM context, and because the overhead of the
   establishing a real, underlying mechanism context is avoided.





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9.3.  Initiating CCM-MIC Contexts

   The key issue is how to associate an CCM-BIND established security
   context with a new CCM-MIC context, There no existing interfaces
   defined in the GSS-API for associating one GSS context with another.
   This then is the key issue for implementations of CCM-MIC.

   We will assume that GSS-API implementation is in the C programming
   language and therefore the GSS-API C bindings [RFC2744] are being
   used.  The CCM mechanism implementation will have a table that maps
   gss_targ_ctx values to gss_ctx_id_t values (see section 5.19 of
   [RFC2744]).  The latter are GSS-API context handles as returned by
   gss_init_sec_context().  The former are the context handles as
   returned in a response token from the CCM target.  In addition, each
   CCM context has a reference to its underlying mechanism context.

   Let us suppose the application decides it will use CCM-MIC.  CCM-MIC
   has a well known mechanism OID which the application can check for.
   The point where the initiator calls GSS_Init_sec_context(), is a
   logical place to associate an existing CCM-BIND context with a new
   CCM-MIC context.  Here is where special CCM handling is necessary in
   order to associate a security context with a CCM context.  We discuss
   several approaches.

   1.   The first approach is for the CCM-MIC's GSS_Init_sec_context()
        entry point to pass as the claimant_cred_handle the
        output_context_handle as returned by GSS_Init_sec_context() for
        a previously created CCM-BIND context.  Such an approach may
        work well with applications that normally pass
        GSS_C_NO_CREDENTIAL as the claimant_cred_handle.

   2.   The second approach derives from the observation that normally,
        the first time GSS_Init_sec_context() is called, the input_token
        field is NULL and the initial context_handle (type gss_ctx_id_t)
        is also NULL.  The input_token is supposed to be the token
        received from the target's context acceptance routine, which has
        the XDR type CCM_MIC_resp_t.  Overloading the input_token is one
        way.  By passing in a non-null input_token, and a NULL pointer
        to the context_handle (using the C bindings calling conventions
        for gss_init_sec_context()), this will tell the CCM-MIC
        initiator that input_token containing information to to
        associate a new CCM-MIC context with an existing CCM-BIND
        context.  In the C programming language, we could thus have have
        input_token containing:

           typedef struct {
                gss_ctx_id_t context_ptr;
           } CCM_MIC_initiator_bootstrap_t;



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        The CCM entry point for creating contexts on the initiator side
        would, if being called for the first time (*context_handle is
        NULL), interpret the presence of the input token with an invalid
        status as the CCM_MIC_initiator_bootstrap_t.  It would use
        context_ptr to lookup the  corresponding gss_targ_ctx in the
        aforementioned gss_ctx_id_t to gss_targ_ctx mapping table.  It
        would then proceed to generate an output token encoded as XDR
        type CCM_MIC_init_t, described in the section entitled "Initial
        Context Token for CCM-MIC".

   Regardless of the approach taken, the first time GSS_Init_sec_context
   is called, assuming success, it will return GSS_S_CONTINUE_NEEDED,
   because it will need to process the token returned by the target.
   The second time it is called, assuming success, it will return
   GSS_S_COMPLETE.

9.4.  Accepting CCM-MIC Contexts

   The CCM-MIC target receives an opaque gss_targ_ctx value as part of
   the mechanism dependent part of the initial context token.
   Originally, this opaque handle came from the target as a result of
   previously creating a context via a CCM-BIND context exchange.  If
   the opaque handle is still valid, then the target can easily
   determine the original CCM-BIND context, and from that, the CCM-BIND
   mechanism's context.  With the underlying context, GSS_VerifyMIC()
   can be invoked (with a qop_req of CCM_REAL_QOP (1)) to verify the
   mic_nonce of the input token, and GSS_GetMIC() can be used to
   generate the mic_init_tkn field of the output token.  By comparing
   the ctx_sh_number in the initiator's token with highest value
   recorded by the target, the target takes care to ensure that
   initiator has not replayed a short token.

9.5.  Non-Token Generating GSS-API Routines

   Since the CCM module will record the underlying mechanism's context
   pointer in its internal data structures, this provides a simple
   answer to what to do when GSS-API is invoked on a CCM context that
   does not generate any tokens for the GSS peer.  When CCM is called
   for such an operation, it simply re-invokes the GSS-API call, but on
   the recorded underlying context.

9.6.  CCM-MIC and GSS_Delete_sec_context()

   The CCM-MIC entry point for GSS_Delete_sec_context() should not call
   the underlying mechanism's GSS_Delete_sec_context() routine.  If it
   did, this would effectively delete all CCM-MIC context's associating
   with the same underlying mechanism.




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9.7.  GSS Status Codes


9.7.1.  Status Codes for CCM-BIND

   CCM-BIND mechanisms define no minor status codes.  If the underlying
   mechanism is not available, then a CCM-BIND mechanism will return
   GSS_S_BAD_MECH and minor status of zero.  Otherwise, it will return
   whatever major and minor status codes the underlying mechanism
   returns.

9.7.2.  Status Codes for CCM-MIC

   Generally, major and minor status codes for will be whatever major
   and minor status codes the underlying CCM-BIND mechanism returns.
   However, for GSS_Init_sec_context() and GSS_Accept_sec_context(),
   this is not the case because the those operations are invoking
   routines (GSS_Wrap() and GSS_Unwrap()) that have major statuses that
   are not subsets of the legal status returns from
   GSS_Init_sec_context() and GSS_Accept_sec_context().  Moreover, in
   some cases for GSS_Init_sec_context(), the minor and major status are
   driven from the target, and the target's codes will not always be
   among the legal set for GSS_Init_sec_context().

9.7.2.1.  CCM-MIC: GSS_Accept_sec_context() status codes

   The minor status code for GSS_Accept_sec_context is always from the
   set defined in the CCM_MIC_status_t type.  If GSS_Unwrap() reports a
   major status failure, then the minor status will be
   CCM_ERR_TKN_UNWRAP, and the reported major status will what
   GSS_Unwrap() reports, with exceptions as according to the following
   table:
      major status code from GSS_Unwrap      major status code reported
                                             by GSS_Accept_sec_context
                                             to caller.
      -----------------------------------------------------------------
      GSS_S_BAD_SIG                          GSS_S_BAD_SIG
      GSS_S_CONTEXT_EXPIRED                  GSS_S_DEFECTIVE_TOKEN
      GSS_S_GAP_TOKEN                        GSS_S_DEFECTIVE_TOKEN
      GSS_S_UNSEQ_TOKEN                      GSS_S_DUPLICATE_TOKEN


   If GSS_GetMIC() reports a major status failure, then the minor status
   will be CCM_ERR_TKN_GET_MIC, and the reported major status will be
   what GSS_GetMIC() reports, with exceptions as according to the
   following table:
      major status code from GSS_GetMIC      major status code reported
                                             by GSS_Accept_sec_context()
                                             to caller.


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      ------------------------------------------------------------------
      GSS_S_BAD_QOP                          GSS_S_FAILURE
      GSS_S_CONTEXT_EXPIRED                  GSS_S_DEFECTIVE_TOKEN

   The target will always report the actual GSS major and minor codes to
   the initiator.  The initiator will map the GSS major code as
   described in the next subsection.

9.7.2.2.  CCM-MIC: GSS_Init_sec_context() status codes

   The minor status code for GSS_Init_sec_context is always from the set
   defined in the CCM_MIC_status_t type.

   If the minor status code came from the target, then that will always
   be what GSS_Init_sec_context() reports.  The most of the minor codes
   from the target are to be mapped to the major status code as follows:
      minor status code          major status code
      from target                reported to caller of
                                 GSS_Init_sec_context()
      ----------------------------------------------------
      CCM_OK                     GSS_S_COMPLETE
      CCM_ERR_HANDLE_MALFORMED   GSS_S_DEFECTIVE_TOKEN
      CCM_ERR_HANDLE_EXPIRED     GSS_S_CREDENTIALS_EXPIRED
      CCM_ERR_HANDLE_NOT_FOUND   GSS_S_CREDENTIALS_EXPIRED
      CCM_ERR_TKN_REPLAY         GSS_S_DUPLICATE_TOKEN
      CCM_ERR_CHAN_MISMATCH      GSS_S_BAD_BINDINGS
      CCM_ERR_TKN_WRAP           GSS_S_FAILURE
      CCM_ERR_TKN_VER_MIC        GSS_S_FAILURE

   Note that in the above table CCM_ERR_TKN_WRAP and CCM_ERR_TKN_VER_MIC
   MUST not be returned by the target.  But if they are, then the
   initiator reports GSS_S_FAILURE.

   If the minor status code from the target is CCM_ERR_TKN_UNWRAP or
   CCM_ERR_TKN_GET_MIC, then the target will also report the major
   status code it got from GSS_Unwrap() or GSS_GetMIC().  The major
   status from the target will be be reported by GSS_Init_sec_context()
   to its caller with exceptions as according to the following table:
      major status code from target          major status code reported
                                             by GSS_Init_sec_context()
                                             to caller
      -----------------------------------------------------------------
      GSS_S_BAD_QOP                          GSS_S_FAILURE
      GSS_S_BAD_SIG                          GSS_S_BAD_SIG
      GSS_S_CONTEXT_EXPIRED                  GSS_S_DEFECTIVE_TOKEN
      GSS_S_GAP_TOKEN                        GSS_S_DEFECTIVE_TOKEN
      GSS_S_UNSEQ_TOKEN                      GSS_S_DUPLICATE_TOKEN




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   If GSS_Wrap() fails on the initiator, then the minor status will be
   CCM_ERR_TKN_WRAP, and the major status will what GSS_Wrap() reports,
   with exceptions as according to the following table:
      major status code from GSS_Wrap      major status code reported
                                           by GSS_Init_sec_context()
                                           to caller
      ---------------------------------------------------------------
      GSS_S_CONTEXT_EXPIRED                GSS_S_DEFECTIVE_TOKEN
                                              or
                                           GSS_S_DEFECTIVE_CREDENTIAL

      GSS_S_BAD_QOP                        GSS_S_FAILURE

   If GSS_VerifyMIC() fails on the initiator, then the minor status will
   be CCM_ERR_TKN_VER_MIC, and the major status will what
   GSS_VerifyMIC() reports, with exceptions as according to the
   following table:
      major status code from GSS_VerifyMIC  major status code reported
                                            by GSS_Init_sec_context()
                                            to caller
      ---------------------------------------------------------------
      GSS_S_CONTEXT_EXPIRED                 GSS_S_DEFECTIVE_TOKEN
      GSS_S_GAP_TOKEN                       GSS_S_DEFECTIVE_TOKEN
      GSS_S_UNSEQ_TOKEN                     GSS_S_DUPLICATE_TOKEN

9.8.  Channel Bindings on the Target

   When an application invokes GSS_Accept_sec_context() on a CCM token,
   it won't know if channel bindings are required or not.  Of course, it
   could inspect the OID of the input_token and determine the channel
   bindings directly if it is a CCM-BIND token, but normally
   applications will not parse the mechanism OID in an input token.  And
   in any case, such inspection for a CCM-MIC token provides no
   information about channel bindings to the target application.

   The application on the target will have to try
   GSS_Accept_sec_context() without channel bindings.  If the target CCM
   mechanism requires channel bindings (as indicated by the
   GSS_S_BAD_BINDINGS), then the application will have to re-invoke
   GSS_Accept_sec_context() with the right channel bindings.  If the
   channel bindings are the wrong type, then the CCM mechanism will
   indicate GSS_S_BAD_BINDINGS again.  The application will have to
   iterate through all the valid types of bindings.  The application can
   avoid this iteration if the bindings includes both, address and key
   bindings if at all possible.  The CCM mechanisms should use only
   those parts of the application-provided bindings that they care for.





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10.  Advice for NFSv4 Implementors

   The NFSv4.0 specification does not mandate CCM, so clients and
   servers should not insist on its use.  When a server wants a client
   to try to use CCM, it can return a NFS4ERR_WRONGSEC error to the
   client.  The client will then follow up with a SECINFO request.  The
   response to the SECINFO request should list first the CCM-BIND
   mechanisms it supports, second the CCM-MIC mechanism (if supported),
   and finally, the conventional security flavors the server will accept
   for access to file object.  If the client supports CCM, it will use
   it.  Otherwise, it will have to stick with a conventional flavor.

   Since the CCM-MIC OID is general, rather than a separate CCM-MIC OID
   for every real mechanism, the NFS server will have be careful to make
   sure that a CCM-MIC context is authorized access an object.  For
   example suppose /export is exported such that SPKM-3 is the
   authorized underlying mechanism, and CCM-NULL + SPKM-3 and CCM-MIC
   are similarly authorized to access /export.  Suppose CCM-NULL is
   created over a Kerberos V5 context, and then CCM-MIC is used to
   derived a context from the CCM-NULL context.  If the NFS server
   simply records that the OID of CCM-MIC is authorized to access
   /export, then Kerberos V5 authenticated users will be mistakenly
   allowed access.  Instead, the server needs to examine what context
   the CCM-MIC context is associated with, and check that context's OID
   against the authorized list of OIDs for /export.

11.  Man in the Middle Attacks without CCM-KEY

   In this example, NFS with/ RPCSEC_GSS will be the application, and
   IPsec the secure channel.

   Man in the middle (MITM) avoidance means making sure that the client
   and server are the same at both layers, NFS and IPsec, but since the
   principal names at the one layer will be radically different from the
   names at the other, how can one be certain that there is no MITM at
   the IPsec layer before leaving it to IPsec to provide session
   protection to the NFS layer?  The answer is to use channel bindings,
   which, conceptually, are an exchange, at the NFS/GSS layer, of
   signatures of the principal names or session ID/keys involved at the
   IPsec layer.

   Consider an attacker who can cause a client's IPsec stack to
   establish an SA with the attacker, instead of the server intended by
   the NFS layer (this is accomplished by spoofing the DNS server).
   Suppose further that the attacker can fool the client's IPsec layer
   without also fooling its NFS/RPCSEC_GSS layer (for example, if
   Kerberos V5 is being used as the real mechanism, and avoids the use
   of DNS to canonicalize the server principal name -- admittedly, this
   avoidance is unlikely -- a DNS spoof attack will be detected by the


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   NFS client, because the Kerberos Key Distribution Center (KDC)
   generates tickets associated with pairs of principals, not host
   names).  Suppose that the attacker's host is in part of the site's
   IPsec infrastructure (perhaps the attacker broke into that host).
   Then the attacker might be able to act as a MITM between the client
   and the server who gets all the plain text and even gets to modify
   it, if CCM-NULL is wrapping Kerberos V5 at the RPCSEC_GSS level.
   Both, the client and the server would see that IPsec is in use
   between them, but they would each see a different ID for its IPsec
   peer.  Channel bindings are used to prove that the client and server
   each see the same two peer names at the lower (in this case, IPsec)
   layer, and therefore with CCM-KEY there is no MITM.

   DNSSEC would of course defeat the attack, but DNSSEC was not, at the
   time this document was written, in widespread use.

12.  Security Considerations

   There are many considerations for the use CCM, since it is reducing
   security at one protocol layer in trade for equivalent security at
   another layer.  In this discussion, we will assume that cryptography
   is being used in the application and lower protocol layers.

   *    CCM should not be used whenever the combined key
        strength/algorithm strength of the lower protocol layer securing
        the connection is weaker than what the underlying GSS context
        can provide.

   *    CCM should not be used if the lower level protocol does not
        offer comparable or superior security services to that the
        application would achieve with GSS.  For example, if the lower
        level protocol offers integrity, but the application wants
        privacy, then CCM is inappropriate.

   *    The use of CCM contexts over secured connections can be
        characterized nearly secure instead of as secure as using the
        underlying GSS context for protecting each application message
        procedure call.  The reason is that applications can multiplex
        the traffic of multiple principals over a single connection and
        so the ciphertext in the traffic is encrypted with multiple
        session keys.  Whereas, a secure connection method such as IPsec
        is protected with per host session keys.  Therefore, an attacker
        has more cipher text per session key to perform cryptanalysis
        via connections protected with IPsec, versus connections
        protected with GSS.

   *    Related to the previous bullet, the management of private keys
        for a secure channel is often outside the control of the user of
        CCM.  If the secure channel's private keys are compromised, then


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        all users of the secure channel are compromised.

   *    CCM contexts created during one session or transport connection
        SHOULD not be used for subsequent sessions or transport
        connections.  In other words, full initiator to target
        authentication SHOULD occur each time a session or transport
        connection is established.  Otherwise, there is nothing
        preventing an attacker from using a CCM context from one
        authenticated session or connection to trivially establish
        another, unauthenticated session or connection.  For efficiency,
        a CCM-BIND context from a previous session MAY be used to
        establish a CCM-MIC context.

        If the application protocol using CCM has no concept of a
        session and does not use a connection oriented transport, then
        there is no sequence of state transitions that tie the CCM
        context creation steps with the subsequent message traffic of
        the application protocol.  Thus it can be hard to assert that
        the subsequent message traffic is truly originated by the CCM
        initiator's principal.  For this reason, CCM SHOULD NOT be used
        with applications that do not have sessions or do not use
        connection oriented transports.

   *    The underlying secure channel SHOULD be end to end, from
        initiator to the target.  It is permissible for the user to
        configure the underlying secure channel to not be end to end,
        but this should only be done if user has confidence in the
        intermediate end points.  For example, suppose the application
        is being used behind a firewall that performs network address
        translation.  It is possible to have an IPsec secure channel
        from the initiator to the firewall, and a second secure channel
        from the firewall to the target, but not from the initiator to
        the target.  So, if the firewall is compromised by an attacker
        in the middle, the use of CCM to avoid per message
        authentication is useless.  Furthermore, without channel
        bindings mandated by CCM-KEY, it is not possible for the
        initiator and target to enforce end to end channel security.  Of
        course, if the initiator's node created a IP-layer tunnel
        between it and the target, end to end channel security would be
        achieved, but without the use of CCM-KEY, the initiator and
        target applications would have no way of knowing that.

   *    It has been stated that it is not uncommon to find IPsec
        deployments where multiple nodes share common private keys
        [Black].  The use of CCM is discouraged in such environments,
        since the compromise of one node compromises all the other nodes
        sharing the same private key.

   *    Applications using CCM MUST ensure that the binding between the


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        CCM context and the secure channel is legitimate for each
        message that references the CCM context.  In other words, the
        referenced CCM context in a message MUST be established in the
        same secure channel as the message.  The use of CCM-KEY enforces
        this binding.

   *    When the same secure channel is multiplexing traffic for
        multiple users, the initiator has to ensure the CCM context is
        only accessible to the initiator principal that has established
        it in the first place.  One possible way to ensure that is by
        placing CCM contexts in the privileged address space offering
        only controlled indexed access.

   *    CCM does not unnecessarily inflate the scope of the trust
        domain, as does for example AUTH_SYS [RFC1831] over IPSec.  By
        requiring the authentication in the CCM context initialization
        (using a previously established context), the trust domain does
        not extend to the client.

   *    Both the traditional mechanisms and CCM rely on the security of
        the client to protect locally logged on users.  Compromise of
        the client impacts all users on the same client.  CCM does not
        make the problem worse.

   *    The CCM context MUST be established over the same secure channel
        that the subsequent message traffic will be using.  This way,
        the binding between the initial authentication and the
        subsequent traffic is ensured.  Again, the use of CCM-KEY is one
        way to assert this binding.

   *    The section entitled "CCM-KEY and Anonymous IPsec", suggests a
        method for simulating anonymous IPsec via self-signed
        certificates.  If one is careless, this is will neuter all IPsec
        authentication, a real problem for those applications not using
        CCM-KEY.  The use of the self-signed certificates in IPsec
        should be restricted by port in the IPsec Security Policy
        Database (SPD) only to those application using CCM-KEY.  Note
        however, that port selector support is OPTIONAL in IPsec.

   *    If an application is using IPsec and is not using CCM-KEY, then
        then the site where the application is deployed should configure
        the IPsec SPD to carefully limit the ports and nodes that are
        allowed create security  associations to application targets.

   *    CCM-KEY's IPsec bindings use public SA information, and CCM-
        ADDR's bindings are simply public network addresses.  If the
        secure channel is IPsec, and non-anonymous certificates are used
        with IKE, then a MITM cannot spoof the target's and initiator's
        IP addresses, because the attacker will presumably be unable to


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        spoof the Certificate Authority that signed the certificates.
        Thus, when IPsec is used as the secure channel, and non-
        anonymous certificates are used with IKE, CCM-ADDR is as secure
        as CCM-KEY.

   *    CCM contexts should not be used forever without re-
        authenticating periodically via the underlying mechanism.  One
        rational approach is for the CCM context to persist no longer
        than the underlying mechanism context.  Implementing this via
        the GSS-API is simple.  Applications can periodically invoke
        gss_context_time() to find out how long the context will be
        valid.  Moreover, CCM can enforce this by invoking
        gss_context_time() and the system time of day API to get an
        expiration date when the CCM mechanism is established.  Each
        subsequent call can check the time of day against the
        expiration, and if expired, return GSS_S_CONTEXT_EXPIRED.


13.  IANA Considerations

   XXX Note 1 to IANA: The CCM-BIND mechanism OID prefixes and the CCM-
   MIC mechanism OID must be assigned and registered by IANA.  Please
   look for TBD1 in this document and notify the RFC Editor what value
   you have assigned.

   XXX Note 1 to RFC Editor: When IANA has made the OID assignments,
   please do the following:

   *    Delete the "XXX Note 1 to RFC Editor: ..." paragraph.

   *    Replace occurrences of TBD1 with the value assigned by IANA.

   *    Replace the "XXX Note 1 to IANA: ..." paragraph with:
             OIDs for the CCM-BIND mechanism prefix, and for the CCM-MIC
             mechanism have been assigned by, and registered with IANA,
             with this document as the reference.


   XXX Note 2 to IANA: Please assign RPC flavor numbers for values
   currently place held in this document as TBD2 through TBD10.  Also
   please establish the registry that RFC2623 mandates.

   XXX Note 2 to RFC Editor: When IANA has made the RPC flavor number
   assignments, please do the following:

   *    Delete the "XXX Note 2 to RFC Editor: ..." paragraph.

   *    Replace occurrences of TBD2 through and including TBD10 withe
        flavor number assignments from IANA.


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   Section 6, "IANA Considerations" of [RFC2623] established a registry
   for mapping GSS mechanism OIDs to RPC pseudo flavor numbers.  This
   registry was augmented in the NFSv4 specification [RFC3530] with
   several more entries.  This document adds the following entries to
   the registry:

    1 == number of pseudo flavor
    2 == name of pseudo flavor
    3 == mechanism's OID
    4 == quality of protection
    5 == RPCSEC_GSS service

   1          2               3                4 5
   --------------------------------------------------------------
   TBD2  ccm-mic         1.3.6.1.5.5.TBD1.2    0 rpc_gss_svc_none

   TBD3  ccm-null-krb5   1.3.6.1.5.5.TBD1.1.1. 0 rpc_gss_svc_none
                         1.2.840.113554.1.2.2

   TBD4  ccm-addr-krb5   1.3.6.1.5.5.TBD1.1.2. 0 rpc_gss_svc_none
                         1.2.840.113554.1.2.2

   TBD5  ccm-key-krb5    1.3.6.1.5.5.TBD1.1.3. 0 rpc_gss_svc_none
                         1.2.840.113554.1.2.2

   TBD6  ccm-null-spkm3  1.3.6.1.5.5.TBD1.1.1. 0 rpc_gss_svc_none
                         1.3.6.1.5.5.1.3

   TBD6  ccm-addr-spkm3  1.3.6.1.5.5.TBD1.1.2. 0 rpc_gss_svc_none
                         1.3.6.1.5.5.1.3

   TBD7  ccm-key-spkm3   1.3.6.1.5.5.TBD1.1.3. 0 rpc_gss_svc_none
                         1.3.6.1.5.5.1.3

   TBD8  ccm-null-lipkey 1.3.6.1.5.5.TBD1.1.1. 0 rpc_gss_svc_none
                         1.3.6.1.5.5.1.3

   TBD9  ccm-addr-lipkey 1.3.6.1.5.5.TBD1.1.2. 0 rpc_gss_svc_none
                         1.3.6.1.5.5.1.3

   TBD10 ccm-addr-lipkey 1.3.6.1.5.5.TBD1.1.3. 0 rpc_gss_svc_none
                         1.3.6.1.5.5.1.3


14.  Acknowledgements

   Dave Noveck, for the observation that NFS version 4 servers could
   downgrade from integrity service to plain authentication service if
   IPsec was enabled.  David Black, Peng Dai, Sam Hartman, and Julian


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   Satran, for their critical comments.  Much of the text for the
   "Security Considerations" section comes directly from David and Peng.

15.  Normative References


   [RFC1832]
        R. Srinivasan, RFC1832, "XDR: External Data Representation
        Standard", August, 1995.

   [RFC2025]
        C. Adams, RFC2025: "The Simple Public-Key GSS-API Mechanism
        (SPKM)," October 1996, Status: Standards Track.

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

   [RFC2401]
        S. Kent, R. Atkinson, RFC2401, "Security Architecture for the
        Internet Protocol ", November, 1998.

   [RFC2409]
        D. Harkins and D. Carrel, RFC2119: "The Internet Key Exchange
        (IKE)," November 1998.

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

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

   [RFC2847]
        M. Eisler, RFC2847: "LIPKEY - A Low Infrastructure Public Key
        Mechanism Using SPKM," June 2000, Status: Standards Track.

   [FIPS]U.S. Department of Commerce / National Institute of Standards
        and Technology, FIPS PUB 180-1, "Secure Hash Standard", May 11,
        1993.

   [IKEv2]
        C. Kaufman, draft-ietf-ipsec-ikev2-07.txt: "Internet Key
        Exchange (IKEv2) Protocol," A work in progress, April 2003.

        XXX - Note 3 to RFC Editor: In the event this work in progress
        is not approved for publication when the CCM document is, then
        the sections of the CCM document that refer to IKEv2 in a


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        normative manner are to be removed for submission as a separate
        document.

   [SSHv2]
        T. Ylonen et. al., draft-ietf-secsh-transport-15.txt: "SSH
        Transport Layer Protocol," A work in progress, September 2002.

        XXX - Note 4 to RFC Editor: In the event this work in progress
        is not approved for publication when the CCM document is, then
        the sections of the CCM document that refer to SSHv2 in a
        normative manner are to be removed for submission as a separate
        document.

16.  Informative References


   [RFC1831]
        R. Srinivasan, RFC1831, "RPC: Remote Procedure Call Protocol
        Specification Version 2", August, 1995.

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

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

   [RFC2623]
        M. Eisler, RFC2623, "NFS Version 2 and Version 3 Security Issues
        and the NFS Protocol's Use of RPCSEC_GSS and Kerberos V5", June
        1999.

   [RFC3530]
        S. Shepler, B. Callaghan, D. Robinson, R. Thurlow, C.  Beame, M.
        Eisler, D. Noveck, RFC3530, "Network File System (NFS) version 4
        Protocol", April 2003.

   [Black]
        D. Black, EMail message on the NFSv4 working group alias,
        February 28, 2003.

   [DAFS]
        Mark Wittle (Editor), "DAFS Direct Access File System Protocol,
        Version: 1.00", September 1, 2001.

17.  Authors' Addresses

   Mike Eisler


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   5765 Chase Point Circle
   Colorado Springs, CO 80919
   USA

   Phone: 719-599-9026
   EMail: mike@eisler.com


   Nicolas Williams
   Sun Microsystems, Inc.
   5300 Riata Trace CT
   Austin, TX 78727
   USA

   EMail: nicolas.williams@sun.com

18.  IPR Notices

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.


19.  Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this


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   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the  purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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