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NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                G. Chander
Updates: 4279 (if approved)                        Microsoft Corporation
Intended status: Standards Track                               J. Altman
Expires: January 26, 2008                          Secure Endpoints Inc.
                                                            S. Santesson
                                                   Microsoft Corporation
                                                           July 25, 2007

     Flexible Key Agreement for Transport Layer Security (FKA-TLS)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on January 26, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document defines extensions to RFC 4279, "Pre-Shared Key
   Ciphersuites for Transport Layer Security (TLS)", to enable dynamic
   key sharing in distributed environments using a Generic Security
   Service Application Program Interface (GSS-API) mechanism, and then

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   import that shared key as the "Pre-Shared Key" to complete the TLS

   This is a modular approach to perform authentication and key exchange
   based on off-shelf libraries.  And it obviates the need of pair-wise
   key sharing by enabling the use of the widely-deployed Kerberos alike
   trust infrastructures that are highly scalable and robust.
   Furthermore, conforming implementations can provide server
   authentication without the use of certificates.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Protocol Definition  . . . . . . . . . . . . . . . . . . . . .  3
   4.  Choosing GSS-API Mechanisms  . . . . . . . . . . . . . . . . .  8
   5.  Client Authentication  . . . . . . . . . . . . . . . . . . . .  8
   6.  Protecting GSS-API Authentication Data . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 11
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 11
   Appendix A.  An FKA-TLS Example: Kerberos TLS  . . . . . . . . . . 13
   Appendix B.  Additional Use Cases for FXA-TLS  . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  Introduction

   [RFC4279] defines Transport Layer Security (TLS) based on pre-shared
   keys (PSK).  This assumes a pair-wise key sharing scheme that is less
   scalable and more costly to manage in comparison with a trusted third
   party scheme such as Kerberos [RFC4120].  In addition, off-shelf GSS-
   API libraries that allow dynamic key sharing are not currently
   accessible to TLS applications.  Lastly, [RFC4279] does not provide
   true mutual authentication against the server.

   This document extends [RFC4279] to establish a shared key, and
   optionally provide client or server authentication, by using off-
   shelf GSS-API libraries, and the established shared key is then
   imported as "PSK" to [RFC4279].  No new key cipher suite is defined
   in this document.

   As an example usage scenario, Kerberos [RFC4121] is a GSS-API
   mechanism that can be selected to establish a shared key between a
   client and a server based on either asymmetric keys [RFC4556] or
   symmetric keys [RFC4120].  By using the extensions defined in this
   document, a TLS connection is secured using the Kerberos version 5
   mechanism exposed as a generic security service via GSS-API.

   With regard to the previous work for the Kerberos support in TLS,
   [RFC2712] defines "Addition of Kerberos Cipher Suites to Transport
   Layer Security (TLS)" which has not been widely implemented due to
   violations of Kerberos Version 5 library abstraction layers,
   incompatible implementations from two major distributions (Sun Java
   and OpenSSL), and its lack of support for credential delegation.
   This document defines a generic extensible method that addresses the
   limitations associated with [RFC2712] and integrates Kerberos and
   TLS.  Relying on [RFC4121] for Kerberos Version 5 support will
   significantly reduce the challenges associated with implementing this
   protocol as a replacement for [RFC2712].

2.  Conventions Used in This Document

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

3.  Protocol Definition

   In this protocol, the on-demand key exchange is implemented by
   encapsulating the GSS security context establishment within the TLS
   handshake messages when PSK cipher suites are requested in the

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   extended ClientHello message.

   The gss_api TLS extension is defined according to [RFC3546].  The
   extension data carries GSS-API token within the TLS hello messages.

     enum {
         gss_api(TBD), (65535)
     } ExtensionType;

   The client MUST NOT include a gss_api TLS extension if there is no
   PSK ciphersuite [RFC4279] included in the cipher_suites field of the
   client hello message.

   Initially the client computes the gss_api TLS extension data by
   calling GSS_Init_sec_context() [RFC2743] to establish a security
   context.  The TLS client MUST set the mutual_req_flag and identify
   the server by targ_name so that mutual authentication is performed in
   the course of context establishment.  The extension_data from the
   client contains the output token of GSS_Init_sec_context().

   If a GSS-API context cannot be established, the gss_api TLS extension
   MUST NOT be included in the client hello message and it is a matter
   of local policy on the client whether to continue or reject the TLS
   authentication as if the gss_api TLS extension is not supported.

   If the mutual authentication is not available on the established GSS-
   API context, the PSK key exchange described in Section 2 of [RFC4279]
   MUST NOT be selected, and the DHE_PSK or RSA_PSK key exchange MUST be
   negotiated instead in order to authenticate the server.

   Upon receipt of the gss_api TLS extension from the client, and if the
   server supports the gss_api TLS extension, the server calls
   GSS_Accept_sec_context() with the client GSS-API output token in the
   client's extension data as the input token.  If
   GSS_Accept_sec_context() returns a token successfully, the server
   responds by including a gss_api TLS extension in the server hello
   message and places the output token in the extension_data.  If
   GSS_Accept_sec_context() fails, it is a matter of local policy on the
   server whether to continue or reject the TLS authentication as if the
   gss_api TLS extension is not supported.

   The server MUST ignore a TLS gss_api extension in the extended
   ClientHello if its selected CipherSuite is not a PSK CipherSuite
   [RFC4279], and the server MUST NOT include a gss_api TLS extension in
   the server hello message.

   If after the exchange of extended ClientHello and extended
   ServerHello with the gss_api extension, at least one more additional

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   GSS token is required in order to complete the GSS security context
   establishment, the additional GSS-API token is encapsulated in a new
   TLS Handshake message called the token_transfer message.

         enum {
             token_transfer(TBD), (255)
         } HandshakeType;

         struct {
             HandshakeType msg_type;    /* handshake type */
             uint24 length;             /* bytes in message */
             select (HandshakeType) {
                 case token_transfer: /* NEW */
             } body;
         } Handshake;

         enum {
             gss_api_token(1), (255)
         } TokenTransferType;

         struct {
               TokenTransferType token_type; /* token type */
               opaque token<0..2^16-1>;
         } TokenTransfer;

   The TokenTransfer structure is filled out as follows:

   o  The token_type is gss_api_token.

   o  The token field contains the GSS-API context establishment tokens
      from the client and the server.

   The client calls GSS_Init_sec_context() with the token in the
   TokenTransfer stucture from the server as the input token, and then
   places the output token, if any, into the TokenTransfer message and
   sends the handshake message to the server.  The server calls
   GSS_Accept_sec_context() with the token in the TokenTransfer
   structure from the client as the input token, and then places the
   output token, if any, into the TokenTransfer message and sends the
   handshake message to the client.

   This loop repeats until either the context fails to establish or the
   context is established successfully.  To prevent an infinite loop,
   both the client and the server MUST have a policy to limit the
   maximum number of GSS-API context establishment calls for a given
   session.  The recommended value is a total of five (5) calls
   including the GSS_Init_sec_context() and GSS_Accept_sec_context()

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   from both the client and server.  Exceeding the maximum number of
   calls is to be treated as a GSS security context establishment
   failure.  It is RECOMMENDED that the client and server enforce the
   same maximum number

   If the GSS-API context fails to establish, it is a matter of local
   policy whether to continue or reject the TLS authentication as if the
   gss_api TLS extension is not supported.

   When the last GSS-API context establishment token is sent by the
   client or when the GSS-API context fails to establish on the client
   side and the local policy allows the TLS authentication to proceed as
   if the TLS gss_api extension is not supported, the client sends an
   empty TokenTransfer handshake message.

   If the GSS-API context fails to establish and local policy allows the
   TLS authentication continue as if the gss_api TLS extension is not
   supported, the server MAY send another ServerHello message in order
   to choose a different cipher suite.  The client then MUST expect the
   second ServerHello message from the server before the session is
   established.  The additional ServerHello message MUST only differ
   from the first ServerHello message in the choice of CipherSuite and
   it MUST NOT include a TLS gss_api extension.  The second ServerHello
   MUST NOT be present if there is no TokenTransfer message.

   If the client and the server establish a security context
   successfully, both the client and the server call GSS_Pseudo_random()
   [RFC4401] to compute a sufficiently long shared secret with the same
   value based on the negotiated cipher suite (see details below), and
   then proceed according to [RFC4279] using this shared secret value as
   the "PSK".

   When the shared key is established using a GSS-API mechanism as
   described in this document, the identity of the server and the
   identity of the client MUST be obtained from the GSS security
   context.  In this case, the PSK identity MUST be processed as

   o  The PSK identity as defined in Section 5.1 of [RFC4279] MUST be
      specified as an empty string.

   o  If the server key exchange message is present, the PSK identity
      hint as defined in Section 5.2 of [RFC4279] MUST be empty, and it
      MUST be ignored by the client.

   The input parameters to GSS_Pseudo_random() to compute the shared
   secret value MUST be provided as follows:

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   o  The context is the handle to the GSS-API context established in
      the given session.

   o  The prf_key is GSS_C_PRF_KEY_FULL.

   o  The prf_in contains the UTF8 encoding of the string "GSS-API TLS

   o  The desired_output_len is 64.  In other words, the output keying
      mastering size is 64 in bytes.  Note that this is the maximum PSK
      length required to be supported by implementations conforming to

   The following text art summaries the protocol message flow.

        Client                                               Server

        ClientHello                -------->
                                  <--------*            ServerHello
        TokenTransfer*             -------->
                                  <--------          TokenTransfer*
        TokenTransfer*             -------->
                                  <--------         ServerHelloDone
        Finished                   -------->
                                  <--------                Finished
        Application Data          <-------->       Application Data

          Fig. 1. Message flow for a full handshake

       * Indicates optional or situation-dependent messages that are
         not always sent.

   There could be multiple TokenTransfer handshake messages, and the
   last TokenTransfer message, if present, is always sent from the
   client to the server and it can carry an empty token.

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4.  Choosing GSS-API Mechanisms

   If more than one GSS-API mechanism is shared between the client and
   the server, it is RECOMMENDED to deploy a pseudo GSS-API mechanism
   such as [RFC4178] to choose a mutually preferred GSS-API mechanism.

   When Kerberos is selected as the GSS-API mechanism, the extensions
   defined in [KRB-ANON] can perform server authentication without
   client authentication, thus provide the functional equivalence to the
   certificate-based TLS [RFC4346].

   If the Kerberos client does not have access to the KDC but the server
   does, [IAKERB] can be chosen to tunnel the Kerberos authentication
   exchange within the TLS handshake messages.

5.  Client Authentication

   If the GSS-API mechanism in the gss_api TLS extension provides client
   authentication [RFC2743], the CertificateRequest, the client
   Certificate and the CertificateVerify handshake messages MUST NOT be
   present.  This is illustrated in Appendix A.

6.  Protecting GSS-API Authentication Data

   GSS-API [RFC2743] provides security services to callers in a generic
   fashion, supportable with a range of underlying mechanisms and
   technologies and hence allowing source-level portability of
   applications to different environments.  For example, Kerberos is a
   GSS-API mechanism defined in [RFC4121].  It is possible to design a
   GSS-API mechanism that can be used with FKA-TLS in order to, for
   example, provide client authentication, and is so weak that its GSS-
   API token MUST NOT be in clear text over the open network.  A good
   example is a GSS-API mechanism that implements basic authentication.
   Although such mechanisms are unlikely to be standardized and will be
   encouraged in no circumstance, they exist for practical reasons.  In
   addition, it is generally beneficial to provide privacy protection
   for mechanisms that send client identities in the clear.

   In order to provide a standard way for protecting weak GSS-API data
   for use over FKA-TLS, TLSWrap is defined in this section as a pseudo
   GSS-API mechanism that wraps around the real GSS-API authentication
   context establishment tokens.  This pseudo GSS-API mechanism does not
   provide per-message security.  The real GSS-API mechanism protected
   by TLSWrap may provide per-message security after the context is

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   The syntax of the initial TLSWrap token follows the
   initialContextToken syntax defined in Section 3.1 of [RFC2743].  The
   TLSWrap pseudo mechanism is identified by the Object Identifier
   iso.org.dod.internet.security.mechanism.tls-wrap (
   Subsequent TLSWrap tokens MUST NOT be encapsulated in this GSS-API
   generic token framing.

   TLSWrap encapsulates the TLS handshake and data protection in its
   context establishment tokens.

   The innerContextToken [RFC2743] for the initial TLSWrap context token
   contains the ClientHello message encoded according to [RFC4346].  No
   PSK ciphersuite can be included in the client hello message.  The
   targ_name is used by the client to identify the server and it follows
   the name forms defined in Section 4 of [PKU2U].

   Upon receipt of the initial TLSWrap context token, the GSS-API server
   processes the client hello message.  The output GSS-API context token
   for TLSWrap contains the ServerHello message and the ServerHelloDone
   potentially with the optional handshake messages in the order as
   defined in [RFC4346].

   The GSS-API client then processes the server reply and returns the
   ClientKeyExchange message and the Finished message potentially with
   the optional handshake messages in the order as defined in [RFC4346].
   The client places the real GSS-API authentication mechanism token as
   an application data record right after the TLS Finished message in
   the same GSS-API context token for TLSWrap.  Because the real
   mechanism token is placed after the ChangeCipherSpec message, the
   GSS-API data for the real mechanism is encrypted.  If the GSS-API
   server is not authenticated at this point of the TLS handshake for
   TLSWrap, the TLSWrap context establishment MUST fail and the real
   authentication mechanism token MUST not be returned.

   The GSS-API server in turn processes the client reply and returns the
   TLS Finished message, the server places the reply token from the real
   authentication mechanism, if present, as an application data record.

   If additional TLS messages are needed before the application data,
   these additional TLS messages are encapsulated in the context token
   of TLSWrap in the same manner how the client hello message and the
   server hello message are encapsulated as described above.

   If additional tokens are required by the real authentication
   mechanism in order to establish the context, these tokens are placed
   as an application data record, encoded according to [RFC4346] and
   then returned as TLSWrap GSS-API context tokens, with one TLSWrap
   context token per each real mechanism context token.  The real

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   mechanism context tokens are decrypted by TLSWrap and then supply to
   the real mechanism to complete the context establishment.

7.  Security Considerations

   As described in Section 3, when the shared key is established using a
   GSS-API mechanism as described in this document, the identity of the
   server MUST be obtained from the GSS security context and the
   identity of the client MUST be obtained from the GSS security
   context.  Authentication methods such as GSS security context and
   X.509 certificate mixed MUST NOT conflict.  Such confusion about the
   identity will interfere with the ability to properly determine the
   client's authorization privileges, thus potentially result in a
   security weakness.

   When Kerberos as defined in [RFC4120] is used to establish the share
   key, it is vulnerable to offline dictionary attacks.  The threat is
   mitigated by deploying Kerberos FAST [KRB-FAST].

   Shared symmetric keys obtained from mutual calls to
   GSS_Pseudo_random() are not susceptible to off-line dictionary
   attacks in the same way that traditional pre-shared keys are.  The
   strength of the generated keys are determined based upon the security
   properties of the selected GSS mechanism.  Implementers MUST take
   into account the Security Considerations associated with the GSS
   mechanisms they decide to support.

8.  Acknowledgements

   Ari Medvinsky was one of the designers of the original TLS Kerberos
   version 5 CipherSuite and contributed to the first two revisions of
   this protocol specification.

   Raghu Malpani provided insightful comments and was very helpful along
   the way.

   Ryan Hurst contributed significantly to the use cases of FKA-TLS.

   Love Hornquist Astrand, Nicolas Williams and Martin Rex provided
   helpful comments while reviewing early revisions of this document.

9.  IANA Considerations

   A new handshake message token_transfer is defined according to
   [RFC4346] and a new TLS extension called the gss_api extension is

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   defined according to [RFC3546].  The registry needs to be updated to
   include these new types.

   This document defines the type of the transfer tokens in Section 3, a
   registry need to be setup and the allocation policy is "Specification

10.  References

10.1.  Normative References

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

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

   [RFC3546]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 3546, June 2003.

   [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.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

10.2.  Informative References

   [IAKERB]   Zhu, L., "Initial and Pass Through Authentication Using
              Kerberos V5 and the GSS-API", draft-zhu-ws-kerb-03.txt
              (work in progress), 2007.

              Zhu, L. and P. Leach, "Kerberos Anonymity Support",
              draft-ietf-krb-wg-anon-04.txt (work in progress), 2007.

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              Zhu, L. and S. Hartman, "A Generalized Framework for
              Kerberos Pre-Authentication",
              draft-ietf-krb-wg-preauth-framework-06.txt (work in
              progress), 2007.

   [PKU2U]    Zhu, L., Altman, J., and A. Medvinsky, "Public Key
              Cryptography Based User-to-User Authentication - (PKU2U)",
              draft-zhu-pku2u-02.txt (work in progress), 2007.

   [RFC2487]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              TLS", RFC 2487, January 1999.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2712]  Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              October 1999.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 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.

   [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.

   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510,
              June 2006.

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.

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   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, June 2006.

Appendix A.  An FKA-TLS Example: Kerberos TLS

   This section provides a non-normative description of the message flow
   when Kerberos Version 5 is used to established the shared secret
   according to [RFC4121] and that shared secret is then used to secure
   the TLS connection according to FKA-TLS defined in this document.

          Client                                               Server

          ClientHello(with AP-REQ)  -------->
                                             ServerHello(with AP-REP)
                                   <--------          ServerHelloDone
          Finished                  -------->
                                   <--------                Finished
          Application Data         <-------->       Application Data

             Fig. 2. Kerberos FKA-TLS example message flow

   In this successful authentication sample, the TLS client sends the
   Kerberos AP-REQ [RFC4120] in the inital context token according to
   [RFC4121].  The initial GSS-API context token from the GSS-API client
   contains the Object Identifier that signifies the Kerberos mechanism
   and it is encapsulated in the gss_api TLS extension in the client
   hello message.  The TLS client always requests mutual authentication,
   and the TLS server then sends a GSS-API context token that contains
   the AP-REP [RFC4120] according to [RFC4121].  The TLS server's GSS-
   API context token is encapsulated in the gss_api TLS extension in the
   server hello message.  The GSS-API context is established at that
   point and both sides can derive the shared secret value according to

   In this example, the ServerKeyExchange handshake message is not
   needed and it is not present.  And according to Section 5 none of the
   CertificateRequest, the client Certificate or the CertificateVerify
   handshake messages is present.

Appendix B.  Additional Use Cases for FXA-TLS

   TLS runs on layers beneath a wide range of application protocols such

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   as LDAP [RFC4510], SMTP [RFC2487], and XMPP [RFC3920] and above a
   reliable transport protocol.  TLS can add security to any protocol
   that uses reliable connections (such as TCP).  TLS is also
   increasingly being used as the standard method for protecting SIP
   [RFC3261] application signaling.  TLS can provide authentication and
   encryption of the SIP signaling associated with VOIP (Voice over IP)
   and other SIP-based applications.

   Today these applications use public key certificates to verify the
   identity of endpoints.

   However, it is overwhelmingly complex to manage the assurance level
   of the certificates when deploying PKI and such complexity has
   gradually eroded the confidence for the PKI-based systems in general.
   In addition, the perceived overhead of deploying and managing
   certificates is fairly high.  As a result, the industry badly needs
   the ability to secure TLS connections by leveraging the existing
   credential infrastructure.  For many customers that means Kerberos.
   It is highly desirable to enable PKI-less deployments yet still offer
   strong authentication.

   Having Kerberos/GSS-API in the layer above TLS means all TLS
   applications need to be changed in the protocol level.  In many
   cases, such changes are not technically feasible.  For example,
   [RFC4559] provides integration with Kerberos in the HTTP level.  It
   suffers from a couple of drawbacks, most notably it only supports
   single-round-trip GSS-API mechanisms and it lacks of channel bindings
   to the underlying TLS connection which makes in unsuitable for
   deployment in situations where proxies exists.  Furthermore,
   [RFC4559] lacks of session-based re-authentication (comparing with
   TLS).  The root causes of these problems are inherent to the HTTP
   protocol and can't be fixed trivially.

   Consequently, It is a better solution to integrate Kerberos/GSS-API
   in the TLS layer.  Such integration allows the existing
   infrastructure work seamlessly with TLS for the products based on
   them in ways that were not practical to do before.  For instance, an
   increasing number of client and server products support TLS natively,
   but many still lack support.  As an alternative, users may wish to
   use standalone TLS products that rely on being able to obtain a TLS
   connection immediately, by simply connecting to a separate port
   reserved for the purpose.  For example, by default the TCP port for
   HTTPS is 443, to distinguish it from HTTP on port 80.  TLS can also
   be used to tunnel an entire network stack to create a VPN, as is the
   case with OpenVPN.  Many vendors now marry TLS's encryption and
   authentication capabilities with authorization.  There has also been
   substantial development since the late 1990s in creating client
   technology outside of the browser to enable support for client/server

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   applications.  When compared against traditional IPSec VPN
   technologies, TLS has some inherent advantages in firewall and NAT
   traversal that make it easier to administer for large remote-access

   PSK-TLS as defined in [RFC4279] is a good start but this document
   finishes the job by making it more deployable.  FKA-TLS also fixes
   the mutual-authentication problem in [RFC4279] in the cases where the
   PSK can be shared among services on the same host.

Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: lzhu@microsoft.com

   Girish Chander
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: gchander@microsoft.com

   Jeffrey Altman
   Secure Endpoints Inc.
   255 W 94th St
   New York, NY  10025

   Email: jaltman@secure-endpoints.com

   Stefan Santesson
   Microsoft Corporation
   Tuborg Boulevard 12
   2900 Hellerup, WA

   Email: stefans@microsoft.com

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