


Network Working Group                                          L. Morand
Internet-Draft                                   France Telecom - Orange
Intended status: Informational                         September 7, 2010
Expires: March 11, 2011


 Hypertext Transfer Protocol (HTTP) Digest Authentication Using GSM 2G
                 Authentication and Key Agreement (AKA)
                   draft-morand-http-digest-2g-aka-01

Abstract

   This memo specifies a one-time password generation mechanism for
   Hypertext Transfer Protocol (HTTP) Digest access authentication based
   on Global System for Mobile Communications (GSM) authentication and
   key generation functions A3 and A8, also known as GSM AKA or 2G AKA.
   The HTTP Authentication Framework includes two authentication
   schemes: Basic and Digest.  Both schemes employ a shared secret based
   mechanism for access authentication.  The GSM AKA mechanism performs
   user authentication and session key distribution in GSM and Universal
   Mobile Telecommunications System (UMTS) networks.  GSM AKA is a
   challenge-response based mechanism that uses symmetric cryptography.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on March 11, 2011.

Copyright Notice




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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction and Motivation  . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  GSM 2G AKA Mechanism Overview  . . . . . . . . . . . . . . . .  4
   5.  Specification of Digest 2G AKA . . . . . . . . . . . . . . . .  6
     5.1.  Algorithm Directive  . . . . . . . . . . . . . . . . . . .  6
     5.2.  Creating a Challenge . . . . . . . . . . . . . . . . . . .  7
     5.3.  Client Authentication  . . . . . . . . . . . . . . . . . .  7
     5.4.  Server Authentication  . . . . . . . . . . . . . . . . . .  8
   6.  Example of Digest 2G AKA operations  . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     8.1.  Authentication of Clients using Digest 2G AKA  . . . . . . 11
     8.2.  Limited Use of Nonce Values  . . . . . . . . . . . . . . . 11
     8.3.  Multiple Authentication Schemes and Algorithms . . . . . . 12
     8.4.  Online Dictionary Attacks  . . . . . . . . . . . . . . . . 12
     8.5.  Session Protection . . . . . . . . . . . . . . . . . . . . 12
     8.6.  Replay Protection  . . . . . . . . . . . . . . . . . . . . 13
     8.7.  Mutual Authentication  . . . . . . . . . . . . . . . . . . 13
     8.8.  Flooding the Authentication Centre . . . . . . . . . . . . 13
     8.9.  AKA Security . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14









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1.  Introduction and Motivation

   The Hypertext Transfer Protocol (HTTP) Authentication Framework,
   described in [RFC2617], includes two authentication schemes: Basic
   and Digest.  Both schemes employ a shared secret based mechanism for
   access authentication.  The Basic scheme is inherently insecure in
   that it transmits user credentials in plain text.  The Digest scheme
   improves security by hiding user credentials with cryptographic
   hashes, and additionally by providing limited message integrity.

   The 2G AKA functions [TS55.205] perform authentication and session
   key distribution in Global System for Mobile Communication (GSM) and
   Universal Mobile Telecommunications System (UMTS) networks. 2G AKA is
   a challenge-response based mechanism that uses symmetric
   cryptography.  GSM AKA is typically run in a GSM Subscriber Identity
   Module (SIM), which resides in a smart card like device that also
   provides tamper resistant storage of shared secrets.  The 3G
   Authentication and Key Agreement (AKA) mechanism, also known as UMTS
   AKA, relying on the use of the UMTS Subscriber Identity Module (USIM)
   instead of the GSM SIM, is most closely associated with UMTS;
   however, mobile operators commonly distribute GSM SIMs with UMTS
   mobile phones, resulting in the use of GSM 2G AKA in place of UMTS
   AKA.

   This document specifies a mapping of GSM AKA parameters onto HTTP
   Digest authentication.  In essence, this mapping enables the usage of
   GSM 2G AKA as a one-time password generation mechanism for Digest
   authentication.

   This document is based heavily on [RFC3310] which specified a mapping
   of Authentication and Key Agreement (AKA) onto HTTP Digest
   authentication.  While Digest AKA can be generally used when the
   mobile phones are equipped with a UMTS SIM card, it may be useful for
   mobile operators who have not yet fully deployed USIMs and have still
   millions of SIMs deployed in the network.  Digest 2G AKA allows
   access to applications in a more secure way than would be possible
   with the use of passwords or with GSM without enhancements.

   Moreover, as the Session Initiation Protocol (SIP) [RFC3261]
   Authentication Framework closely follows the HTTP Authentication
   Framework, Digest 2G AKA is directly applicable to SIP as well as to
   any other embodiment of HTTP Digest.


2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in [RFC2119].


3.  Terminology

   This section explains the terminology used in this document.


   AuC  Authentication Center.

   AKA  Authentication and Key Agreement.

   GSM  Global System for Mobile Communication.

   IMS  IP Multimedia Subsystem.

   ISIM  IMS Subscriber Identity Module.

   Kc Cypher Key. Ki Subscriber Key.

   RAND  Random Challenge.

   SIM  Subscriber Identity Module.

   SRES  Signed Authentication Response.

   UMTS  Universal Mobile Telecommunications System.

   USIM  UMTS Subscriber Identity Module.


4.  GSM 2G AKA Mechanism Overview

   This chapter describes the GSM AKA mechanism based on a shared secret
   key (Ki) and the use of A3/A8 algorithms:


   1.  A shared secret Ki is established beforehand between the SIM and
       the Authentication Center (AuC).  The secret is stored in the
       SIM, which resides on a smart card like, tamper resistant device.

   2.  The AuC of the home network produces an authentication vector
       (AV), based on the shared secret Ki and a generated random value
       (RAND).  Using Ki and RAND, the A3 algorihm is used for
       calculating the expected response (SRES) and the A8 algorithm is
       used for calculating the ciffering key Kc.





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   3.  The authentication vector (RAND, SRES, Kc) is downloaded to a
       server.  Optionally, the server can also download a batch of AVs,
       containing more than one authentication vector generated by
       adding different RAND values.

   4.  The server creates an authentication request, which contains the
       random challenge RAND.

   5.  The authentication request is delivered to the client.

   6.  The client produces a signed authentication response RES, using
       the shared secret Ki and the random challenge RAND.

   7.  The authentication response RES is delivered to the server.

   8.  The server compares the authentication response RES with the
       expected response SRES.  If the two match, the user has been
       successfully authenticated, and the session key Kc can be used
       for protecting further communication between the client and the
       server































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        SIM                       Server                         AuC
        (Ki)                                                     (Ki)

         |                          |                             |
         |--- Request (IMSI) ------>|                             |
         |                          | Authen. Data Request (IMSI) |
         |                          |---------------------------->|
         |                          |                             |
         |                          |           +-----------------+----+
         |                          |           | IMSI --> Ki          |
         |                          |           | A3(RAND,Ki) = SRES   |
         |                          |           | A8(RAND,Ki) = Kc     |
         |                          |           +----------------+-----+
         |                          |                            |
         |                          |<--- AV (RAND, SRES, Kc) ---|
         |<-- Auth. Request (RAND) -|                            |
         |                          |                            |
   +-----+---------------+          |                            |
   | A3(RAND,Ki) = SRES* |          |                            |
   | A8(RAND,Ki) = Kc    |          |                            |
   +-----+---------------+          |                            |
         |                          |                            |
         |- Auth. Response (SRES*)->|                            |
         |                          |                            |
         |                  +-------+------+                     |
         |                  | SRES*= SRES? |                     |
         |                  +-------+------+                     |
         |                          |                            |

                                 Figure 1. GSM 2G AKA overview


5.  Specification of Digest 2G AKA

   In general, the Digest GSM AKA operation is identical to the Digest
   operation in [RFC2617].  This chapter specifies the parts in which
   Digest 2G AKA extends the Digest operation.  The notation used in the
   Augmented BNF definitions for the new and modified syntax elements in
   this section is as used in SIP [RFC3261], and any elements not
   defined in this section are as defined in SIP and the documents to
   which it refers.

5.1.  Algorithm Directive

   In order to direct the client into using 2G AKA for authentication
   instead of the standard password system, the RFC 2617 defined
   algorithm directive is overloaded in Digest 2G AKA:




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      algorithm = "algorithm" EQUAL ( 2GAKA-namespace / algorithm-value
      )

      2GAKA-namespace = "2GAKA" "-" algorithm-value

      algorithm-value = ( "MD5" / "MD5-sess" / token )

   algorithm

      A string indicating the algorithm used in producing the digest and
      the checksum.  If the directive is not understood, the nonce
      SHOULD be ignored, and another challenge (if one is present)
      should be used instead.  Reuse of the same SRES value in
      authenticating subsequent requests and responses is NOT
      RECOMMENDED.  An SRES value SHOULD only be used as a one-time
      password, and algorithms such as "MD5-sess", which limit the
      amount of material hashed with a single key, by producing a
      session key for authentication, SHOULD NOT be used.

5.2.  Creating a Challenge

   In order to deliver the GSM AKA authentication challenge to the
   client in Digest 2G AKA, the nonce directive defined in [RFC2617] is
   extended:

      nonce = "nonce" EQUAL ( 2GAKA-nonce / nonce-value )

      2GAKA-nonce = LDQUOT 2GAKA-nonce-value RDQUOT

      2GAKA-nonce-value = <base64 encoding of RAND>

   nonce

      A parameter which is populated with the Base64 [RFC2045] encoding
      of the 2G AKA authentication random challenge RAND.

5.3.  Client Authentication

   When a client receives a Digest 2G AKA authentication challenge, it
   extracts the RAND from the "nonce" parameter and runs the A3-A8
   algorithms with the RAND challenge and shared secret Ki.

   The resulting A3-A8 SRES parameter is treated as a "password" when
   calculating the response directive of [RFC2617].  Due to the fact
   that the SRES parameter is 32 bits and the response directive of
   [RFC2617] is defined as 32 hex digits, SRES is encoded in the low
   order (i.e. rightmost) 32 bits of "response", padded with leading
   zeroes.



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   Example:

      SRES="0000000000000000000000007018d8a1"

5.4.  Server Authentication

   With Digest 2G AKA, the server MUST use the expected response SRES
   received in the authentication vector as "password" when calculating
   the "response-auth" of the "Authentication-Info" header defined in
   [RFC2617].


6.  Example of Digest 2G AKA operations

   Figure 2 below describes a message flow describing a Digest 2G AKA
   process of authenticating a SIP request, namely the SIP REGISTER
   request.


































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      SIM                        HTTP Server                       AuC
      (Ki)                                                         (Ki)

       | 1) GET (IMSI)              |                               |
       |--------------------------->|                               |
       |                            | 2) Authen. Data Request (IMSI)|
       |                            |------------------------------>|
       |                            |                               |
       |                            |             +-----------------+--+
       |                            |             | IMSI --> Ki        |
       |                            |             | A3(RAND,Ki) = SRES |
       |                            |             | A8(RAND,Ki) = Kc   |
       |                            |             +-----------------+--+
       |                            |                               |
       |                            |       3) AV (RAND, SRES, Kc)  |
       |                            |<------------------------------|
       | 4) 401 Unauthorized (RAND) |                               |
       |<---------------------------|                               |
       |                            |                               |
 +-----+---------------+            |                               |
 | A3(RAND,Ki) = SRES* |            |                               |
 | A8(RAND,Ki) = Kc    |            |                               |
 +-----+---------------+            |                               |
       |                            |                               |
       | 5) GET (IMSI, RAND, SRES)  |                               |
       |--------------------------->|                               |
       |                            |                               |
       |                    +-------+------+                        |
       |                    | SRES*= SRES? |                        |
       |                    +-------+------+                        |
       |                            |                               |
       |                  6) 200 OK |                               |
       |<---------------------------|                               |
       |                            |                               |


       Figure 2: Message flow representing a successful authentication



     1) Initial request

         GET / HTTP/1.1
         Authorization: Digest
            username="user1_private@home1.net",
            realm="service1.home1.net",
            nonce="",
            uri="/",



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            response=""

     2) Request to the AuC for 2G AKA authentication vector (AV) for the
        given IMSI

     3) Response from the AuC providing 2G AKA AV (RAND, SRES, Kc)
        associated with the IMSI

     4) Response containing a challenge

         HTTP/1.1 401 Unauthorized
         WWW-Authenticate: Digest
             realm="service1.home1.net",
             nonce="base64(RAND)",
             qop="auth,auth-int",
             opaque="6dae728da9089dab9112373c9f0a9731",
             algorithm=2GAKA-MD5

     5) Request containing credentials

         GET / HTTP/1.1
         Authorization: Digest
            username="user1_private@home1.net",
            realm="service1.home1.net",
            nonce="base64(RAND)",
            uri="/",
            nc=00000001,
            cnonce="0b8f29d6",
            response="6629fae49393a05397450978507c4ef1",
            opaque="6dae728da9089dab9112373c9f0a9731",
            algorithm=2GAKA-MD5

     6) Successful response

         HTTP/1.1 200 OK
         Authentication-Info:
            qop=auth-int,
            rspauth="6629fae49394a05397450978507c4ef1",
            cnonce="6629fae49393a05397450978507c4ef1",
            nc=00000001,
            opaque="6dae728da9089dab9112373c9f0a9731",
            nonce="base64(RAND)"


7.  IANA Considerations

   This document has no actions for IANA.




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   Note to RFC Editor: this section may be removed on publication as an
   RFC.


8.  Security Considerations

   In general, Digest 2G AKA is vulnerable to the same security threats
   as HTTP authentication [RFC2617].  This chapter discusses the
   relevant exceptions.

8.1.  Authentication of Clients using Digest 2G AKA

   2G AKA is typically -- though this isn't a theoretical limitation --
   run on a SIM application that usually resides in a tamper resistant
   smart card.  Interfaces to the SIM exist, which enable the host
   device to request authentication to be performed on the card.
   However, these interfaces do not allow access to the long-term secret
   outside the SIM, and the authentication can only be performed if the
   device accessing the SIM has knowledge of a PIN code, shared between
   the user and the SIM.  Such PIN codes are typically obtained from
   user input, and are usually required when the device is powered on.

   The use of tamper resistant cards with secure interfaces implies that
   Digest 2G AKA is typically more secure than regular Digest
   implementations, as neither possession of the host device nor Trojan
   Horses in the software give access to the long-term secret.  Where a
   PIN scheme is used, the user is also authenticated when the device is
   powered on.  However, there may be a difference in the resulting
   security of Digest 2G AKA, compared to traditional Digest
   implementations, depending on whether those implementations cache/
   store passwords that are received from the user.

8.2.  Limited Use of Nonce Values

   The Digest scheme uses server-specified nonce values to seed the
   generation of the request-digest value.  The server is free to
   construct the nonce in such a way that it may only be used from a
   particular client, for a particular resource, for a limited period of
   time or number or uses, or any other restrictions.  Doing so
   strengthens the protection provided against, for example, replay
   attacks.

   Digest 2G AKA limits the applicability of a nonce value to a
   particular SIM.  Typically, the SIM is accessible only to one client
   device at a time.  However, the nonce values are strong and secure
   even though limited to a particular SIM.  Additionally, this requires
   that the server is provided with the client identity before an
   authentication challenge can be generated.  If a client identity is



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   not available, an additional round trip is needed to acquire it.

8.3.  Multiple Authentication Schemes and Algorithms

   In HTTP authentication, a user agent MUST choose the strongest
   authentication scheme it understands and request credentials from the
   user, based upon that challenge.

   In general, using passwords generated by Digest 2G AKA with other
   HTTP authentication schemes is not recommended even though the realm
   values or protection domains would coincide.  In these cases, a
   password should be requested from the end-user instead.  Digest 2G
   AKA passwords MUST NOT be re-used with such HTTP authentication
   schemes, which send the password in the clear.  In particular, 2G AKA
   passwords must not be re-used with HTTP Basic.

   The same principle must be applied within a scheme if several
   algorithms are supported.  A client receiving an HTTP Digest
   challenge with several available algorithms MUST choose the strongest
   algorithm it understands.  For example, Digest with "2GAKA-MD5" would
   be stronger than Digest with "MD5".

8.4.  Online Dictionary Attacks

   Since user-selected passwords are typically quite simple, it has been
   proposed that servers should not accept passwords for HTTP Digest
   which are in the dictionary [RFC2617].  This potential threat does
   not exist in HTTP Digest 2G AKA because the algorithm will use SIM
   originated passwords.  However, the end-user must still be careful
   with PIN codes.  Even though HTTP Digest 2G AKA password requests are
   never displayed to the end-user, the end-user will be authenticated
   to the SIM via a PIN code.  Commonly known initial PIN codes are
   typically installed to the SIM during manufacturing and if the end-
   users do not change them, there is a danger than an unauthorized user
   may be able to use the device.  Naturally this requires that the
   unauthorized user has access to the physical device, and that the
   end-user has not changed the initial PIN code.  For this reason, end-
   users are strongly encouraged to change their PIN codes when they
   receive a SIM.

8.5.  Session Protection

   Digest 2G AKA is able to generate an additional session key for
   integrity (Kc) protection.  Even though this document does not
   specify the use of these additional keys, they may be used for
   creating additional security within HTTP authentication or some other
   security mechanisms.




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8.6.  Replay Protection

   The generation of RAND used as one-time or very limited-use nonces
   and the use of the integrity protection of qop=auth-int will limit
   the possibility of replay attacks.

   In GSM, the network is allowed to re-use the RAND challenge in
   consecutive authentication exchanges.  This is not allowed in Digest
   2G AKA.  The server is mandated to use fresh triplets (RAND
   challenges) in consecutive authentication exchanges.  Digest 2G AKA
   does not mandate any means for the client to check if the RANDs are
   fresh, so the security of the scheme leans on the secrecy of the
   triplets.  However, the peer MAY employ implementation-specific
   mechanisms to remember some of the previously used RANDs, and the
   client MAY check the freshness of the server's RANDs.

8.7.  Mutual Authentication

   With Digest 2G AKA, network authentication is performed only after UE
   authentication, in contrary to Digest AKA [RFC3310] in which the UE
   authenticates the network before responding to the challenge.  To
   prevent an impersonation attack of the server to the UE, the
   authentication of the server to the UE SHOULD be improved by
   protecting the communication with TLS.  An attacker succeeds only if
   he can break both, the certificate-based TLS authentication to the UE
   and mutual authentication provided by HTTP Digest using a password
   derived from GSM procedures.  One way to break TLS is to compromise
   the certificate.

8.8.  Flooding the Authentication Centre

   The server typically obtains authentication vectors from the
   Authentication Centre (AuC).  Digest 2G AKA introduces a new usage
   for the AuC.  The protocols between the server and the AuC are out of
   the scope of this document.  However, it should be noted that a
   malicious client may generate a lot of protocol requests to mount a
   denial of service attack.  The server implementation SHOULD take this
   into account and SHOULD take steps to limit the traffic that it
   generates towards the AuC, preventing the attacker from flooding the
   AuC and from extending the denial of service attack from Digest 2G
   AKA to other users of the AuC.

8.9.  AKA Security

   Evolutions of GSM networks, specifically Universal Mobile
   Telecommunications System (UMTS) and IP Multimedia System (IMS)
   networks, use an enhanced shared secret based mechanism for
   authentication known as Authentication and Key Agreement (AKA).  In



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   these networks, AKA is typically run in a UMTS Services Identity
   Module (USIM) or IP Multimedia Services Identity Module (ISIM).  GSM
   phones can also be equipped with a USIM or ISIM.  In that case,
   Digest AKA as described in [RFC3310] is used for authentication as
   opposed to Digest 2G AKA.


9.  Acknowledgements

   This memo is based on an initial draft written by Brett Wallis
   (draft-ietf-http-digest-auth-a3a8-01).


10.  References

10.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

10.2.  Informative References

   [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.

   [RFC3310]  Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer
              Protocol (HTTP) Digest Authentication Using Authentication
              and Key Agreement (AKA)", RFC 3310, September 2002.

   [TS55.205]
              "Specification of the GSM- MILENAGE algorithms (Release
              9)", December 2009.








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Author's Address

   Lionel Morand
   France Telecom - Orange
   38-40 rue du general Leclerc
   Issy-Les-Moulineaux Cedex 9  92794
   France

   Phone: +33 1 4529 6257
   Email: lionel.morand@orange-ftgroup.com









































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