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Versions: 00 01 02 RFC 4169

Network Working Group                                        V. Torvinen
Internet-Draft                                                  J. Arkko
Expires: March 1, 2004                                        M. Näslund
                                                                Ericsson
                                                       September 1, 2003


     Hypertext Transfer Protocol (HTTP) Digest Authentication Using
            Authentication and Key Agreement (AKA) Version-2
                draft-torvinen-http-digest-aka-v2-01.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/ietf/1id-abstracts.txt.

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

   This Internet-Draft will expire on March 1, 2004.

Copyright Notice

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

Abstract

   HTTP Digest is known to be vulnerable to man-in-the-middle attacks if
   the client fails to authenticate the server in TLS, or if the same
   passwords are used for authentication in some other context without
   TLS. This is a general problem that exist not just with HTTP Digest
   but also with other IETF protocols that use tunneled authentication.
   This document defines version 2 of the HTTP Digest AKA algorithm.
   This algorithm can be implemented in a way that it is resistant to
   the man-in-the-middle attack.





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Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1 Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  HTTP Digest AKAv2  . . . . . . . . . . . . . . . . . . . . . .  8
   3.1 Password generation  . . . . . . . . . . . . . . . . . . . . .  8
   3.2 Session keys . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Example Digest AKAv2 Operation . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.1 Multiple Authentication Schemes and Algorithms . . . . . . . . 11
   5.2 Session Protection . . . . . . . . . . . . . . . . . . . . . . 11
   5.3 Man-in-the-middle attacks  . . . . . . . . . . . . . . . . . . 11
   5.4 Entropy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   6.1 Registration Information . . . . . . . . . . . . . . . . . . . 14
       Normative References . . . . . . . . . . . . . . . . . . . . . 15
       Informative References . . . . . . . . . . . . . . . . . . . . 16
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
       Intellectual Property and Copyright Statements . . . . . . . . 18































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1. Requirements notation

   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].














































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2. Introduction

   The Hypertext Transfer Protocol (HTTP) Digest Authentication,
   described in [RFC2617], has been extended in [RFC3310] to support
   Authentication and Key Agreement (AKA) mechanism [AKA-REF]. AKA
   mechanism performs authentication and session key distribution in
   Universal Mobile Telecommunications System (UMTS) networks. HTTP
   Digest AKA enables the usage of AKA as a one-time password generation
   mechanism for Digest authentication.

   HTTP Digest is known to be vulnerable to man-in-the-middle attacks,
   even when run inside TLS, if the same HTTP Digest authentication
   credentials are used in some other context without TLS. The attacker
   may initiate a TLS session with a server, and when the server
   challenges the attacker with HTTP Digest, the attacker masquerades
   the server to the victim. If the victim responds to the challenge,
   the attacker is able to use this response towards the server in HTTP
   Digest. Note that this attack is an instance of general attack that
   affects a number of IETF protocols such as PIC. The general problem
   is discussed in [Asokan-Niemi-Nyberg] and
   [Puthenkulam-binding-draft].

   Because of the previous vulnerability, the use of HTTP Digest "AKAv1"
   should be limited to the situations where the client is able to
   demonstrate that in addition to the AKA response, it possess the AKA
   session keys. This is possible, for example, if the underlying
   security protocol uses the AKA generated session keys to protect the
   authentication response. This is the case for example in the 3GPP IP
   Multimedia Core Network Subsystem (IMS) where HTTP Digest "AKAv1" is
   currently applied. However, HTTP Digest "AKAv1" should not be used
   with tunnelled security protocols that do not utilize the AKA session
   keys. For example, the use of HTTP Digest "AKAv1" is not necessarily
   secure with TLS if the server side is authenticated using
   certificates and the client side is authenticated using HTTP Digest
   AKA.

   There are at least four potential solutions to the problem:

   1.  The use of the authentication credentials is limited to one
       application only. However, this would increase the total number
       of authentication credentials for an end-user, and would cause
       scalability problems in the server side.

   2.  The keys used in the underlying security protocols are somehow
       bind to the keys used in the tunneled authentication protocol.
       However, this would cause problems with the current
       implementations of underlying security protocols. For example, it
       is not possible to use the session keys from TLS at application



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       layer. Furthermore, this solution would only solve the problem
       when HTTP Digest is used over one hop, and leave the problem of
       using HTTP Digest via multiple hops, e.g. via proxy servers,
       unsolved.

   3.  Authentication credentials are used in cryptographically
       different way for each media and/or access network. However, it
       may be difficult to know which underlying media is used below the
       application.

   4.  Authentication credentials are used in cryptographically
       different way for each application.

   This document specifies a new algorithm version for HTTP Digest AKA,
   i.e. "AKAv2". "AKAv2" specifies a cryptographically different way to
   use AKA credentials in use cases that are based either on HTTP Digest
   authentication or UMTS authentication (cf. approach 4 above). The
   only difference to "AKAv1" is that in addition to AKA response RES
   the AKA related session keys, IK and CK, are also used as the
   password for HTTP Digest. AKAv2 is immune to man-in-the-middle attack
   described above. However, if AKAv2 is used in some environment both
   with and without some underlying security, such as TLS, the problem
   still exists.

   New HTTP Digest AKA algorithm versions can be registered in IANA
   based on Expert Review. Documentation of new algorithm versions is
   not mandated as RFCs. However, "AKAv2" is documented as an RFC
   because the use of different AKA algorithm versions includes security
   implications that the implementators should be aware of. The
   extension version and security implications are presented in this
   document.

2.1 Terminology

   This chapter explains the terminology used in this document.

   AKA

      Authentication and Key Agreement.

      AKA is a challenge-response based mechanism that uses symmetric
      cryptography. AKA can be run in a UMTS IM Services Identity Module
      (ISIM) or in UMTS Subscriber Identity Module (USIM), which reside
      on a smart card like device that also provides tamper resistant
      storage of shared secrets.

   CK




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      Cipher Key. An AKA session key for encryption.

   CK'

      Cipher Key. HTTP Digest AKAv2 session key for encryption. CK' is
      derived from CK using a pseudo-random function.

   IK

      Integrity Key. An AKA session key for integrity check.

   IK'

      Integrity Key. HTTP Digest AKAv2 session key for integrity check.
      IK' is derived from IK using a pseudo-random function.

   ISIM

      IP Multimedia Services Identity Module. Sometimes ISIM is
      implemented using USIM.

   RES

      Authentication Response. Generated by the ISIM.

   PRF

      Pseudo-random function that is used to construct the AKAv2
      password and related session keys IK' and CK'. In this document,
      PRF is presented in format KD(secret, data) denoting a keyed
      digest algorithm (KD) performed to the data "data" with the secret
      "secret".

   SIM

      Subscriber Identity Module. GSM counter part for ISIM and USIM.

   UMTS

      Universal Mobile Telecommunications System.

   USIM

      UMTS Subscriber Identity Module. UMTS counter part for ISIM and
      SIM.

   XRES




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      Expected Authentication Response. In a successful authentication
      this is equal to RES.

















































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3. HTTP Digest AKAv2

   In general, the Digest AKAv2 operation is identical to the Digest
   AKAv1 operation described in  [RFC3310].  This chapter specifies the
   parts in which Digest AKAv2 is different from Digest AKAv1 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
   [RFC3310].

   In order to direct the client into using AKAv2 for authentication
   instead of other AKA versions or other HTTP Digest algorithms, the
   AKA version directive of [RFC3310] shall have the following new
   value:


             aka-version         =  "AKAv2"

   The AKA version directive is used as a part of the algorithm field as
   defined in [RFC3310].


             Example:  algorithm=AKAv2-MD5


3.1 Password generation

   The client shall use base64 encoded [RFC2045] parameters
   PRF(RES||IK||CK, "http-digest-akav2-password") as a "password" when
   calculating the HTTP Digest response directive for AKAv2.

   The server shall use base64 encoded [RFC2045] parameters
   PRF(XRES||IK||CK, "http-digest-akav2-password") as a "password" when
   checking the HTTP Digest response or when calculating the
   "response-auth" of the "Authentication-Info" header.

   The pseudo-random function (PRF) used to construct the HTTP Digest
   password is equal to HMAC [RFC2104] using the hash algorithm that is
   used in producing the digest and the checksum. For example, if the
   algorithm is AKAv2-MD5, then the PRF is HMAC_MD5.

   The string "http-digest-akav2-password" included in the key
   derivation is case sensitive.

3.2 Session keys

   Even though HTTP Digest AKA framework does not specify the use of the
   session keys IK and CK for confidentiality and integrity protection,



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   the keys may be used for creating additional security within HTTP
   authentication or some other security mechanism. However, the
   original session keys IK and CK MUST NOT be directly used for such
   additional security in "AKAv2". Instead, session keys IK' and CK' are
   derived from the original keys IK and CK in the following way:

      IK' = PRF(IK, "http-digest-akav2-integritykey")

      CK' = PRF(CK, "http-digest-akav2-cipherkey")

   If the session keys are used with some other applications than HTTP
   authentication, they MUST use some other application specific string
   than "http-digest-akav2-integritykey" or
   "http-digest-akav2-cipherkey" to mask the original session keys.

   The pseudo-random function (PRF) used to construct the HTTP Digest
   session keys is equal to HMAC [RFC2104] using the hash algorithm that
   is used in producing the digest and the checksum. For example, if the
   algorithm is AKAv2-MD5, then the PRF is HMAC_MD5. The algorithm MUST
   be used in HMAC format as defined in [RFC2104].

   The strings "http-digest-akav2-integritykey" and
   "http-digest-akav2-cipherkey" included in the key derivation are case
   sensitive.



























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4. Example Digest AKAv2 Operation

   This document does not introduce any changes to the operations of
   HTTP Digest or HTTP Digest AKA. Examples defined in [RFC3310] applies
   directly to AKAv2 with the following two exceptions:

   1.  The algorithm directive has a prefix "AKAv2" instead of "AKAv1".

   2.  The HTTP Digest password is derived from base64 encoded
       PRF(RES||IK||CK, "http-digest-akav2-password") or
       PRF(XRES||IK||CK, "http-digest-akav2-password") instead of (RES)
       or (XRES) respectively.

   3.  The optional session keys are derived from PRF(IK,
       "http-digest-akav2-integritykey") and PRF(CK,
       "http-digest-akav2-cipherkey") instead of IK and CK respectively.

   Note that the password in "AKAv1" is in binary format. "AKAv2"
   password is base64 encoded [RFC2045].
































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5. Security Considerations

5.1 Multiple Authentication Schemes and Algorithms

   The rules for an user agent for choosing among multiple
   authentication schemes and algorithms are as defined in [RFC3310]
   except that the user agent MUST choose "AKAv2" if both "AKAv1" and
   "AKAv2" are present.

   Since HTTP Digest is known to be vulnerable for bidding-down attack
   in environments where multiple authentication schemes and/or
   algorithms are used, the system implementators should pay special
   attention for scenarios where both "AKAv1" and "AKAv2" are used.
   Especially if the AKA generated sessions keys or some other
   additional security measures to authenticate the clients, such as
   client certificates, are not used, the use of both AKA algorithm
   versions should be avoided.

5.2 Session Protection

   Even though "AKAv2" uses the additional integrity (IK) and
   confidentiality (CK) keys as a part of HTTP Digest AKA password,
   these session keys may still be used for creating additional security
   within HTTP authentication or some other security mechanism. This
   recommendation is based on the assumption that algorithms used in
   HTTP Digest, such as MD5, are sufficiently strong one-way functions,
   and consequently HTTP Digest responses leak no or very little
   computational information about IK and CK. Furthermore, the session
   keys are masked into IK' and CK' before they can be used for session
   protection.

5.3 Man-in-the-middle attacks

   [Asokan-Niemi-Nyberg] describe a "man-in-the-middle" attack related
   to tunnelled authentication protocols. The attack can occur e.g. in
   EAP context or any similar contexts where tunnelled authentication is
   used and where the same authentication credentials are used without
   protection in some other context or the client fails to authenticate
   the server.

   For example, the use of TLS with HTTP Digest authentication (i.e. TLS
   for server authentication, and subsequent use of HTTP Digest for
   client authentication) is an instance of such scenario. HTTP
   challenges and responses can be fetched from and to different TLS
   tunnels without noticing where they originally came from. Especially,
   the attack is easy to perform if the client fails to authenticate the
   server. If the same HTTP credentials are used with unsecured
   connection, the attack is also easy to perform.



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   This is how the "man-in-the-middle" attack works with HTTP Digest and
   TLS if the victim (i.e. the client) fails to authenticate the server:

   1.  The victim contacts the attacker using TLS. If the attacker has a
       valid server certificate, the client may continue talking to the
       attacker and use some HTTP authentication compatible protocol,
       such as Session Initiation Protocol (SIP).

   2.  The attacker contacts some real proxy/server also using TLS and
       some HTTP authentication compatible protocol. The proxy/server
       responds to the attacker with HTTP Authentication challenge.

   3.  The attacker forwards the HTTP Authentication challenge from the
       proxy/server to the victim. If the victim is not careful, and
       check that the identity in the server certificate in TLS matches
       the realm in the HTTP authentication challenge, it may send a new
       request which carries a valid response to the HTTP Authentication
       challenge.

   4.  The attacker may use the response with the victims HTTP Digest
       username and password to authenticate itself to the proxy/server.

   The man-in-the-middle attack is not possible if the client compares
   the identities in the TLS server certificate and the HTTP Digest
   authentication challenge. Note that with HTTP Basic, the client would
   send the password to the attacker.

   Another variant of the "man-in-the-middle" attack is the so-called
   "interleaving attack". This attack is possible if the HTTP Digest
   authentication credentials are used in several contexts, and in one
   of them without protection.

   This is how the attack could proceed:

   1.  The attacker establishes a TLS tunnel to the proxy/server using
       one-way server authentication. The attacker sends a request to
       the proxy/server.

   2.  The proxy/server challenges the attacker with HTTP Digest
       challenge.

   3.  The attacker challenges the victim in some other context using
       the challenge carried in the HTTP Digest challenge. The HTTP
       Digest challenge need to be modified to the format used in the
       protocol of this other context.

   4.  The victim responds with a response.




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   5.  The attacker uses the response from the other context for
       authentication in HTTP Digest.

   6.  The proxy/server accepts the response, and delivers the service
       to the attacker.

   In some circumstances, HTTP Digest AKAv1 may be vulnerable for the
   interleaving attack. In particular, if ISIM is implemented using USIM
   the HTTP Digest AKAv1 should not be used with tunneled security
   protocols unless the AKA related session keys, IK and CK, are somehow
   used with the solution.

   HTTP Digest AKAv2 is not vulnerable for this interleaving attack, and
   it can be used with tunneled security protocols without using the
   related AKA session keys.

5.4 Entropy

   AKAv1 passwords should only be used as one-time passwords if the
   entropy of the used RES value is limited (e.g., only 32 bits). For
   this reason, the reuse of the same RES value in authenticating
   subsequent requests and responses is not recommended. Furthermore,
   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 with AKAv1.

   Passwords generated using AKAv2 can more securely be used for
   authenticating subsequent requests and responses because the
   concatenation of AKA credentials (i.e. RES||IK||CK) makes the
   passwords significantly longer, and the pseudo-random function
   heuristically provides an entropy equal to the length of this string,
   or, the length of the PRF output, whichever is the shortest. The user
   agent does not need to assume that AKAv2 passwords are limited to
   one-time use only, and it may try to re-use the AKAv2 passwords with
   the server. However, note that AKAv2 passwords can not be re-used
   with HTTP Digest AKAv2 algorithm because such authentication
   challenge will automatically generate a fresh password. AKAv2
   passwords can be used with other HTTP Digest algorithms, such as
   "MD5".

   The underlying AKA protocol (e.g. UMTS AKA) has been designed to keep
   CK and IK confidential but will typically send RES in the clear. We
   note that even if (by some unfortunate missuse of AKA) RES values
   were revealed, the inclusion of RES in PRF(RES||IK||CK) is still
   benefitial as it makes pre-calculated dictionaries of IK||CK values
   rather useless (though such dictionaries are anyway infeasible for
   typical sizes of IK and CK).




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6. IANA Considerations

   This document specifies a new aka-version, "AKAv2", to the
   aka-version namespace maintained by IANA. The allocation of new
   aka-versions is up to Expert Review as outlined in [RFC2434].

6.1 Registration Information

   To: ietf-digest-aka@iana.org

   Subject: Registration of a new AKA version

   Version identifier: "AKAv2"

   Contacts for further information: vesa.torvinen@ericsson.com,
   jari.arkko@ericsson.com or mats.naslund@ericsson.com



































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Normative References

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

   [RFC2104]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
              Keyed-Hashing for Message Authentication", RFC 2104,
              February 1997.

   [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.

   [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.


























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Informative References

   [AKA-REF]  3rd Generation Partnership Project, "Security Architecture
              (Release 4)", TS 33.102, December 2001.

   [Asokan-Niemi-Nyberg]
              Asokan, N., Niemi, V. and K. Nyberg, "Man-in-the-Middle in
              Tunnelled Authentication Protocols", Cryptology ePrint
              Archive, http://eprint.iacr.org Report 2002/163, October
              2002.

   [Puthenkulam-binding-draft]
              Puthenkulam, J., Lortz, V., Palekar, A. and D. Simon, "The
              Compound Authentication Binding Problem", IETF, Work in
              progress draft-puthenkulam-eap-binding-02.txt, March 2003.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.


Authors' Addresses

   Vesa Torvinen
   Ericsson
   Joukahaisenkatu 1
   Turku  FIN 20520
   Finland

   Phone: +358 40 7230822
   EMail: vesa.torvinen@ericsson.com


   Jari Arkko
   Ericsson
   Hirsalantie 1
   Jorvas  FIN 02420
   Finland

   Phone: +358 40 5079256
   EMail: jari.arkko@ericsson.com










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   Mats Näslund
   Ericsson
   Torshamnsgatan 23
   Stockholm  SE 16480
   Sweden

   Phone: +46 8 58533739
   EMail: mats.naslund@ericsson.com











































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Intellectual Property Statement

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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































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