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Network Working Group                                        V. TorvinenRequest for Comments: 4169                             Turku PolytechnicCategory: Informational                                         J. Arkko                                                              M. Naslund                                                                Ericsson                                                           November 2005Hypertext Transfer Protocol (HTTP) Digest Authentication UsingAuthentication and Key Agreement (AKA) Version-2Status of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   HTTP Digest, as specified inRFC 2617, 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   exists not just with HTTP Digest, but also with other IETF protocols   that use tunneled authentication.  This document specifies version 2   of the HTTP Digest AKA algorithm (RFC 3310).  This algorithm can be   implemented in a way that it is resistant to the man-in-the-middle   attack.Torvinen                     Informational                      [Page 1]

RFC 4169                   HTTP Digest AKAv2               November 2005Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . .42.  HTTP Digest AKAv2  . . . . . . . . . . . . . . . . . . . . . .52.1.  Password generation  . . . . . . . . . . . . . . . . . .62.2.  Session keys . . . . . . . . . . . . . . . . . . . . . .63.  Example Digest AKAv2 Operation . . . . . . . . . . . . . . . .74.  Security Considerations  . . . . . . . . . . . . . . . . . . .74.1.  Multiple Authentication Schemes and Algorithms . . . . .74.2.  Session Protection . . . . . . . . . . . . . . . . . . .74.3.  Man-in-the-middle attacks  . . . . . . . . . . . . . . .84.4.  Entropy  . . . . . . . . . . . . . . . . . . . . . . . .95.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .105.1.  Registration Information . . . . . . . . . . . . . . . .106.  References . . . . . . . . . . . . . . . . . . . . . . . . . .116.1.  Normative References . . . . . . . . . . . . . . . . . .116.2.  Informative References . . . . . . . . . . . . . . . . .111.  Introduction   The Hypertext Transfer Protocol (HTTP) Digest Authentication,   described in [4], has been extended in [6] to support the   Authentication and Key Agreement (AKA) mechanism [7].  The AKA   mechanism performs authentication and session key agreement 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 a general attack   that affects a number of IETF protocols, such as PIC.  The general   problem is discussed in [8] and [9].   Because of the vulnerability described above, the use of HTTP Digest   "AKAv1" should be limited to the situations in which the client is   able to demonstrate that, in addition to the AKA response, it   possesses 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 DigestTorvinen                     Informational                      [Page 2]

RFC 4169                   HTTP Digest AKAv2               November 2005   "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.  In general, this approach is good and can be       recommended from the security point of view.  However, this will       increase the total number of authentication credentials for an       end-user, and may cause scalability problems in the server side.   2.  The keys used in the underlying security protocols are somehow       bound 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 the       application layer.  Furthermore, this solution would only solve       the problem when HTTP Digest is used over one hop, and would       leave the problem of using HTTP Digest via multiple hops (e.g.,       via proxy servers) unsolved.   3.  Authentication credentials are used in a 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 a 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 on either HTTP   Digest authentication or UMTS authentication (cf. approach 4 above).   The only difference to "AKAv1" is that, in addition to an AKA   response RES, the AKA related session keys, IK and CK, are also used   as the password for HTTP Digest.  AKAv2 is immune to the   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 with 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 of which the implementors should be aware.  TheTorvinen                     Informational                      [Page 3]

RFC 4169                   HTTP Digest AKAv2               November 2005   extension version and security implications are presented in this   document.1.1.  Terminology   This chapter explains the terminology 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 [3].   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      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.Torvinen                     Informational                      [Page 4]

RFC 4169                   HTTP Digest AKAv2               November 2005   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 the 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      Expected Authentication Response.  In a successful authentication,      this is equal to RES.2.  HTTP Digest AKAv2   In general, the Digest AKAv2 operation is identical to the Digest   AKAv1 operation described in [6].  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 [5], and   any elements not defined in this section are as defined in [6].   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 [6] 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 [6].      Example:  algorithm=AKAv2-MD5Torvinen                     Informational                      [Page 5]

RFC 4169                   HTTP Digest AKAv2               November 20052.1.  Password Generation   The client shall use base64 encoded [1] 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 [1] 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 [2] 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.2.2.  Session keys   Even though the HTTP Digest AKA framework does not specify the use of   the session keys IK and CK for confidentiality and integrity   protection, 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   re-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")   Any application using the HTTP authentication framework is allowed to   use these masked session keys.  The unmasked session keys MAY also be   re-used in some other context if application-specific strings other   than "http-digest-akav2-integritykey" or   "http-digest-akav2-cipherkey" are used to mask the original session   keys.   The pseudo-random function (PRF) used to construct the HTTP Digest   session keys is equal to HMAC [2] 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 the HMAC format, as defined in [2].Torvinen                     Informational                      [Page 6]

RFC 4169                   HTTP Digest AKAv2               November 2005   The strings "http-digest-akav2-integritykey" and "http-digest-akav2-   cipherkey" included in the key derivation are case sensitive.3.  Example Digest AKAv2 Operation   This document does not introduce any changes to the operations of   HTTP Digest or HTTP Digest AKA.  Examples defined in [6] apply   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.  The "AKAv2"   password is base64 encoded [1].4.  Security Considerations4.1.  Multiple Authentication Schemes and Algorithms   The rules for a user agent for choosing among multiple authentication   schemes and algorithms are as defined in [6], 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 attacks   in environments where multiple authentication schemes and/or   algorithms are used, the system implementors should pay special   attention to scenarios in which both "AKAv1" and "AKAv2" are used.   The use of both AKA algorithm versions should be avoided, especially   if the AKA generated sessions keys or some other additional security   measures to authenticate the clients (e.g., client certificates) are   not used.4.2.  Session Protection   Even though "AKAv2" uses the additional integrity (IK) and   confidentiality (CK) keys as a part of the 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 littleTorvinen                     Informational                      [Page 7]

RFC 4169                   HTTP Digest AKAv2               November 2005   computational information about IK and CK.  Furthermore, the session   keys are masked into IK' and CK' before they can be used for session   protection.4.3.  Man-in-the-Middle Attacks   Reference [8] describes a "man-in-the-middle" attack related to   tunnelled authentication protocols.  The attack can occur in an 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 their origin.  The attack is especially easy   to perform if the client fails to authenticate the server.  If the   same HTTP credentials are used with an unsecured connection, the   attack is also easy to perform.   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 the Session Initiation Protocol (SIP).   2.  The attacker contacts a real proxy/server also using TLS and an       HTTP-authentication-compatible protocol.  The proxy/server       responds to the attacker with the 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       does not check whether the identity in the server certificate in       TLS matches the realm in the HTTP authentication challenge, it       may send a new request that 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.Torvinen                     Informational                      [Page 8]

RFC 4169                   HTTP Digest AKAv2               November 2005   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 the 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 needs to be modified to the format used in the       protocol of this other context.   4.  The victim responds with a response.   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 to this interleaving attack, and   it can be used with tunneled security protocols without using the   related AKA session keys.4.4.  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 re-use of the same RES value in authenticating   subsequent requests and responses is not recommended.  Furthermore,Torvinen                     Informational                      [Page 9]

RFC 4169                   HTTP Digest AKAv2               November 2005   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 cannot be re-used   with the HTTP Digest AKAv2 algorithm because such an 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 misuse of AKA) RES   values were revealed, the inclusion of RES in PRF(RES||IK||CK) is   still beneficial, as it makes pre-calculated dictionaries of IK||CK   values rather useless (though such dictionaries are infeasible for   typical sizes of IK and CK).5.  IANA Considerations   This document specifies a new aka-version, "AKAv2", to the   aka-version namespace maintained by IANA.  The procedure for   allocation of new aka-versions is defined in [6].5.1.  Registration Information   To: ietf-digest-aka@iana.org   Subject: Registration of a new AKA version   Version identifier: "AKAv2"   Contacts for further information: Vesa.Torvinen@turkuamk.fi,   jari.arkko@ericsson.com, or mats.naslund@ericsson.comTorvinen                     Informational                     [Page 10]

RFC 4169                   HTTP Digest AKAv2               November 20056.  References6.1.  Normative References   [1]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail        Extensions (MIME) Part One: Format of Internet Message Bodies",RFC 2045, November 1996.   [2]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing        for Message Authentication",RFC 2104, February 1997.   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [4]  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.   [5]  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.   [6]  Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer        Protocol (HTTP) Digest Authentication Using Authentication and        Key Agreement (AKA)",RFC 3310, September 2002.6.2.  Informative References   [7]  3rd Generation Partnership Project, "Security Architecture        (Release 4)", TS 33.102, December 2001.   [8]  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.   [9]  Puthenkulam, J., Lortz, V., Palekar, A., and D. Simon, "The        Compound Authentication Binding Problem", Work in Progress,        March 2003.Torvinen                     Informational                     [Page 11]

RFC 4169                   HTTP Digest AKAv2               November 2005Authors' Addresses   Vesa Torvinen   Turku Polytechnic   Ylhaistentie 2   Salo  FIN 24130   Finland   Phone: +358 10 5536210   EMail: vesa.torvinen@turkuamk.fi   Jari Arkko   Ericsson   Hirsalantie 1   Jorvas  FIN 02420   Finland   Phone: +358 40 5079256   EMail: jari.arkko@ericsson.com   Mats Naeslund   Ericsson   Torshamnsgatan 23   Stockholm  SE 16480   Sweden   Phone: +46 8 58533739   EMail: mats.naslund@ericsson.comTorvinen                     Informational                     [Page 12]

RFC 4169                   HTTP Digest AKAv2               November 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights 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; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Torvinen                     Informational                     [Page 13]