Network Working Group V. Torvinen Internet-Draft J. Arkko Expires: December 15, 2003 Ericsson June 16, 2003 Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA) Version-2 draft-torvinen-http-digest-aka-v2-00.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 December 15, 2003. 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, even when run inside TLS, if the same passwords are used for authentication in some other context without TLS. This is a general problem that affects not just HTTP digest but also other IETF protocols. However, for a class of strong algorithms the attack is avoidable. This document defines version 2 of the HTTP Digest AKA algorithm. Unlike previous versions of HTTP Digest such as MD5 or AKAv1, this algorithm is immune to the man-in-the-middle attack. Torvinen & Arkko Expires December 15, 2003 [Page 1] Internet-Draft HTTP Digest AKAv2 June 2003 Table of Contents 1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Digest password generation in AKAv2 . . . . . . . . . . . . . 7 4. Example Digest AKAv2 Operation . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5.1 Multiple Authentication Schemes and Algorithms . . . . . . . . 9 5.2 Session Protection . . . . . . . . . . . . . . . . . . . . . . 9 5.3 Man-in-the-middle attacks . . . . . . . . . . . . . . . . . . 9 5.4 Entropy . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 6.1 Registration Information . . . . . . . . . . . . . . . . . . . 12 Normative References . . . . . . . . . . . . . . . . . . . . . 13 Informative References . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . 15 Torvinen & Arkko Expires December 15, 2003 [Page 2] Internet-Draft HTTP Digest AKAv2 June 2003 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]. Torvinen & Arkko Expires December 15, 2003 [Page 3] Internet-Draft HTTP Digest AKAv2 June 2003 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 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 Torvinen & Arkko Expires December 15, 2003 [Page 4] Internet-Draft HTTP Digest AKAv2 June 2003 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 applications 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 Torvinen & Arkko Expires December 15, 2003 [Page 5] Internet-Draft HTTP Digest AKAv2 June 2003 Cipher Key. An AKA session key for encryption. IK Integrity Key. An AKA session key for integrity check. ISIM IP Multimedia Services Identity Module. Sometimes ISIM is implemented using USIM. RES Authentication Response. Generated by the ISIM. 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. Torvinen & Arkko Expires December 15, 2003 [Page 6] Internet-Draft HTTP Digest AKAv2 June 2003 3. Digest password generation in 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 The client shall use the concatenated AKA parameters (RES||IK||CK) as a "password" when calculating the HTTP Digest response directive for AKAv2. The server shall use the concatenated AKA parameters (XRES||IK||CK) as a "password" when checking the HTTP Digest response or when calculating the "response-auth" of the "Authentication-Info" header. Torvinen & Arkko Expires December 15, 2003 [Page 7] Internet-Draft HTTP Digest AKAv2 June 2003 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 (RES||IK||CK) or (XRES||IK||CK) instead of (RES) or (XRES) respectively. Torvinen & Arkko Expires December 15, 2003 [Page 8] Internet-Draft HTTP Digest AKAv2 June 2003 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. 5.3 Man-in-the-middle attacks [Asokan-Niemi-Nyberg] describe a "man-in-the-middle" attack related to tunnelled authentication protocols. [Asokan-Niemi-Nyberg] discuss the attack mostly in EAP context; however, it can exist in 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. This is how the "man-in-the-middle" attack works with HTTP Digest and Torvinen & Arkko Expires December 15, 2003 [Page 9] Internet-Draft HTTP Digest AKAv2 June 2003 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. 5. The attacker uses the response from the other context for Torvinen & Arkko Expires December 15, 2003 [Page 10] Internet-Draft HTTP Digest AKAv2 June 2003 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 interleaving attack. 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. 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, the length of the RES still matters because the attacker may try to use pre-calculated dictionaries to guess the (RES||IK||CK). The longer the RES is, the more difficult it is for the attacker to guess the (RES||IK||CK). Torvinen & Arkko Expires December 15, 2003 [Page 11] Internet-Draft HTTP Digest AKAv2 June 2003 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.fi or jari.arkko@ericsson.com Torvinen & Arkko Expires December 15, 2003 [Page 12] Internet-Draft HTTP Digest AKAv2 June 2003 Normative References [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. Torvinen & Arkko Expires December 15, 2003 [Page 13] Internet-Draft HTTP Digest AKAv2 June 2003 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.fi Jari Arkko Ericsson Hirsalantie 1 Jorvas FIN 02420 Finland Phone: +358 40 5079256 EMail: jari.arkko@ericsson.com Torvinen & Arkko Expires December 15, 2003 [Page 14] Internet-Draft HTTP Digest AKAv2 June 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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