Network Working Group Abhijit Menon-Sen Internet-Draft Oryx Mail Systems GmbH Intended Status: Proposed Standard Chris Newman Expires: September 6, 2007 Sun Microsystems March 2007 Salted Challenge Response Authentication Mechanism (SCRAM) draft-newman-auth-scram-04.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering 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 expires in September 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract The secure authentication mechanism most widely deployed and used by Internet application protocols, is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism which could be addressed by the use of a challenge Menon-Sen and Newman Expires September 2007 [Page 1] Internet-draft March 2007 response authentication mechanism protected by TLS. Unfortunately, the challenge response mechanisms presently on the standards track all fail to meet requirements necessary for widespread deployment and have had success only in limited use. This specification describes the Salted Challenge Response Authentication Mechanism (SCRAM) which addresses the deployability requirements. When used in combination with Transport Layer Security or an equivalent security layer, this mechanism could improve the status-quo for application protocol authentication and provide a suitable choice for a mandatory-to-implement mechanism for future application protocol standards. 1. Conventions 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 [RFC2119]. Formal syntax is defined by [RFC4234] including the core rules defined in Appendix B of [RFC4234]. Example lines prefaced by "C:" are sent by the client and ones prefaced by "S:" by the server. If a single "C:" or "S:" label applies to multiple lines, then the line breaks between those lines are for editorial clarity only and are not part of the actual protocol exchange. 1.1. Terminology This document uses several terms defined in [RFC2828] ("Internet Security Glossary") including the following: authentication, authentication exchange, authentication information, brute force, challenge-response, cryptographic hash function, dictionary attack, eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, one-way encryption function, password, replay attack and salt. Readers not familiar with these terms should use that glossary as a reference. Some clarifications and additional definitions follow: - Authentication information: Information used to verify an identity claimed by a SCRAM client. The authentication information for a SCRAM identity consists of salt, and for each cryptographic hash function supported, it includes the "StoredKey" and the "ServerKey" (the latter two are defined in the algorithm Menon-Sen and Newman Expires September 2007 [Page 2] Internet-draft March 2007 overview). - Authentication database: The database used to look up the authentication information associated with a particular identity. For application protocols, LDAPv3 (see [RFC4510]) is frequently used as the authentication database. For network-level protocols such as PPP or 802.11x, the use of RADIUS is more common. - Base64: An encoding mechanism defined in [RFC2045] which converts an octet string input to a textual output string which can be easily displayed to a human. Although MIME permits arbitrary insertion of whitespace and other characters, use of base64 in SCRAM is restricted to the canonical form with no whitespace. - Octet: An 8-bit byte. - Octet string: A sequence of 8-bit bytes. - Salt: A random octet string that is combined with a password before applying a one-way encryption function. This value is used to protect passwords that are stored in an authentication database. 1.2. Notation The pseudocode description of the algorithm uses the following notations: - ":=": The variable on the left hand side represents the octet string resulting from the expression on the right hand side. - "+": Octet string concatenation. - "[ ]": A portion of an expression enclosed in "[" and "]" may not be included in the result under some circumstances. See the associated text for a description of those circumstances. - HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in [RFC2104]) using the octet string represented by "key" as the key and the octet string "str" as the input string. The size of the result is the hash result size for the hash function in use. For example, it is 20 octets for SHA-1 (see [RFC3174]). - H(str): Apply the cryptographic hash function to the octet string "str", producing an octet string as a result. The size of the result depends on the hash result size for the hash function in use. Menon-Sen and Newman Expires September 2007 [Page 3] Internet-draft March 2007 - Hi(str): Apply the cryptographic hash function to the octet string "str", then repeat the application on the output string for a number of iterations equal to the integer i minus 1. - XOR: Apply the exclusive-or operation to combine the octet string on the left of this operator with the octet string on the right of this operator. The length of the output and each of the two inputs will be the same for this use. 2. Introduction This specification describes the Salted Challenge Response Authentication Mechanism (SCRAM) which addresses the requirements necessary to deploy a challenge-response mechanism more widely than past attempts. When used in combination with Transport Layer Security (TLS, see [RFC4346]) or an equivalent security layer, this mechanism could improve the status-quo for application protocol authentication and provide a suitable choice for a mandatory-to- implement mechanism for future application protocol standards. For simplicity, this mechanism does not presently include negotiation of a security layer. It is intended to be used with an external security layer such as that provided by TLS or SSH. SCRAM provides the following protocol features: - The authentication information stored in the authentication database is not sufficient by itself to impersonate the client. The information is salted to prevent a pre-stored dictionary attack if the database is stolen. - The server does not gain the ability to impersonate the client to other servers (with an exception for server-authorized proxies). - The mechanism permits the use of a server-authorized proxy without requiring that proxy to have super-user rights with the back-end server. - A standard attribute is defined to enable storage of the authentication information in LDAPv3 (see [RFC4510]). - Bindings to several authentication frameworks are provided so the mechanism is not limited to a small subset of protocols. - Both the client and server can be authenticated by the protocol. - The cryptographic hash function used to authenticate can be Menon-Sen and Newman Expires September 2007 [Page 4] Internet-draft March 2007 upgraded gracefully without breaking backwards compatibility or risking downgrade attacks. For an in-depth discussion of why other challenge response mechanisms are not considered sufficient, see appendix A. For more information about the motivations behind the design of this mechanism, see appendix B. Comments regarding this draft may be sent either to the ietf- sasl@imc.org mailing list or to the authors. 3. SCRAM Algorithm Overview Here is a psuedocode overview of the complete SCRAM algorithm: SaltedPassword := Hi(HMAC(password, salt)) ClientKey := H(SaltedPassword) StoredKey := H(ClientKey) Message := [FirstClientMessage + "," + FirstServerMessage + "," +] FinalClientMessageExcludingP ClientSignature := HMAC(StoredKey, Message) ClientProof := ClientKey XOR ClientSignature ServerKey := HMAC(SaltedPassword, salt) ServerSignature := HMAC(ServerKey, Message) The client has access to the user's password and once it gets the value of the salt and the value of i from the server, it can determine all of the above values. The authentication information which the server stores consists of the "salt", the "StoredKey" and the "ServerKey". The server authenticates the client by computing the ClientSignature, exclusive-oring that with the ClientProof to recover the ClientKey and verifying the correctness of the ClientKey by applying the hash function and comparing the result to the StoredKey. 4. SCRAM Authentication Exchange The SCRAM protocol is a textual protocol with a comma-separated list of attribute value pairs. Each attribute has a name with one letter. An example of a simple protocol exchange follows: C: r=ClientNonce,n=Chris Newman S: r=ClientNonceServerNonce,i=128,s=PxR/wv+epq C: r=ClientNonceServerNonce, c=QWBv+XbwcaI/wp,p=WxPv/siO5l+qxN4 Menon-Sen and Newman Expires September 2007 [Page 5] Internet-draft March 2007 S: v=WxPv/siO5l+qxN4 The client sends the first message to provide the user name whose password will be used, a random and unique nonce and a list of the cryptographic hash functions the client supports. The server responds by appending additional randomness on the end of the nonce, providing the list of hash functions it supports, and providing the base64-encoded salt from that user's authentication information. The client then authenticates by repeating the nonce, stating the resource to which it is authenticating, an optional base64-encoded channel binding (discussed later) and a proof that it has the user's password. Unless the client has choosen to skip the server authentication step (discussed later), the server responds with a verification proof that the server has the appropriate password verifier. A more detailed description of the protocol elements follows: - a: This attribute can be provided in the client's first message. It specifies the authorization identity. When this is omitted or empty, it is presumed to be the same as the "n" attribute. When this is different from the "n" attribute, then the "n" attribute specifies the user name whose password will be used for authentication, while the "a" attribute specifies the user whose identity will be associated with the connection subsequent to successful authentication and authorization. This is typically used when an administrative user wishes to impersonate another user to perform some management role for that user. It can also be used by proxies in some situations (see appendix A for more information). The syntax of this field is the same as that of the "n" field with respect to quoting of '=' and ','. - c: This base64-encoded string is the channel binding. Whether this is included and the meaning of the contents of this string depends on the external security layer used with this mechanism. See section XXX for more details. If this is present, the authentication MUST fail unless this value is successfully verified. In other words, if it appears when not expected then an automatic authentication failure results. This is necessary to detect a man-in-the-middle attack on the security layer (see security considerations for details). - n: This is the name of the user whose password is used for authentication. In most cases, the "a" attribute is omitted and this also specifies the identity which will be associated with the connection subsequent to authentication and authorization. If the actual name contains the characters ',' or '=', then a quoting mechanism is applied which converts these characters to '=2C' and Menon-Sen and Newman Expires September 2007 [Page 6] Internet-draft March 2007 '=3D' respectively. If the server receives a name string which contains '=' not followed by either '2C' or '3D', then the server MUST fail the authentication. - p: This base64-encoded string provides the client's proof of authentication. This is the last attribute in the message. See the algorithm overview for the computation. - r: This is a sequence of random printable characters excluding ',' which forms the nonce used as input to the hash function. No quoting is applied to this string (unless the binding of SCRAM to a particular protocol states otherwise). The client provides the first part of the string and the server provides the second part. It is important that this be different for each authentication and it is important that the client verifies the first part of this string used in all messages was provided by the client. - s: This is a base64-encoded string which represents the salt used by the server for this user. - v: This is used to verify the server has access to the user's authentication information. If the client supplies the "v" attribute with an empty value in the message with a "p", that indicates the client does not want the server to provide verification. When the server includes this in the final message, it is always the last attribute in the message. See the next section for the computation. 5. Security Considerations If the authentication exchange is performed without a strong security layer, then a passive eavesdropper can gain sufficient information to mount an offline dictionary or brute-force attack which can be used to recover the user's password. The amount of time necessary for this attack depends on the cryptographic hash function selected, the strength of the password and the iteration count supplied by the server. An external security layer with strong encryption will prevent this attack. If the external security layer used to protect the SCRAM exchange uses an anonymous key exchange, then the SCRAM channel binding mechanism can be used to detect a man-in-the-middle attack on the security layer and cause the authentication to fail as a result. However, the man-in-the-middle attacker will have gained sufficient information to mount an offline dictionary or brute-force attack. For this reason, SCRAM includes the ability to increase the iteration count over time. Menon-Sen and Newman Expires September 2007 [Page 7] Internet-draft March 2007 If the authentication information is stolen from the authentication database, then an offline dictionary or brute-force attack can be used to recover the user's password. The use of salt mitigates this attack somewhat by requiring a separate attack on each password. Authentication mechanisms which protect against this attack are available (e.g., the EKE class of mechanisms), but the patent situation is presently unclear. If an attacker obtains the authentication information from the authentication repository and eavesdrops on one authentication exchange, the attacker gains the ability to impersonate that user to all servers providing SCRAM access using the same password and salt. 6. IANA considerations (Hash function names registry, SASL mechanism registration.) 7. Acknowedgements (Frank Ellermann already?) 8. Normative References [RFC2045] Freed, Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, Innosoft, November 1996. [RFC2104] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message Authentication", IBM, February 1997. [RFC2119] Bradner, "Key words for use in RFCs to Indicate pRequirement Levels", RFC 2119, Harvard University, March 1997. [RFC3174] Eastlake, Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, Motorola, September 2001 [RFC3986] Berners-Lee, Fielding, Masinter, "Uniform Resource Identifier (URI): Generic Syntax", RFC 3986, W3C/MIT, January 2005. [RFC4234] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, Brandenburg Internetworking, Demon Internet Ltd, October 2005. Menon-Sen and Newman Expires September 2007 [Page 8] Internet-draft March 2007 [RFC4346] Dierks, Rescorla, "The Transport Layer Security (TLS) Protocol, Version 1.1", RFC 4346, Brandenburg Internetworking, April 2006. [RFC4422] Melnikov, Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, Isode Limited, June 2006. 9. Informative References [RFC1939] Myers, Rose, "Post Office Protocol - Version 3", RFC 1939, Carnegie Mellon, May 1996. [RFC2195] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension for Simple Challenge/Response", RFC 2195, MCI, September 1997. [RFC2202] Cheng, Glenn, "Test Cases for HMAC-MD5 and HMAC-SHA-1", RFC 2202, IBM, September 1997 [RFC2289] Haller, Metz, Nesser, Straw, "A One-Time Password System", RFC 2289, STD0061, February 1998. [RFC2828] Shirey, "Internet Security Glossary", RFC 2828, FYI 0036, May 2000. [RFC4086] Eastlake, Schiller, Crocker, "Randomness Requirements for Security", RFC 4086, BCP 0106, Motorola Laboratories, June 2005. [RFC4120] Neuman, Yo, Hartman, Raebun, "The Kerberos Network Authentication Service (V5)", RFC 4120, USC-ISI, July 2005. [RFC4510] Zeilenga, "Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map", RFC 4510, June 2006. [DIGEST-MD5] Melnikov, "Using Digest Authentication as a SASL Mechanism", draft-ietf-sasl-rfc2831bis-11.txt, Isode Ltd., November 2006 10. Authors' Addresses Abhijit Menon-Sen Oryx Mail Systems GmbH Email: ams@oryx.com Menon-Sen and Newman Expires September 2007 [Page 9] Internet-draft March 2007 Chris Newman Sun Microsystems 1050 Lakes Drive West Covina, CA 91790 USA Email: chris.newman@sun.com Appendix A: Other Authentication Mechanisms (To be written.) Appendix B: Design Motivations (To be written.) Appendix C: SCRAM Examples (To be written.) Appendix D: SCRAM Interoperability Testing (To be written.) Intellectual Property Statement 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 in BCP 78 and BCP 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 at http://www.ietf.org/ipr. Menon-Sen and Newman Expires September 2007 [Page 10] Internet-draft March 2007 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. Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 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, THE IETF TRUST 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Menon-Sen and Newman Expires September 2007 [Page 11] Internet-draft March 2007 (RFC Editor: Please delete everything after this point) Open Issues Appendixes A, B and C need to be written. D too, and the framework implemented. Should the title include the acronym SASL to help the greppers? Changes since -03 - Seven years have passed, in which it became clear that DIGEST-MD5 suffered from unacceptably bad interoperability, so SCRAM-MD5 is now bad from the dead. - Be hash agnostic, so MD5 can be replaced more easily - General simplification. Menon-Sen and Newman Expires September 2007 [Page 12]