Network Working Group G. Zorn Internet-Draft H. Zhou Updates: 2865 (if approved) J. Salowey Expires: January 5, 2005 Cisco Systems July 7, 2004 Session Key Transport in RADIUS draft-zorn-radius-keyreq-02.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 January 5, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes an extension to the RADIUS protocol designed to support requests for, and delivery of, session key material between RADIUS clients and servers. The messages described in this document may be used for a wide variety of keying applications. Zorn, et al. Expires January 5, 2005 [Page 1] Internet-Draft Session Key Transport in RADIUS July 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Specification of Requirements . . . . . . . . . . . . . . . . 3 3. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Packet Types . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1 Key-Request . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 Key-Response . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Normative References . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . 11 Zorn, et al. Expires January 5, 2005 [Page 2] Internet-Draft Session Key Transport in RADIUS July 2004 1. Introduction AAA servers are a central repository of authentication credentials. Since authentication is typically a prerequisite to key distribution, it is natural for AAA servers to be able to deliver keys to clients for various purposes. One example of this is the provisioning of the link layer encryption keys used in IEEE 802.11 and 802.1X. There is a wide variety of additional uses for key distribution in RADIUS. This document describes an extension to the RADIUS protocol designed to support requests for, and delivery of, session key material between RADIUS clients and servers. One example of the applicability of these extensions is the case where the end of a session key's lifetime [KEYWRAP] is approaching, but the messages described in this document may be used for a wide variety of keying applications. Discussion of this draft may be directed to the authors. 2. Specification of Requirements 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]. 3. Packet Format Exactly one RADIUS packet is encapsulated in the UDP Data field [RFC0768] where the UDP Destination Port field indicates 1812 (decimal). When a reply is generated, the source and destination ports are reversed. A summary of the RADIUS data format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Authenticator | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attributes ... Zorn, et al. Expires January 5, 2005 [Page 3] Internet-Draft Session Key Transport in RADIUS July 2004 +-+-+-+-+-+-+-+-+-+-+-+-+- Code The Code field is one octet, and identifies the type of RADIUS packet. When a packet is received with an invalid Code field, it is silently discarded. The RADIUS Codes (decimal) defined in this document are as follows: Key-Request Key-Response Identifier The Identifier field is one octet, and aids in matching requests and replies. The RADIUS server can detect a duplicate request if it has the same client source IP address, source UDP port and Identifier within a short span of time. Length The Length field is two octets. It indicates the length of the packet including the Code, Identifier, Length, Authenticator and Attribute fields. Octets outside the range of the Length field MUST be treated as padding and ignored on reception. If the packet is shorter than the Length field indicates, it MUST be silently discarded. The minimum length is 20 and maximum length is 4096. Authenticator The Authenticator field is sixteen (16) octets. The most significant octet is transmitted first. This value is used to authenticate the reply from the RADIUS server. Request Authenticator In Key-Request packets, the Authenticator value is a 16 octet random number, called the Request Authenticator. The value SHOULD be unpredictable and unique over the lifetime of a secret (the password shared between the client and the RADIUS server), since repetition of an authenticator value in conjunction with the same secret would permit an attacker to reply with a previously intercepted response. Since it is expected that the same secret MAY be used to authenticate with Zorn, et al. Expires January 5, 2005 [Page 4] Internet-Draft Session Key Transport in RADIUS July 2004 servers in disparate geographic regions, the Request Authenticator field SHOULD exhibit global and temporal uniqueness. The Authenticator value in a Key-Request packet SHOULD also be unpredictable, lest an attacker trick a server into responding to a predicted future request, and then use the response to masquerade as that server to a future notification packet. Although protocols such as RADIUS are incapable of protecting against theft of an authenticated session via real-time active wiretapping attacks, generation of unique unpredictable requests can protect against a wide range of active attacks against authentication. Response Authenticator The value of the Authenticator field in the Key-Response packet is called the Response Authenticator, and contains a one-way MD5 hash calculated over a stream of octets consisting of: the RADIUS packet, beginning with the Code field, including the Identifier, the Length, the Request Authenticator field from the Key-Request packet, and the response Attributes, followed by the shared secret. That is, Acknowledgement Auth = MD5(Code+ID+Length+RequestAuth+Attributes+Secret) where '+' denotes concatenation. Administrative Note The secret shared between the client and the RADIUS server SHOULD be at least as large and unguessable as a well- chosen password. It is preferred that the secret be at least 16 octets. This is to ensure a sufficiently large range for the secret to provide protection against exhaustive search attacks. The secret MUST NOT be empty (length 0) since this would allow packets to be trivially forged. A RADIUS server MUST use the source IP address of the RADIUS UDP packet to decide which shared secret to use, so that RADIUS requests can be proxied. When using a forwarding proxy, the proxy must be able to alter the packet as it passes through in each direction - when the proxy forwards the request, the proxy MAY add a Proxy-State Attribute, and when the proxy forwards a response, it MUST remove its Zorn, et al. Expires January 5, 2005 [Page 5] Internet-Draft Session Key Transport in RADIUS July 2004 Proxy-State Attribute if it added one. Proxy-State is always added or removed after any other Proxy-States, but no other assumptions regarding its location within the list of attributes can be made. Since Access-Accept and Access-Reject replies are authenticated on the entire packet contents, the stripping of the Proxy-State attribute invalidates the signature in the packet - so the proxy has to re-sign it. Further details of RADIUS proxy implementation are outside the scope of this document. 4. Packet Types The RADIUS Packet type is determined by the Code field in the first octet of the Packet. 4.1 Key-Request Description Key-Request packets are sent to a RADIUS server to request the delivery of a session key. A RADIUS client wishing to request the delivery of a session key MUST transmit a RADIUS packet with the Code field set to (Key-Request). Upon receipt of an Key-Request packet from a valid client, the server MUST reply using either a Key-Response message or a Server-Error-Notification message [ERRMSG]. A Key-Request message MUST contain either a NAS-IP-Address Attribute [RFC2865] or a NAS-Identifier Attribute [RFC2865] or both. To help defeat spoofing attacks, a Key-Request message MUST contain either a Message-Authenticator Attribute [RFC2869] or a Message-Authentication-Code Attribute [KEYWRAP]. A Key-Request message SHOULD contain a Session-Id Attribute [LOGOFF] if one was returned from the server in the Access-Accept message for the session; if no Session-Id Attribute is included, the packet MUST contain a User-Name Attribute and such additional Attributes as are necessary to positively identify a given user session (e.g., Service-Type [RFC2865], Calling-Station-Id [RFC2865], etc.). A Key-Request packet MAY contain one or more Key Attributes [KEYWRAP] in order to request a key with a particular identifier or encrypted using a particular algorithm. Zorn, et al. Expires January 5, 2005 [Page 6] Internet-Draft Session Key Transport in RADIUS July 2004 A summary of the Key-Request packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Request Authenticator | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attributes ... +-+-+-+-+-+-+-+-+-+-+-+-+- Code for Key-Request Identifier The Identifier field MUST be changed whenever the content of the Attributes field changes, and whenever a valid reply has been received for a previous request. For retransmissions, the Identifier MUST remain unchanged. Request Authenticator The Request Authenticator value MUST be changed each time a new Identifier is used. Attributes The Attribute field is variable in length, and contains the list of required Attributes, as well as any desired optional Attributes. 4.2 Key-Response Description A Key-Response packet is sent by a RADIUS server in response to a Key-Request packet. A RADIUS server wishing to transfer keying material to a client in response to a Key-Request packet MUST transmit a RADIUS packet with the Code field set to Zorn, et al. Expires January 5, 2005 [Page 7] Internet-Draft Session Key Transport in RADIUS July 2004 (Key-Response). At least one Key Attribute [KEYWRAP] MUST be included in a Key-Response packet. A summary of the Key-Response packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Response Authenticator | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attributes ... +-+-+-+-+-+-+-+-+-+-+-+-+- Code for Key-Response Identifier The Identifier field is a copy of the Identifier field of the Key-Request packet which caused this Key-Response packet to be created. Response Authenticator The Response Authenticator value is calculated from the Key-Request packet, as described above. Attributes The Attribute field is variable in length, and MAY contain any desired optional Attributes in addition to the required Attributes. 5. IANA Considerations The criteria to be used by the Internet Assigned Numbers Authority (IANA) for assignment of numbers within namespaces defined within Zorn, et al. Expires January 5, 2005 [Page 8] Internet-Draft Session Key Transport in RADIUS July 2004 this document are identical to those given in [RFC3575]. 6. Security Considerations If the Key-Response packet is unauthenticated or if the shared secret is compromised, an attacker might be able to modify or replace the keying material. 7 Normative References [ERRMSG] Zorn, G., "RADIUS Error Messages", draft-zorn-radius-err-msg-01.txt (work in progress), July 2004. [KEYWRAP] Zorn, G., Zhang, T., Walker, J. and J. Salowey, "RADIUS Attributes for Key Delivery", draft-zorn-radius-keywrap-01.txt (work in progress), July 2004. [LOGOFF] Zorn, G., "User Session Tracking in RADIUS", draft-zorn-radius-logoff-03.txt (work in progress), July 2004. [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC2869] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions", RFC 2869, June 2000. [RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote Authentication Dial In User Service)", RFC 3575, July 2003. Zorn, et al. Expires January 5, 2005 [Page 9] Internet-Draft Session Key Transport in RADIUS July 2004 Authors' Addresses Glen Zorn Cisco Systems 2901 Third Avenue Suite 600 Seattle, WA 98121 US Phone: +1 (425) 344-8113 EMail: gwz@cisco.com Hao Zhou Cisco Systems 4125 Highlander Parkway REQ01/3/ Richfield, OH 44286 US Phone: +1 (330) 523-2132 EMail: hzhou@cisco.com Joseph Salowey Cisco Systems 2901 Third Avenue SEA1/6/ Seattle, WA 98121 US Phone: +1 (206) 256-3380 EMail: jsalowey@cisco.com Zorn, et al. Expires January 5, 2005 [Page 10] Internet-Draft Session Key Transport in RADIUS July 2004 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. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Zorn, et al. Expires January 5, 2005 [Page 11]