Network Working Group Scott Bradner Harvard University Allison Mankin USC/ISI Jeffrey I. Schiller Massachusetts Institute of Technology September 2002 A Framework for Purpose Built Keys (PBK) Status of this Memo This document is an Internet-Draft and is subject to the 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. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This memo considers the need to authenticate the source of a network communication where the actual identity of the source is not important but it is important to be sure that the source can not be spoofed and that successive messages in the communication come from the same source. This memo defines the use of specially generated public/private key pairs, known as Purpose Built Keys (PBKs), to provide this assurance. This memo is not a full specification of a Bradner, Mankin & Schiller [Page 1] Purpose Built Keys Framework September 2002 PBK protocol, but rather a model or framework for development of PBK in applications. 1.0 Introduction There are many cases in Internet protocols where cryptographic mechanisms can add significant security improvement. However most such mechanisms rely on associating keys to entities, ultimately requiring an enterprise-wide, multi-enterprise, or even more widely deployed Public Key Infrastructure (PKI). In the absence of security mechanisms, many protocols are continuously vulnerable to attack. However there are many circumstances where we can improve overall security by narrowing the window of vulnerability, so that if we assume that some operation is performed securely, we can secure all future transactions. There are also cases where the actual identity of the initiator of a network communication is not an important piece of information, yet it is important to know that successive packets are from that same source. One example of this is in mobile IPv6. Mobile IPv6 contains a rebinding option that enables a mobile node to tell the other end of a communication that the IP address for the mobile node has changed. It is clearly important to know that any such rebinding request actually came from the correct mobile node even if the identity of the user of that mobile node does not need to be known. Note that it is not that the identity of the user here is unimportant to the network (the node user may well authenticate to an AAA service or other access manager at the start of network activity), but rather that it is unimportant to accomplish that level of authentication for the purpose of rebinding. This memo describes the use of a temporary public/private key pair that is generated by a host for each case where the consistency of authentication needs to be assured. For example, a new key pair would be generated before each mobile IP session and discarded when the session was complete. This use of these host-generated temporary keys is confined to the parties in a communication and does not require that the keys be registered with or known by any third party. Thus this mechanism does not require that any support infrastructure exist outside of the protocol support in the corresponding hosts and it can be deployed incrementally as host support becomes available. It also scales well Bradner, Mankin & Schiller [Page 2] Purpose Built Keys Framework September 2002 since the operations are confined to the end systems involved in the communication. By not using registered keys, this mechanism preserves user anonymity as long as the identity of the users are not obtained by some other process during the communication. By using a challenge-response confirmation step, this mechanism can work in environments where the IP addresses in the packet stream could be modified in the path between the correspondents. The challenge-response makes it much harder for a man-in-the-middle attacker to issue requests in the name of a correspondent. The PBK mechanism does not require the use of a reliable protocol. It is intended to used with transport or application protocols. It differs from IPSec in that it is applied on demand by an application or by a transport protocol. When this mechanism is used with applications the PBK's public key can be used in an identity for a web-cookie like function, but the use is under the control of the node that initiates the connection rather than under the control of the server. 2.0 Conceptual Overview Following is a conceptual step-by-step description of the PBK process when operating below the transport layer. First some definitions: initiating node: the node initiating the conversation receiving node: the node at the other end of the conversation Before an initiating node initiates a connection during which it will need to prove that it is the same node that started the connection, it creates a public/private key pair for use during the connection. This is known as a purpose-built key (PBK) pair. The initiating node then creates a Purpose-Built ID (PBID) by performing a cryptographic hash of the public part of the PBK. This PBID will be used as an identity token for the node. The initiating node then initiates the connection. The PBID is sent along with the initial packets in the connection. In IPv6 this could be done in an end-to-end option header, in IPv4 as a header option. (These option ideas are for transport level use of the PBK - if the PBK was used from within HTTP or another application, the PBID's location would be in the application.). The PBID does not need to appear in all of the packets; it just has to be reliably conveyed to Bradner, Mankin & Schiller [Page 3] Purpose Built Keys Framework September 2002 the receiving node. Reliability may be obtained by carrying it on enough packets so that a return packet indicates it was received eventually. This is the simplest approach; depending on requirements and the application, the PBID may well be transported reliably. The receiving node stores the PBID and the source IP address that were in the received packet in a table. At some time in the connection before the proof of identity is needed, the initiating node sends its public key to the receiving node. This again could be done in IP-level options or in an application-level exchange. The receiving node verifies that the received public key hashes to the previously provided PBID. When the initiating node wants to perform some operation, such as a mobile IPv6 address rebinding, it sends the operation request along with the PBID. The message is signed using the private part of the PBK. If replay protection is necessary, a nonce value (a monotonically increasing value) or timestamp may be included with the operation request. When the receiving node gets such an operation request it verifies the digital signature and returns a challenge packet. The challenge packet is sent to the IP address that was in the source IP address field of the packet that contained the request. The challenge packet contains a random number test value generated by the receiving node. When the initiating node receives the challenge packet it encrypts the test value in its private key and sends the result back to receiving node. When the receiving node gets the challenge response it decrypts the test value using the stored public key associated with the PBID. If the results match then the receiving node can be sure that the node that sent the operation request was the correct initiating node. The PBKs would normally be discarded at the end of the communication but in those cases where a continuity of identity is needed over multiple sessions the PBKs could be retained until the requirement was over. 3.0 Notes on the design The hash of the public key is used as the PBID so that the relationship between an offered PBID and public key can be established. If a receiving node is in possession of the private key and the hash of the corresponding public key matches an offered PBID, it can be sure that it has the correct PBID for that public key. Bradner, Mankin & Schiller [Page 4] Purpose Built Keys Framework September 2002 Retransmission algorithms, where they are needed, must be conformant with RFC 2914 [RFC2914]. The challenge / response exchange has to take place synchronized within the data stream if the processing of packets after the operation request would be different that before the operation request, as it would be for mobile IPv6. This would mean suspending normal transmission until the challenge / response exchange was completed. The challenge is sent to the source address in the packet and this address is not included in the digital signature on the operation request packet so that this mechanism can work through any address modifying devices that may be in the path. In the cases where commands could be issued by both ends of a communication, as would be the case in mobile IPv6 if both ends were mobile, separate PBKs would be created by each end and the mechanism would be run independently by each end. 4.0 Security Considerations This whole document is about security. Specifically the memo discusses how to perform authenticated operations in an environment where there is no existing security infrastructure or an environment where network addresses might change during the course of the communication. In the absence of a security infrastructure such as a PKI, it is not always possible to authenticate one party to another. In the absence of any cryptographic security mechanism, internet transactions are continuously at risk of compromise. With PBKs it is possible to leverage an initial "leap of faith" so that presuming an initial transaction has not been tampered with (say the exchange of PBID's at the beginning of an association between two parties), future transactions can be secured. 5.0 Acknowledgements Some of these same concepts are explored in [HIP.] 6.0 Author's Addresses Scott Bradner Harvard University Cambridge MA 02138 Phone +1 617 495 3864 Bradner, Mankin & Schiller [Page 5] Purpose Built Keys Framework September 2002 email sob@harvard.edu Allison Mankin University of Southern California, Information Sciences Institute 4350 N. Fairfax Drive, Suite 620 Arlington, VA 22203 Phone: +1 703 812 3706 email: mankin@isi.edu Jeffrey I. Schiller Massachusetts Institute of Technology MIT Room W92-190 77 Massachusetts Avenue Cambridge, MA 02139-4307 Phone: +1 617 253 0161 email: jis@mit.edu Informative References [RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914, September 2000. [HIP] Moskowitz, R., "Host Identity Payload Architecture", "Host Identity Payload Protocol", http://homebase.htt-consult.com/~hip, 2001. Full Copyright statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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Bradner, Mankin & Schiller [Page 6] Purpose Built Keys Framework September 2002 This document and the information contained herein is provided on An "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Bradner, Mankin & Schiller [Page 7]