SIPPING C. Jennings Internet-Draft Cisco Systems Expires: January 9, 2005 July 11, 2004 SIP Computational Puzzles draft-jennings-sip-hashcash-00 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 9, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract SPAM has been a frustrating problem in communications and also in SIP. Forcing the client requesting the service to perform a calculation that limits the rate and increases the cost of requests is one of the techniques that may help manage this problem. This draft defines a way to allow a UAS to ask the UAC to compute a computationally expensive hash based function and present the result to the UAS. This draft is a very incomplete and more of a sketch of a solution than a final draft. Jennings Expires January 9, 2005 [Page 1] Internet-Draft SIP Puzzles July 2004 1. Introduction This draft defines a mechanism for a proxy or UAS to request that a UAC compute the solution to a puzzle. The puzzle is based on finding a value called the pre-image that when hashed with SHA 1 results in a specific value referred to as the image. The puzzle provides a number, k, that indicates how how many bit of the pre-image have not been provided, the pre-image with some of the bits replaced with 0, and the image. The proxy or UAS can send a 419 response to a SIP request and include a puzzle header that provides the puzzle to compute. The UAC can compute a solution to this puzzle and resubmit the request with the solution in the puzzle header. 2. Requirements Allow a UAS to request a variable amount of work from a UAC. Make sure this work cannot be used for attacking other systems. 3. Puzzles The following is a non-normative way for a UAS or proxy to construct a puzzle. The following strings are concatenated: 1) a secret that only this device knows and would typically be a crypto random string of bits; 2) the current time, rounded to the nearest minute; 3) the URI of the request, the Call-ID, the From tags, and the branch tag for a proxy or the To tag for a UAS. The string is hashed with SHA1 to form the pre-image. The pre-image is appended to the string "z9hG4bK" and the SHA1 hash of this is computed to get the value of the image. This concatenation is done so that this mechanism cannot be used as a distributed computation to reverse arbitrary hashed values. A value k indicates how many bits of the pre-image are to be removed. The value k could be a configurable parameter or could be dynamically discovered by the software based on how long a hash should take and the speed of the computer it was running on. In the latter case, the resulting software would automatically choose larger values of k as the computer got faster. The low order k bits of the pre-image are set to zero. The puzzle consists of k, the pre-image (with the low order bits set to zero), and the image. The normative definition of a puzzle is as follows. A puzzle is three values, k, pre-image, and image. There MUST exist a value X such that all but the k low order bits of X match the pre-image, and the SHA 1 hash of the concatenation of "z9hG4bK" and X results in a value that MUST be equal to the image. The value X is the solution to the puzzle. Jennings Expires January 9, 2005 [Page 2] Internet-Draft SIP Puzzles July 2004 4. Semantics A proxy or UAS MAY reject a request with a 419 status code and request that a particular computation be performed. The puzzle is constructed as described in section X. A UAC that receives a 419 request must compute the result that satisfies the challenge and resubmit the request with the computed answer. If the UAC knows that it is routing the request through a proxy that will compute the answer for the UAC, it MAY leave the answer blank. A proxy that receives a request that contains a puzzle but does not have a solution to the puzzle MAY compute the solution and modify the header. 5. Example 6. Syntax A new header called Puzzle carries the puzzle and solution information. It has a parameter k that has the number of bits as a text encoded number, a parameter p that carries the pre image base 64 encoded, and a parameter i that carries the image value base 64 encoded. The solution is put in the same header but the value of k is 0 when it is a solution. Example puzzle Puzzle: k=10;p="XPokF1n0+NG6iwRcYzeXuETrtDo="; i="XPokF1n0+NG6iwRcYzeXuETrtDo=" Example solution Puzzle: k=0;p="XPokF1n0+NG6iwRcYzeXuETrtDo="; i="XPokF1n0+NG6iwRcYzeXuETrtDo=" 7. Security Considerations TODO - many things left to do here. E wants to send spam to A. Calls A, get challenge. Encourages others to call E. When B calls E, E passes on the puzzle to B. B solves it and sends result to E who sends it to A. Jennings Expires January 9, 2005 [Page 3] Internet-Draft SIP Puzzles July 2004 8. IANA TODO - Define new header. TODO - Define 419 status code. 9. Open Issues Put puzzle in header or body? 10. References 10.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] 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. [3] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. 10.2 Informational References [4] Black, A., "http://www.hashcash.org/", June 2004. Author's Address Cullen Jennings Cisco Systems 170 West Tasman Drive MS: SJC-21/2 San Jose, CA 95134 USA Phone: +1 408 902-3341 EMail: fluffy@cisco.com Jennings Expires January 9, 2005 [Page 4] Internet-Draft SIP Puzzles 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. 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. Disclaimer of Validity 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. Copyright Statement Copyright (C) The Internet Society (2004). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Jennings Expires January 9, 2005 [Page 5]