| Network Working Group | C. Jennings |
| INTERNET DRAFT | Cisco Systems |
| <draft-jennings-sip-hashcash-01> | February 2005 |
| Category: Standards Track | |
| Expires: August 2005 |
SIP Computational Puzzles
draft-jennings-sip-hashcash-01
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One of the techniques used in SPAM prevention and various solutions for denial of service attacks is to force the SIP client requesting a service to perform a calculation that limits the rate and increases the cost of the request. 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. Although the computation is expensive for the UAC to compute, it is cheap for the UAS to verify. The solution also allows for proxies to compute and check the puzzle on behalf of the UAC or UAS.
This specification extends RFC 3261 [3] and 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 SHA1 [4], results in a specific value referred to as the image. The goal is for the UAC to find a pre-image that will SHA1 hash to the correct image. The UAS provides a partial pre-image with some of the low order bits set to zero, together with the number of bits in the pre-image that have been set to zero.
The UAS provides the puzzle information using a 419 response, and the UAC resubmits the request along with the solution to the puzzle. The high level flow of information is shown below.
UAC UAS | Request | |------------------------->| | | | 419 with Puzzle | |<-------------------------| | | | Request with Solution | |------------------------->| | |
This specification defines the 419 response code along with a new header, called Puzzle, to carry the puzzle and solution.
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 RFC 2119 [2].
The normative definition of a puzzle is as follows. A puzzle is four values: an integer number referred to as work, a pre-image string, an image string, and a integer number referred to as value. There MUST exist a value X such that all but the "work" number of low order bits of X match the pre-image string, and the SHA1 hash of the string formed by the concatenation of "z9hG4bK" and X results in a value Y, where the "value" number of low order bits of Y are the same as those bits in the image string. The SHA1 hash is computed as described in RFC 3174 [4]. The value X is the solution to the puzzle. The 'work' number of low order bits of the pre-image MUST be zero.
This can all be described in a more mathematical way. The notation low(v,x) returns the low v low order bits of x, and zero(v,x) returns x with the low v bits set to zero. The | operator signifies string concatenation. The solution to the puzzle can be considered finding an X such that both the following are true:
low( value, image ) = low( value, sha1( "z9hG4bK" | X )
zero( work, X ) = zero( work, pre-image )
The pre-image forms a constraint on X. The value of X is the same pre-image other than the low 'work' bits that are set to zero in the pre-image. The 'value' is the number of bits that match in the solution and is typically set to 160, which is the full size of the SHA1 hash result.
The following is a non-normative way for a UAS or proxy to construct a puzzle. The following strings are concatenated:
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. A value 'work' indicates how many bits of the pre-image are to be removed. The value 'work' could be a configurable parameter, or it 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 'work' as computers get faster. The low order 'work' bits of the pre-image are set to zero. The puzzle consists of the chosen value of 'work', the pre-image (with the low order bits set to zero), the image, and the 'value'. The 'value' would typically be set to 160 as this is the size of the SHA1 hash.
When a UAS wishes to challenge a request, it MAY create a puzzle, encode this puzzle in a Puzzle header field value, and return the puzzle in a 419 response.
When a UAC receives a 419 response, it needs to look at the 'work' and 'value' requested and decide whether or not to try to solve this puzzle. This decision can be made based on the programmed policy and possibly human input. The UAC should not tackle a puzzle that will take longer than the age of the universe to solve. If the UAC chooses to try to solve the puzzle then it proceeds along the following steps:
If the loop terminates without a solution being found, the puzzle was bad and the 419 response MUST be considered as an error response.
Once the solution to the puzzle, X, is found, a new request is formed by copying the old request and adding an additional puzzle header field value. The new puzzle header field value MUST have the 'work' set to 0, the pre-image set to the value X, the image set to the value of the image in the original puzzle, and the value parameter set to the same as the value parameter in the original puzzle. Note that if a request was challenged by one proxy and a new request was generated with a solution, and then this request was challenged by a second proxy, a third request would be generated that had two Puzzle header field values. If a UAC, through some out of band mechanism, knows that it will be challenged and what the puzzle will be, it MAY include the appropriate puzzle header field value in the initial request.
SIP allows proxies to act as UASs when generating 4xx responses. This same mechanism can be used to allow a proxy to generate the challenge on behalf of a UAS in its domain.
Proxies may also act on behalf of the UAC and compute the solution to a puzzle on behalf of the UAC in either a request or response that passes through the proxy. Typically a proxy would only do this for a UAC that had authenticated to the proxy and for which the proxy had a service relationship.
TBD
The Puzzle header field carries the puzzle and solution information. It has a parameter called 'work' that has the number of bits of the pre-image that have been set to zero for this puzzle. It has a parameter called 'pre' that carries the pre-image string base64 encoded, and a parameter called 'image' that carries the image string base64 encoded. In addition there is a parameter called 'value' that indicates how many bits of the resulting hash will match the 'image' string. The base64 encoding is done as described in RFC 3548 [1].
When the header field value is carrying a solution to a puzzle, the work parameter will be set to zero.
Example:
Puzzle: work=10; pre="XPokF1n0+NG6iwRcYzeXuETrtDo=";
image="XPokF1n0+NG6iwRcYzeXuETrtDo="; value=160
The ABNF for the header is:
Puzzle = "Puzzle" HCOLON puzzle-parm *(COMMA puzzle-param)
puzzle-param = puzzle-bits SEMI puzzle-pre SEMI puzzle-image
SEMI puzzle-value *( SEMI generic-param )
puzzle-work = "work=" 1*DIGIT
puzzle-value = "value=" 1*DIGIT
puzzle-pre = "pre=" quoted-string
puzzle-image = "image=" quoted-string
This document updates the dreaded Table 2 of RFC 3261 to be:
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Puzzle amr o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Still TBD.
The concatenation with "z9hG4bK" is done so that this mechanism cannot be used as a distributed computation to reverse arbitrary hash values, as that would present a security risk for other hash based security schemes.
TODO - Advice on selecting the size of 'work'.
TODO - Comment on proof of work proven not to work paper.
This specification registers a new header and a new response code. IANA is requested to make the following updates in the registry at: http:///www.iana.org/assignments/sip-parameters
Add the following entry to the header sub-registry.
Header Name compact Reference ----------------- ------- --------- Puzzle [RFCXXXX]
Add the following entry to the response code sub-registry under the "Request Failure 4xx" heading.
419 Puzzle Required [RFCXXXX]
This approach was motivated by [5].
| [1] | Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003. |
| [2] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [3] | 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. |
| [4] | Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. |
| [5] | Black, A, "http://www.hashcash.org/", February 2005. |
| Cullen Jennings | |
| Cisco Systems | |
| 170 West Tasman Drive MS: SJC-21/2 |
|
| San Jose, CA 95134 | |
| USA | |
| Phone: | +1 408 421 9990 |
| EMail: | fluffy@cisco.com |
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