Re: what fields to hash with hashcash (Re: A Trial Balloon to Ban Email?)

At 04:53 PM 05/15/2003 +0100, ken wrote:

So if this was implemented we get incentive to design a new kind of hashing algorithm, one designed to be difficult to run, because all it is needed for is to prove that someone bothered enough to spend the time. Also it needs to map one plaintext to many valid hashes of course as others said thats easier when you include the "from:" in the hash or allow some arbitrary field.

The hash is easy to do - Given a target "T", provide a string "X" for Bit(i,SHA1(X)) == Bit(i,SHA1(T)) for i=1...n, and Substring(SHA1(X),N+1,160) != Substring(SHA1(T),N+1,160). You'll need to try roughly 2**N inputs to find one.