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

[Bill Stewart]

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.