Partial hash collision based postage scheme

HashCash
This along with the javascript hashing example is interesting because the client expends a small amount of effort to complete a transaction. Application to email and other resources is discussed.

Hash cash is payment in burnt CPU cycles by calculating n-bit partial hash collisions on chosen texts.

The idea of using partial hashes is that they can be made arbitrarily expensive to compute (by choosing the desired number of bits of collision), and yet can be verified instantly. This can be used as the basis for an ecash system measured in burnt CPU cycles. Such cash systems can be used to throttle systematic abuses of un-metered internet resources.