The Digest::Hashcash Perl module calculates 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 e-cash system measured in burnt CPU cycles. Such cash
systems can be used to throttle systematic abuses of un-metered internet
resources.
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