Fighting spam: moderately hard memory-bound computations
Mike Burrows
Microsoft Research
A resource may be abused if its users incur little or
no cost. For example, e-mail abuse is rampant because
sending an e-mail has negligible cost for the sender. It
has been suggested that such abuse may be discouraged by
introducing an artificial cost in the form of a moderately
expensive computation. Thus, the sender of an e-mail
might be required to pay by computing for a few seconds
before the e-mail is accepted. Unfortunately, because of
sharp disparities across computer systems, this approach
may be ineffective against malicious users with high-end
systems, prohibitively slow for legitimate users with
low-end systems, or both. Starting from this observation,
we discuss moderately hard functions that most recent
systems will evaluate at about the same speed. For this
purpose, we consider memory-bound computations. This
talk describes a family of moderately hard, memory-bound
functions, and explains how to use them for protecting
against abuses. The talk also includes a brief description
of a related network service for spam deterrence.
Joint work with Martin Abadi, Mark Manasse, and Ted Wobber
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