The original "Pricing Via Processing or Combatting Junk Mail" paper by Dwork & Naor from 1992 is http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.75.798

The original "Pricing Via Processing or Combatting Junk Mail" paper by Dwork & Naor from 1992 is http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.75.798&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.75.... And the follow-on "On Memory-Bound Functions for Fighting Spam" (which is based on the notion that "since memory access speeds vary across machines much less than do CPU speeds, memory-bound functions may behave more equitably than CPU-bound functions") is: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.85.9764&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.85....