ash ▁because ▁hash c ash ▁pred ates ▁( M arch ▁ 1 9 9 7 ) ▁the ▁client - p uzz les ▁publication ▁( F ebru ary ▁ 1 9 9 9 ) ▁and ▁the ▁client - p uzz les ▁pat ent ▁fil ing ▁US 7 1 9 7 6 3 9 ▁( F ebru ary ▁ 2 0 0 0 ). ▁ ▁See ▁also ▁ ▁Pen ny ▁Black ▁( res earch ▁project ) ▁ ▁Notes ▁ ▁References ▁ ▁Adam ▁Back , ▁" Hash c ash ▁- ▁A ▁Den ial ▁of ▁Service ▁C ounter - Me asure ", ▁technical ▁report , ▁August ▁ 2 0 0 2 ▁( PDF ). ▁ ▁Ben ▁Laur ie ▁and ▁Richard ▁Clay ton , ▁"' Proof - of - Work ' ▁Pro ves ▁Not ▁to ▁Work ", ▁W E IS ▁ 0 4 . ▁( PDF ). ▁ ▁D work , ▁C . ▁and ▁Na or , ▁M . ▁( 1 9 9 2 ) ▁" P ric ing ▁via ▁Process ing ▁or ▁Com b ating ▁J unk ▁Mail ", ▁C rypto ▁' 9 2 , ▁pp .   1 3 9 – 1 4 7 . ▁( PDF ) ▁ ▁External ▁links ▁ ▁Hash c ash ▁home page ▁ ▁Beat ▁sp am ▁using ▁hash c ash ▁David ▁M ert z ' s ▁article ▁on ▁hash c ash , ▁its ▁applications ▁and ▁an ▁implementation ▁in ▁Python ▁ ▁R SA ▁IP R ▁note ▁to ▁the ▁I ET F ▁about ▁hash c ash ▁( 2 0 0 4 ) ▁