Applications Hashcash

1 applications

1.1 bitcoin mining
1.2 spam filters
1.3 email clients
1.4 email postmark
1.5 blogs

applications
bitcoin mining

in contrast hashcash in mail applications relies on recipients set manually amount of work intended deter malicious senders, bitcoin cryptocurrency network employs different hashing-based proof-of-work challenge enable competitive bitcoin mining. bitcoin miner runs computer program collects unconfirmed transactions coin dealers in network. other data these can form block , earn payment miner, block accepted network when miner discovers trial , error nonce number when included in block yields hash sufficient number of leading 0 bits meet network s difficulty target. blocks accepted miners form bitcoin blockchain growing ledger of every bitcoin transaction since coin s first creation.

while hashcash uses sha-1 hash , requires first 20 of 160 hash bits zero, bitcoin s proof of work uses 2 successive sha-256 hashes , required @ least first 32 of 256 hash bits zero. bitcoin network periodically resets difficulty level keep average rate of block creation @ 6 per hour. of august 2017 (block #478608) bitcoin network has responded deployments of ever faster hashing hardware miners hardening requirement first 72 of 256 hash bits must zero.

spam filters

hashcash used potential solution false positives automated spam filtering systems, legitimate users inconvenienced time takes mine stamp. spamassassin has been able check hashcash stamps since version 2.70, assigning negative score (i.e. less spam) valid, unspent hashcash stamps. however, although hashcash plugin notionally on default, still needs configured list of address patterns must match against hashcash resource field, doesn t work default.

email clients

the penny post software project on sourceforge implements hashcash in mozilla thunderbird email client. project named historical availability of conventional mailing services cost sender 1 penny; see penny post information such mailing services in history.

email postmark

microsoft designed , implemented deprecated open spec, similar , yet incompatible hashcash, email postmark, part of coordinated spam reduction initiative (csri). microsoft email postmark variant of hashcash implemented in microsoft mail infrastructure components exchange, outlook , hotmail. format differences between hashcash , microsoft s email postmark postmark hashes body in addition recipient, , uses modified sha-1 hash function , uses multiple sub-puzzles reduce proof of work variance.

blogs

like e-mail, blogs fall victim comment spam.

some blog owners have used hashcash scripts written in javascript language slow down comment spammers. scripts (such wp-hashcash) claim implement hashcash instead depend on javascript obfuscation force client generate matching key; while require processing power, not use hashcash algorithm or hashcash stamps.