Bitcoin is anyway built to 128-bit security because 256-bit ECDSA is used, which also offers 128-bit security. There is actually no strong reason SHA1 would not have worked also, hashcash relies only on the hash partial preimage resistance property (security up to hash-size, 160-bit with SHA1) and not birthday collision hardness (security up to 80-bit), so the SHA1 hash is big enough. Bitcoin being specified/released in 2008/2009 uses SHA256. In the original 1997 algorithm hashcash used SHA1 because at that time, this was the defacto and NIST recommended hash, and the previous defacto hash MD5 had recently started to show signs of weakness. Wei Dai's B-money Proposal, and Nick Szabo's similar Bit Gold proposal bitcoin precursors, also were proposed in the context of hashcash mining. Hashcash was also used by Hal Finney's bitcoin precursor RPOW as a way to mine coins.

Before bitcoin, hashcash was used by SpamAssasin, and (with an incompatible format) by Microsoft (with the name 'email postmark') in hotmail, exchange, outlook etc and by i2p anonymity network, mixmaster anonymous remailer components and other systems. The Hashcash proof-of-work function was invented in 1997 by Adam Back, and proposed for anti-DoS uses including preventing: anonymous remailer and mail2news gateway abuse, nym name squatting on nymservers (replyable pseudonymous remailer severs), as well as general email anti-spam and general network abuse throttling. Like many cryptographic algorithms hashcash uses a hash function as a building block, in the same way that HMAC, or RSA signatures are defined on a pluggable hash-function (commonly denoted by the naming convention of algorithm-hash: HMAC-SHA1, HMAC-MD5, HMAC-SHA256, RSA-SHA1, etc), hashcash can be instantiated with different functions, hashcash-SHA1 (original), hashcash-SHA256^2 (bitcoin), hashcash-Scrypt(iter=1). All bitcoin miners whether CPU, GPU, FPGA or ASICs are expending their effort creating hashcash proofs-of-work which act as a vote in the blockchain evolution and validate the blockchain transaction log.

2.6 Decentralization: hashcash-Scrypt vs hashcash-SHA256īitcoin uses the hashcash Proof_of_work function as the mining core.

2.3 Work, difficulty & cryptographic security.

If one was to often change some bits in the second half of the data, one could simplify the calculations and hash the first half of the data only once.

Double sha256 hash calculator