{"document_id":"BLC-STUDY-0139","slug":"hashcash","title":"Hashcash: A Denial of Service Counter-Measure","authors":"Adam Back","year":2002,"date_label":"2002","publisher":"Adam Back","paper_type":"Design paper","category":"Consensus","topics":["Consensus"],"source_url":"http://www.hashcash.org/papers/hashcash.pdf","source_label":"hashcash.org","legacy_pdf_path":null,"sha256":null,"sha256_note":"No hash is published. This build does not store those PDF bytes, so a checksum would be invented. Where a legacy path exists, it points at the Internet Archive copy of the old library.","version":"Source cited by the 2002 study. The study is the reading. This page does not add a second essay.","doi":null,"rights_status":"Official external source only","source_status":"Official source","use_status":"Metadata and commentary only. The reading is the study page.","licence":null,"licence_label":"No licence recorded. Rights status under review.","full_text":null,"full_text_reason":"Full text is not published on this page. Public availability is not a licence to republish a document. Blockchain Lab has not recorded a licence that permits an HTML transcription, a training set, or a retrieval corpus of the paper. Text on this page is Blockchain Lab's own wording, or a pointer to a study that is also Blockchain Lab's wording.","editorial_line":"The client finds a partial hash collision. The server checks it cheaply. The stamp is the cost.","canonical_url":"https://blockchainlab.com/whitepaper/hashcash","record_url":"https://blockchainlab.com/research/corpus/papers/hashcash","study_url":"https://blockchainlab.com/whitepaper/hashcash","last_reviewed":"2026-10-01","last_checked":"2026-10-01","relationships":[{"rel":"Canonical reading","label":"Hashcash study","href":"/whitepaper/hashcash"},{"rel":"Related paper","label":"Bitcoin Backbone","href":"/whitepaper/bitcoin-backbone"},{"rel":"Related paper","label":"Selfish Mining","href":"/whitepaper/selfish-mining"}]}