Hashcash: A Denial of Service Counter-Measure

The client finds a partial hash collision. The server checks it cheaply. The stamp is the cost.

Official external source only
Design paper
Official source

Record

Authors: Adam Back
Date: 2002
Publisher: Adam Back
Version: Source cited by the 2002 study. The study is the reading. This page does not add a second essay.
Document ID: BLC-STUDY-0139
DOI: None recorded
Last reviewed: 1 October 2026
Licence: No licence recorded. Rights status under review.

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.

Direct answer

The client finds a partial hash collision. The server checks it cheaply. The stamp is the cost.

Where to read

Official source — hashcash.org
Blockchain Lab study, the canonical readingOriginal commentary. Not a transcription of the paper.

Metadata and commentary only. The reading is the study page.

Full text

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.

What changed

A reading of the public paper. Not a copy, not a benchmark, and not a claim about any later network.

Relationships

Canonical readingHashcash study
Related paperBitcoin Backbone
Related paperSelfish Mining

Cite this record

These citations point at the source document. They are not a licence to copy it.

Author–date

Back, A. (2002). Hashcash: A Denial of Service Counter-Measure. Adam Back. http://www.hashcash.org/papers/hashcash.pdf

BibTeX

@techreport{hashcash2002,
  title = {Hashcash: A Denial of Service Counter-Measure},
  author = {Adam Back},
  year = {2002},
  institution = {Adam Back},
  url = {http://www.hashcash.org/papers/hashcash.pdf},
  note = {Rights status: Official external source only. Blockchain Lab record: https://blockchainlab.com/research/corpus/papers/hashcash}
}

RIS

TY  - RPRT
TI  - Hashcash: A Denial of Service Counter-Measure
AU  - Adam Back
PY  - 2002
PB  - Adam Back
UR  - http://www.hashcash.org/papers/hashcash.pdf
N1  - Rights status: Official external source only
ER  -

CSL-JSON

[
  {
    "type": "report",
    "id": "BLC-STUDY-0139",
    "title": "Hashcash: A Denial of Service Counter-Measure",
    "author": [
      {
        "literal": "Adam Back"
      }
    ],
    "issued": {
      "date-parts": [
        [
          2002
        ]
      ]
    },
    "publisher": "Adam Back",
    "URL": "http://www.hashcash.org/papers/hashcash.pdf",
    "note": "Rights status: Official external source only. Not a licence to republish the paper."
  }
]

JSON BibTeX file RIS file CSL-JSON

Rights record

Source status: Official source
Use status: Metadata and commentary only. The reading is the study page.
Rights: Official external source only
Training and retrieval: Not permitted from this site. There is no rights-cleared full text to retrieve or to train on.

Rights and licensing Corrections