LibraryConsensus2002Design paperCorpus record
Hashcash: A Denial of Service Counter-Measure
Hashcash. Adam Back.
The client finds a partial hash collision. The server checks it cheaply. The stamp is the cost.
A reading of the public paper. Not a copy, not a benchmark, and not a claim about any later network.
When a chain says its puzzle is Hashcash, the part that is Hashcash is the cheap-to-check partial collision. The chain of stamps is a different paper.
- Evidence
- Primary paper
- Re-measured
- No
- Assumptions
- 3
- Records linked
- 3
Code
Source, repository, and what is actually hosted
- hashcash.orghttp://www.hashcash.org/papers/hashcash.pdf
- The source URL is not a GitHub repository. No code remote is invented.
- Full text is not mirrored. Public availability is not a licence to copy the file onto this site.
- Add to a reading list
01 Claim ledger
What the paper is allowed to say
Each row is a sentence already in the study. The status is the same on every row: a model claim, not a live measurement.
Claim 01 · Paper model
The defect
A server that accepts unbounded anonymous requests will be drowned. Pricing the request with a small proof of work was the proposal, years before a chain of blocks.
Claim 02 · Paper model
The proposal
The client finds a partial hash collision. The server checks it cheaply. The stamp is the cost.
Claim 03 · Paper model
The puzzle is minted by the client, not by a miner race.
Double-spending the stamp matters for mail. A ledger of stamps is a later invention.
Claim 04 · Paper model
The bound
Hashcash is not Bitcoin. There is no longest chain in this note.
02 Three cuts
Observation, model, falsifier
A desk does not stop at the summary. Each claim is cut three ways, using only this study's own assumptions and checks. Nothing here is a new figure.
01 The defect
Observation
What the study says
A server that accepts unbounded anonymous requests will be drowned. Pricing the request with a small proof of work was the proposal, years before a chain of blocks.
Falsifier
What would retire it
What is the challenge bound to?
02 The proposal
Observation
What the study says
The client finds a partial hash collision. The server checks it cheaply. The stamp is the cost.
Model
What has to hold
A stamp that can be replayed against many servers has not bound the resource.
Falsifier
What would retire it
Can the stamp be replayed?
03 The puzzle is minted by the client, not by a miner race.
Observation
What the study says
Double-spending the stamp matters for mail. A ledger of stamps is a later invention.
Model
What has to hold
A stamp that can be replayed against many servers has not bound the resource.
Falsifier
What would retire it
Can the stamp be replayed?
04 The bound
03 Sequence
One action, as an operating tape
04 Load-bearing
The argument, and where a pitch drops it
What the paper is for
The cut
When a chain says its puzzle is Hashcash, the part that is Hashcash is the cheap-to-check partial collision. The chain of stamps is a different paper.
Why it carries weight
If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.
Where it is dropped
Hashcash is not Bitcoin. There is no longest chain in this note.
What happened after
The cut
Bitcoin cites Hashcash as the puzzle. The ledger, the difficulty adjustment and the incentive are not this document.
Why it carries weight
If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.
Where it is dropped
Hashcash is not Bitcoin. There is no longest chain in this note.
05 Register
What has to be true
Cryptography · Not re-measured
Hashcash is not Bitcoin. There is no longest chain in this note.
Network · Not re-measured
It does not price electricity for a national network.
Model · Not re-measured
A stamp that can be replayed against many servers has not bound the resource.
06 Divergence
What happened after the paper
Bitcoin cites Hashcash as the puzzle. The ledger, the difficulty adjustment and the incentive are not this document.
A later client, parameter set, or reward formula is a different object. Cite this paper for the mechanism. Cite a primary release for the network. This desk has not re-run the proof.
07 Pre-mortem
What to check before you use the idea
- 0 of 3 marked on this browser. A mark is a reading note, not a pass, a rating, or a recommendation.
08 Anatomy
The paper, in the order a builder needs
The problem it names
A server that accepts unbounded anonymous requests will be drowned. Pricing the request with a small proof of work was the proposal, years before a chain of blocks.
What the design proposes
- The puzzle is minted by the client, not by a miner race.
- Double-spending the stamp matters for mail. A ledger of stamps is a later invention.
- The difficulty is a parameter of annoyance, not of monetary policy.
How the mechanism is specified
What this page does not treat as proven
- Hashcash is not Bitcoin. There is no longest chain in this note.
- It does not price electricity for a national network.
- A stamp that can be replayed against many servers has not bound the resource.
Why the desk still reads it
When a chain says its puzzle is Hashcash, the part that is Hashcash is the cheap-to-check partial collision. The chain of stamps is a different paper.
09 Lexicon
Terms, opened into the record
- Partial collision
- A hash with a run of leading zeros.
- Stamp
- The proof a client attaches to a request.
10 Repository
Every linked record on this page
Underlined words open a page that already exists: a concept, a protocol profile, a failure record, or another paper. If a word is not underlined, this desk does not have a record for it.
Concepts
Protocols
This is Blockchain Lab's reading of a public design paper. It is not the paper, not a copy of it, and not an offer of tokens, equity, custody or a partnership. Later network behaviour can diverge from the text. Nothing here is investment, legal or technical advice.
Research status: Design paper. Last reviewed: 1 October 2026. This is a reading of a public paper, not investment, legal or security advice.