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Hash | Bitcoin Glossary | Mapping Bitcoin

Hash

Cryptography

Also known as: digest

A fixed-length output produced by a hash function from an arbitrary input. In Bitcoin, hashes serve as unique identifiers for blocks and transactions, form the basis of proof of work, and link blocks together in the blockchain.

Overview

A hash is the output of a hash function -- a fixed-size string of characters that uniquely represents an input of any size. In Bitcoin, hashes are used pervasively: to identify transactions, to link blocks together, to create the proof-of-work puzzle, and to derive addresses from public keys. The hash is often described as a "digital fingerprint" because even a tiny change in the input produces a completely different output.

Properties of a Good Hash

Input: "Hello"        → SHA-256 → 185f8db32271fe25f561a6fc938b2e26...
Input: "Hello."       → SHA-256 → 2d8bd7d9bb5f85ba643f0110d50cb506...
Input: "hello"        → SHA-256 → 2cf24dba5fb0a30e26e83b2ac5b9e29e...

Key observations:
1. Fixed length: Always 256 bits (64 hex characters) regardless of input size
2. Deterministic: Same input always produces the same hash
3. Avalanche effect: Tiny input change → completely different hash
4. One-way: Cannot recover input from hash
5. Collision resistant: Practically impossible to find two different
   inputs that produce the same hash

How Bitcoin Uses Hashes

Transaction IDs (TXIDs): Each transaction is identified by the double-SHA-256 hash of its serialized data. This provides a compact, unique identifier for every transaction.

Block hashes: Each block is identified by the double-SHA-256 hash of its 80-byte header. This hash must be below the current target for the block to be valid (proof of work).

Blockchain linking: Each block header contains the hash of the previous block, creating an unbreakable chain:

┌──────────┐    ┌──────────┐    ┌──────────┐
│ Block N  │    │ Block N+1│    │ Block N+2│
│          │    │          │    │          │
│ Prev: ◄──┼────│ Prev: ◄──┼────│ Prev:    │
│ Hash     │    │ Hash     │    │ Hash     │
│          │    │          │    │          │
│ Merkle   │    │ Merkle   │    │ Merkle   │
│ Root     │    │ Root     │    │ Root     │
└──────────┘    └──────────┘    └──────────┘

Merkle trees: Transaction hashes are paired and hashed together repeatedly to form a Merkle tree, with the root hash included in the block header.

Address generation: Bitcoin addresses are derived by hashing public keys through SHA-256 and RIPEMD-160.

Hash Functions Used in Bitcoin

  • SHA-256: Used for mining (double-SHA-256), transaction IDs, and Merkle trees
  • RIPEMD-160: Used in combination with SHA-256 for generating shorter address hashes (HASH160)
  • SHA-512: Used in HMAC-SHA512 for HD wallet key derivation
  • SipHash: Used internally for hash table operations to prevent DoS attacks

Common Misconceptions

  • A hash is not encryption. Encryption is reversible with the right key; hashing is a one-way process.
  • While collisions (two inputs with the same hash) are theoretically possible, finding one for SHA-256 is computationally infeasible with current technology.
  • The term "hash" in Bitcoin mining refers to a single attempt at computing the hash of a block header with a particular nonce value.

Related Terms

Hash FunctionCryptography

A mathematical function that takes an input of any size and produces a fixed-size output (hash) that is deterministic, fast to compute, and practically impossible to reverse. Bitcoin primarily uses SHA-256 and RIPEMD-160 for various purposes including mining and address generation.

SHA-256Cryptography

Secure Hash Algorithm 256-bit, the cryptographic hash function used in Bitcoin's proof-of-work mining and various other protocol operations. SHA-256 produces a 256-bit (32-byte) digest and is applied twice (double SHA-256) in block header hashing.

Proof of WorkMining

A consensus mechanism where miners expend computational energy to find a hash below a target difficulty, proving that real-world resources were consumed to create a block. Proof of work secures the Bitcoin network by making it prohibitively expensive to rewrite blockchain history.

Block HeaderProtocol

An 80-byte data structure at the beginning of each block containing the version number, previous block hash, Merkle root of all transactions, timestamp, difficulty target, and nonce. Miners hash the block header repeatedly to find a valid proof of work.