SHA-256

SHA-256 (‘Secure Hash Algorithm’, 256 bits) is specified by the US standards body NIST in FIPS 180-4. According to the standard, it should be computationally infeasible to find data that matches a given hashGlossaryHash (hash value)A fixed-length digital fingerprint that a hash function calculates from any data. Even the tiniest change produces a completely different value, and the original data can’t be worked out from the hash.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →×, or two inputs with the same hash.^[1] The result is usually written as 64 characters from 0–9 and a–f. There are 2²⁵⁶ possible values, a number with 78 digits.

Where Bitcoin uses SHA-256

• Mining: The 80-byte block header is hashed twice – SHA-256 applied to the result of SHA-256, or SHA-256d for short. The hash must be less than or equal to the target.^[2]
• Linking: Every block header contains this double hash of the previous block header. Anyone who changes an old block therefore has to change all the block headers that follow as well.^[2]
• Identifiers and addresses: Transaction IDs are double SHA-256 hashes too. Many address formats are derived from the public key using SHA-256 and RIPEMD-160.^[3]

What this means for you

You never apply SHA-256 yourself; your wallet and the nodesGlossaryNodeA computer that runs Bitcoin software and exchanges transactions and blocks with other nodes. Together, the nodes form the Bitcoin network; full nodes check every rule themselves.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →× do that. What matters is the asymmetry: all miners together find a matching block header only about once every ten minutes on average – at a current rate of around 969 EH/s (EH/s: quintillions of hashes per second). Any node can check that it matches with a single double hash in a fraction of a second. That’s why falsifying the blockchain after the fact is so expensive.