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Double SHA-256 (hash256)

Apply SHA-256 twice, as Bitcoin does for block headers and transaction ids — the construction that defeats length-extension.

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Double SHA-256

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Why Bitcoin hashes twice

SHA-256 is vulnerable to length extension: knowing SHA-256(secret ‖ m) lets an attacker compute the digest of a longer message without the secret. Hashing the digest again removes that property, because the second pass starts from a fixed initial state.

It is used for block headers, transaction ids and Base58Check checksums. The cost is one extra 32-byte compression, which is negligible.

Frequently asked questions

Is double SHA-256 twice as secure?

No. It closes the length-extension gap; collision resistance is unchanged at roughly 128 bits.

Is this the same as a Bitcoin txid?

The digest is the same computation, but a txid is conventionally displayed byte-reversed. Reverse the byte order if you are comparing with a block explorer.

Related tools

SHA-256 Hash GeneratorCompute a SHA-256 digest of text or a file — the default choice for checksums, signatures and content addressing.RIPEMD-160 Hash GeneratorCompute a RIPEMD-160 digest — the second half of how a Bitcoin address is derived from a public key.MD5 Hash GeneratorGenerate an MD5 checksum of text or a file. Fast and universal, but broken for security — use SHA-256 for anything that matters.SHA-1 Hash GeneratorCompute a SHA-1 digest of text or a file. Still used by Git and legacy systems, but no longer collision-resistant.SHA-384 Hash GeneratorCompute a SHA-384 digest — SHA-512 truncated, used where a 384-bit output is specified.SHA-512 Hash GeneratorCompute a SHA-512 digest of text or a file. Faster than SHA-256 on 64-bit hardware, with twice the output length.