What is double SHA-256?

Double SHA-256, often written SHA-256d or Hash256, is SHA-256 applied to the 32-byte digest of SHA-256. Bitcoin uses it for every block hash, every transaction id, the Merkle tree and the four-byte checksum at the end of a Base58Check address.

Mostly to close the length-extension hole. A single SHA-256 digest lets anyone who knows it continue hashing as if more data followed; hashing the digest again hides the internal state, so the output cannot be extended. Satoshi's design also followed Ferguson and Schneier's advice to double-hash as a conservative margin.

Why does my txid look reversed?

Bitcoin's internal byte order. Block hashes and txids are computed as double SHA-256 and then shown with the bytes reversed, so take the digest from this tool and reverse it two characters at a time to match what an explorer prints.

Is double SHA-256 more secure than SHA-256?

Not meaningfully. It costs twice the work on a short input and the output is as secure as SHA-256's 128-bit collision resistance. Nobody has found a reason to prefer it outside Bitcoin-derived protocols; for a new system, SHA-256 or SHA3-256 is the plain choice.

Is my text or file uploaded anywhere?

Nothing leaves your browser. The hash is computed on your machine, by the browser's Web Crypto API or a WebAssembly build of the algorithm loaded on first use, and the page makes no request with your input. You can load the page, go offline and keep hashing.

How do I get the Double SHA-256 of a file?

Drop the file onto the input or choose it with the button, and the tool reads it in your browser and hashes the bytes. Files up to 1 GB work; past that the browser would need the whole file in memory, so the tool declines. Paste the published digest into the Verify field to compare.

Hex, Base64 or Base64url: which should I use?

Hex is the common form: 64 characters from 0 to 9 and a to f, the same value in upper or lower case. Base64 writes the same 32 bytes in fewer characters, and Base64url swaps + and / for - and _ with no padding. Switch the output and the digest is re-encoded, not recomputed.

Why does my digest differ from the command line?

Text is hashed as UTF-8 bytes, which is what every language and command line does by default. A trailing newline changes the digest, so `echo -n` or this tool's input, with no newline added, matches a command-line tool run on a file without one.

Double SHA-256 hash generator.

Paste text or drop a file and read its double SHA-256 digest, Bitcoin's Hash256, as you type. Double SHA-256 is SHA-256 of the SHA-256 digest, the Hash256 Bitcoin uses for blocks, transactions and address checksums; everything runs in your browser and nothing is uploaded.256 bits64 hex characters2009 published0 bytes uploaded

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How it works

4 steps, no account. It runs in your browser and nothing is uploaded.

Enter text or a file
Type or paste anything, or drop a file onto the input. The digest updates with every keystroke.
Pick the algorithm and format
Double SHA-256 is the default; switch to SHA-256, and read the digest as hex or Base64.
Pick the format
Read the digest as hex in either case, or as Base64 or Base64url; the value is re-encoded, not recomputed.
Verify, copy or download
Paste a published digest to confirm a download, or copy and download your own.

Where double SHA-256 is used

Two hashes, one protocol. Double SHA-256 is Bitcoin's way of naming everything it stores.

Block hashes
A block's hash is SHA-256 of SHA-256 of its 80-byte header, read backwards; mining looks for one below the target.
Transaction ids
A transaction id is the double SHA-256 of the serialised transaction, displayed in reverse byte order.
Merkle trees
Pairs of transaction ids are double-hashed up a Merkle tree to the root in the header, so one hash commits to all of them.
Address checksums
A Base58Check address or WIF key ends in the first four bytes of the double SHA-256 of its payload, catching typos.

Nearby in the hash generators. Every one free, in the browser.

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For personal projects & open source

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Client work, SaaS products and anything you charge for.Commercial use: Personal & open source only
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Complete multi-page sites you can ship as they are.Templates: All 14 templates
React icons from every major library, installable with one command.React icons: 294k+ icons
Presets that restyle every block, in light and dark.Themes: All 59 themes
Lets your AI editor search, preview and install blocks for you.MCP server: Basic usage
Priority requests are answered first.Support: Standard

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For companies with larger teams

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Double SHA-256 hash generator.