Verification lab

What changes when you change the record? Edit the text yourself to explore hashes and the links between blocks.

Inputs are calculated only in this browser, without being saved or sent. Do not enter private keys or personal information.

Change one letter. What happens to the hash?

A hash is a fixed-length value calculated from an input. Here, text becomes UTF-8 bytes and is hashed once with SHA-256. The same input always produces the same result.

Reference input

Bitcoin

SHA-256 (64 hexadecimal digits)

b4056df6691f8dc72e56302ddad345d65fead3ead9299609a826e2344eb63aa4

Start by making the B in “Bitcoin” lowercase. Spaces and line breaks are part of the input too.

SHA-256 (64 hexadecimal digits)

b4056df6691f8dc72e56302ddad345d65fead3ead9299609a826e2344eb63aa4

The input matches the reference, so all 64 digits match.

Change an earlier record. What happens to the chain?

Each block hashes its text together with the previous block’s hash. Change A to A! and inspect the chain to find the mismatch with the reference stored in the next block.

Every reference matches. This checks only the links within this model.

  1. Block 1

    Stored previous hash
    0000000000000000000000000000000000000000000000000000000000000000
    This block’s calculated hash
    ee5a56d8291207c6c21ed6c7c9694e41651c023232bc60f654c0d77fb7fc1c29

    Fixed starting value (64 zeros)

  2. Block 2

    Stored previous hash
    ee5a56d8291207c6c21ed6c7c9694e41651c023232bc60f654c0d77fb7fc1c29
    This block’s calculated hash
    de67a93b548c3f86e4e95b627a30564175b88029bc559b2a3be4431f696a6ce4

    Previous block matches

  3. Block 3

    Stored previous hash
    de67a93b548c3f86e4e95b627a30564175b88029bc559b2a3be4431f696a6ce4
    This block’s calculated hash
    4ad350a24c372cc409a376695831cd39f27428c1416e8949d21256ad08b46ad9

    Previous block matches

The last record has no successor referring to it yet. Editing only that record changes its hash but creates no reference mismatch within this chain.

See exactly what this model hashes

Each hash is a single SHA-256 of the UTF-8 representation of the following JSON array. JSON makes the field boundaries unambiguous. The first reference is 64 zeros. This is not Bitcoin block serialization.

["bitcoin-library-lab-v1", previousHash, text]

What this shows. What it does not.

Linking hashes makes a past change show up in later references. But matching references and an accepted, valid history are different things.

What is reproduced
Real SHA-256 calculations and references to a previous hash, and nothing more. Hashing is not encryption or a digital signature.
What is omitted
Bitcoin’s 80-byte block header, double SHA-256, transactions and Merkle trees, digital signatures, proof of work, node validation, and consensus are not reproduced.
What Bitcoin requires
Actual nodes validate transaction and block rules. Replacing a past block requires redoing proof of work for it and its successors, and competing with the accumulated work of other valid chains.

Explore the mechanisms and original sources

Sources behind the experiment