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Blockchains: a shared ledger nobody owns
A blockchain is a chain of blocks, each containing a batch of transactions. Every block stores the cryptographic hash of the previous block. Because each block’s identity depends on the one before it, altering an old block changes its hash, which breaks every block after it — this is what makes the chain immutable.
Why editing an old block breaks the chain
Hashing visualizer (SHA-256)
Type anything. Watch the hash change completely when you change a single character — that’s the "avalanche effect."
SHA-256 hash
492341cf9522541dadce518b784a9c8b6e2f0050bb29ba2d7375747f071697b0
Length is always 64 hex characters (256 bits) no matter how long your input is. You cannot reverse it back to the input.
Block builder
Pick transactions from the mempool, then "mine" a block. Each block’s hash depends on the previous one — try tampering with an old block and watch the chain break.
Mempool
Target: hash must start with 000 (a toy version of real difficulty).
Chain
- No blocks yet.