2.6 Digital Signatures

A digital signature is the online version of a hand-written signature, except it's far stronger and impossible to forge. It proves that whoever holds a specific secret (the private key) approved a specific message — without ever showing that secret to anyone.

Every Bitcoin address has two matched parts: a private key (the secret only you know) and a public key (which can be shared freely). The private key creates signatures; anyone can use the public key to verify them, but nobody can work backwards from a signature to figure out the key.

When you send Bitcoin, your wallet creates a transaction message saying 'move these coins to this address' and then signs it with your private key. Bitcoin nodes around the world check that signature against your public key before they accept the transaction.

This is the whole reason Bitcoin can exist without a bank in the middle: ownership is proven by math, not by an account at a company.