Private key

A private key is a secret number that gives control over coins at a blockchain address. With it, your wallet creates digital signatures that prove you authorise a transaction, without revealing the key itself. For Bitcoin and Ethereum it is a 256-bit number, which is far too large to guess.

From the private key, software derives a public key and from that an address you can share for receiving funds. The process only works one way: knowing an address or public key does not reveal the private key.

Whoever has the private key controls the funds, and transactions cannot be reversed. If someone else learns your key they can take everything; if you lose it with no backup, the coins are inaccessible forever. Modern wallets derive many keys from a single seed phrase, so protecting that phrase protects all of them.