Private key

The private key is the secret behind your bitcoin: a 256-bit number within a range set by the elliptic curve secp256k1.^[1] From it, the wallet calculates the public keyGlossaryPublic keyCalculated from the private key and used to check your signatures. The calculation only works in one direction: the private key cannot be worked out from the public key.In the glossary →× and, from that, the Bitcoin addressGlossaryBitcoin addressA string of characters like ‘bc1q…’ that someone can send bitcoin to. It is usually derived from a public key, can be shared without risk to your funds and should be generated afresh for every payment.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →×. It doesn’t work the other way round: the private key cannot be calculated from either of them.

What it’s for

When you send bitcoin, your wallet signs the transaction with the private key. Every nodeGlossaryNodeA computer that runs Bitcoin software and exchanges transactions and blocks with other nodes. Together, the nodes form the Bitcoin network; full nodes check every rule themselves.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →× checks the signature using the public key. This is how you prove that you’re allowed to spend, without revealing your secret.

How secure is it?

There are just under 2 to the power of 256 possible keys, a number with 78 digits. Guessing a randomly generated key is practically impossible. The real risks are weak randomness, malware, scams and carelessness.

Modern wallets derive all keys from your seed phraseGlossarySeed phrase (recovery phrase)A sequence of usually 12 or 24 words from which your wallet derives all its private keys. Anyone who knows the words has full access to your bitcoin – so they belong in an offline backup and never in anyone else’s hands.On the learning path: Stage 5 · Step 3 – Backing up your seed phrase →In the glossary →× – whoever knows the words owns your bitcoin. A hardware walletGlossaryHardware walletA small dedicated device that keeps your private keys offline and signs transactions internally. The key never leaves the device, so malware on your computer can’t get at it.On the learning path: Stage 6 · Step 1 – Hardware wallets compared →In the glossary →× signs on the device itself, so the keys never leave it.^[1] More in Keys, addresses & seed phrases.