Keys and addresses

To receive bitcoin, you need an address, and to spend it, you need a private key. A bitcoin address is a short code made from a public key, while the private key is the secret number that proves you own that address. Together, they let anyone pay you and let only you spend what you receive.

A helpful comparison is an account number and a password, although the comparison is rough. An address can only receive coins, and the private key is used to create a signature for each payment, so it is never sent anywhere.

How do the keys connect?

Everything starts with the private key, a random 256-bit number that your wallet creates on your own device. From the private key, the wallet works out the public key using elliptic curve maths. Then it hashes the public key and encodes the result, which gives you the address that people can use to pay you.

The address holds a 20-byte hash of the public key, a version byte and a short checksum. The checksum helps catch typing mistakes, so a mistyped address is usually rejected instead of paying the wrong person. The encoding is Base58, which leaves out characters that look alike, such as 0 and O.

Here is one example set of keys. Do not send coins to it, because its private key is published, and anyone could spend from it:

Private key  ef235aacf90d9f4aadd8c92e4b2562e1d9eb97f0df9ba3b508258739cb013db2
Public key   02b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a8737
Address      1EUXSxuUVy2PC5enGXR1a3yxbEjNWMHuem

Which keys can you share?

You can, and should, share your address with anyone who wants to pay you. Your public key is also safe to share, because it does not reveal the private key. Anyone with your public key can check your signatures, but cannot spend your coins.

The private key is different. Anyone who holds it can spend your coins, whether or not they have your wallet app. Never share it with anyone.

Can someone work out your private key from your address?

No. Each step in the chain is one-way, which means you can move from the private key to the address but never back again. The address is a hash of the public key, so it reveals even less than the public key does. For that reason, an address is safe to post in public, and a payer can check where coins are going without learning anything secret.

Do you need to keep all three?

Not really. The public key and the address both come from the private key, so the private key alone is enough to rebuild them. Most modern wallets go further and back up their keys with a seed phrase, a list of words that can recreate every key in the wallet. The mnemonic seed page explains how that works in detail.

Most people use a wallet that handles keys and addresses for them, so they never have to copy a key by hand. The thing to protect is the private key, or the seed phrase that holds it.

What happens if you lose the private key?

The coins sent to that address stay exactly where they are, and nothing can give you back the private key. Without it, you cannot prove that you own the address, and nobody else can spend the coins either.

This can seem harsh, but it also means there is no back door into your money. There is no password reset, and no support desk can recover your coins for you. Ownership rests entirely with you, which is both the strength and the risk of the system.

Next, private keys looks at the secret number at the start of the chain, how it is made, and why it has to be random.