Public keys, private keys, and addresses

Every wallet is built on three linked pieces: a private key, a public key and an address. One must stay secret and two are safe to share. This lesson shows how they fit together.

The private key

A private key is a very large random number. It is 256 bits long, which Ethereum writes as 64 characters. Your wallet creates it for you.

The private key is what signs transactions. Whoever has it can spend the coins. It never needs to leave your wallet, and nobody else ever needs to see it.

The public key

The public key is calculated from the private key with one-way maths. Going forward is quick. Going backward, from public key to private key, is not possible in practice.

The public key lets the network check your signatures. It cannot create them.

The address

The address is a shorter code worked out from the public key. It is what you give people so they can pay you.

Ethereum addresses have 42 characters and start with 0x.
Bitcoin addresses start with 1, 3 or bc1, depending on their type.

	What it does	Share it?
Private key	Signs transactions	Never
Public key	Lets others verify signatures	Safe
Address	Receives payments	Safe

How a signature works

When you send coins, your wallet uses the private key to produce a signature for that exact transaction. Nodes use your public key to check it. The signature proves the owner of the key approved this payment, and it does so without revealing the key.

A signature cannot be reused. Change the amount or the recipient and it no longer matches.

Can someone guess a key?

The number of possible 256-bit keys is 78 digits long. Guessing one that holds coins is not a realistic attack, even with every computer on Earth.

Real thefts happen another way: people are tricked into revealing keys, or malware copies them. The maths is not the weak point.

Sharing an address

Giving out an address cannot let anyone take your coins. It does have a privacy cost. Anyone who knows your address can see its balance and history.

For this reason, Bitcoin wallets give you a fresh address for each payment. All of them belong to the same wallet. On Ethereum, people usually reuse one address, so everything sent to it is visible together.

One phrase behind many keys

You will rarely see a raw private key. Modern wallets create all your keys from a single list of words, the seed phrase. Lesson 23 explains it.