Glossary
Private key
A private key is a secret number, usually 256 bits long, that proves ownership of a crypto address and is used to sign transactions. Whoever knows it controls the funds.
Also called Secret key, Priv key, Signing key, WIF key
See also
A private key is the secret that gives control over cryptocurrency. On Bitcoin and Ethereum, it is a randomly generated 256-bit number. Whoever knows the private key can sign transactions that move the funds at the matching address, and the network has no way to tell the rightful owner from a thief.
How a private key works
From the private key, wallet software computes a public key using elliptic curve cryptography (the secp256k1 curve on both Bitcoin and Ethereum). From the public key, it derives an address. Each step is easy in one direction and practically impossible to reverse: sharing your address or public key does not reveal your private key.
When you send coins, your wallet signs the transaction with the private key. Nodes check the signature against the public key and accept the transaction if it matches. The key itself never leaves your wallet.
The number of possible keys is about 2 to the power of 256, a figure comparable to the number of atoms in the observable universe. Guessing someone’s key at random is not a realistic attack; stealing it is.
A concrete example
A Bitcoin private key is often displayed in Wallet Import Format, a string of 51 or 52 characters starting with 5, K or L. Most people never see it, because modern wallets derive all their keys from a seed phrase of 12 or 24 words. Backing up those words backs up every key in the wallet.
Why it matters for users
“Not your keys, not your coins” sums it up. If an exchange holds the private keys, you rely on the exchange. If you hold them, you have full control and full responsibility, the core of self-custody.
Risks and good practice
- Never type a private key or seed phrase into a website, a chat or a form, whatever the reason given.
- Store backups offline, ideally on paper or metal, in a safe place.
- Hardware wallets generate and keep keys inside a secure chip so they never touch an internet-connected computer.
- A lost key cannot be reset: without a backup, the funds are gone for good.
Read our guide Seed phrase security for practical storage options.