Private key
A private key is the secret number that signs transfers from your address. It is ownership itself: whoever knows the key controls the coins, and no password change or support ticket can undo that.
How it works
A public key derives mathematically from the private one, and an address from that. The reverse does not work: an address cannot be turned back into a key. That is why an address can be published anywhere.
When you send coins, the wallet signs the transaction with the key. The network checks that signature against the public key and confirms the instruction is genuine, without ever seeing the private key.
In practice you have many keys: the wallet derives them from a seed phrase, one per address. The backup you keep is the phrase, not each individual key.
Why keys are not guessed
A bitcoin private key is a random 256-bit number.
| Possible values | 2 to the 256 |
| Which is about | 10 to the 77 |
| Atoms in the observable universe | 10 to the 80 |
Brute force is out of the question at any scale of computing. So keys are not cracked, they are taken from the owner: phishing, a malicious extension, a screenshot in the cloud, a fake recovery page.
How to keep control
- Never type it into a siteNo legitimate service asks for a private key or seed phrase. The request is the attack.
- Sign on a deviceA hardware wallet signs internally, so the key never appears on an infected computer, not even for a moment.
- Read what you signA signature is not only a transfer. It can grant a contract the right to spend your tokens, and in the interface the two look nearly identical.
- Do not keep everything under one keySplitting across wallets caps the damage: one mistake then costs part of the money rather than all of it.