Seed phrase
Also called: recovery phrase, mnemonic phrase, mnemonic, BIP39 seed phrase, BIP 39 mnemonic, seed words, backup phrase, 12-word phrase, 24-word phrase.
A seed phrase is a list of 12 to 24 words, defined by BIP 39, that encodes the random number from which a wallet derives all of its keys.
A seed phrase (also called a recovery phrase or mnemonic) is a list of 12, 15, 18, 21 or 24 ordinary words that backs up a Bitcoin wallet. Anyone who has the words can recreate every key in the wallet, so they need the same protection as the bitcoin itself.
How a BIP 39 seed phrase works
The format comes from BIP 39 (often written BIP39), by Marek Palatinus, Pavol Rusnak, Aaron Voisine and Sean Bowe. The wallet generates 128 to 256 bits of random entropy and appends a short checksum taken from its SHA-256 hash. The result is split into 11-bit chunks, and each chunk picks one word from a list of 2,048. The checksum is why the last word can't be chosen freely.
| Entropy | Checksum | Words |
|---|---|---|
| 128 bits | 4 bits | 12 |
| 160 bits | 5 bits | 15 |
| 192 bits | 6 bits | 18 |
| 224 bits | 7 bits | 21 |
| 256 bits | 8 bits | 24 |
To turn the words into a wallet, they go through PBKDF2 with HMAC-SHA512 and 2,048 iterations, together with an optional passphrase, producing a 512-bit seed. That seed is the root of an HD wallet (BIP 32), and derivation paths pick out the individual keys.
The English list is designed so the first four letters identify each word. BIP 39 also publishes lists in nine other languages, though it recommends English; browse them in our BIP 39 word list.
Keeping it safe
Write the words on paper or metal. Never type them into a website or take a photo of them. The phrase alone doesn't record which address types the wallet used, which is why output descriptors are often kept alongside it. More in the seed phrase guide.