BIP 39 (standard for seed words)

BIP 39 is an improvement proposal (BIPGlossaryBIP (Bitcoin Improvement Proposal)A publicly documented proposal for Bitcoin – for example for new rules, standards or processes. A BIP is not a decision: whether it gets used is up to users, wallets and node operators themselves.In the glossary →×) from 2013. It defines how a wallet turns a random number into ordinary words – your seed phraseGlossarySeed phrase (recovery phrase)A sequence of usually 12 or 24 words from which your wallet derives all its private keys. Anyone who knows the words has full access to your bitcoin – so they belong in an offline backup and never in anyone else’s hands.On the learning path: Stage 5 · Step 3 – Backing up your seed phrase →In the glossary →×. Words are easier to copy down correctly than numbers. Because so many wallets use the standard, you can usually restore your words in a different wallet too.

How it works

• 128 bits of randomness give 12 words, 256 bits give 24.
• A short checksum in the last word catches most copying mistakes, but not all, and doesn’t correct any.
• The list has 2,048 words. The first four letters are enough to identify each word unambiguously.^[1]

What this means for you

• English words are normal. BIP 39 strongly discourages other languages, because most wallets only support the English list.
• Never make up words yourself. The standard advises against it – invented words are far easier to guess than real randomness.^[1]
• Not every wallet uses BIP 39. Electrum, for example, generates its own seed phrases.^[2] So note which wallet you used to create your backup.

Dive deeperFrom words to key

The words are not yet the key. The wallet turns them – together with an optional passphraseGlossaryPassphrase (the ‘25th word’)An extra word of your choice added to your seed phrase. The same words then produce a completely different wallet. This protects you if someone finds your words – but if you forget the passphrase, access is lost for good.On the learning path: Stage 5 · Step 3 – Backing up your seed phrase →In the glossary →× – into a 512-bit seed using PBKDF2. From this seed, all private keys are created according to BIP 32, each along a derivation pathGlossaryDerivation pathThe route a wallet follows to calculate individual keys from your seed phrase, for example m/84'/0'/0'. If another wallet uses a different path when you restore, it looks empty – even though nothing has been lost.In the glossary →×. Every passphrase produces a valid but different wallet.^[1]