Passphrase
A passphrase (sometimes called the 25th word) is an optional additional word or phrase added to a BIP39 seed phrase during key derivation. It creates an entirely separate wallet, providing an extra layer of security and enabling plausible deniability.
How It Works
When you add a passphrase to your BIP39 seed phrase, it is combined with the mnemonic words during the PBKDF2 key derivation step. This produces a completely different master seed, and therefore a completely different set of keys and addresses. There is no "wrong" passphrase — every passphrase generates a valid wallet. This means there is no way for an attacker to know whether a passphrase exists or what it might be.
This property enables a powerful security strategy: keep a small amount of bitcoin in the wallet without a passphrase (the decoy wallet), and your real holdings behind the passphrase. Under duress, you can reveal the seed phrase and the attacker sees the decoy wallet. This is plausible deniability built into the protocol.
The passphrase should be strong but memorable. It can be any string of characters — a single word, a phrase, or a complex password. The critical trade-off is that the passphrase must be backed up separately from the seed phrase, and if you lose either one, your bitcoin is gone forever. Store the passphrase in a different physical location from the seed phrase for maximum security.
Key Points
- Optional extension to BIP39 that creates an entirely separate wallet from the same seed words
- Every passphrase is valid — there is no error message for a "wrong" passphrase
- Enables plausible deniability with decoy wallets for duress situations
- Must be backed up separately from the seed phrase in a different physical location
- Losing the passphrase means permanent, irrecoverable loss of funds