Your passphrase has two jobs that pull against each other: it must be hard for an attacker to guess and easy for you to enter correctly years from now. This how-to is part of our guide to the BIP39 passphrase.
Why it needs to be strong
If an attacker has your seed phrase, they can test passphrases on their own hardware with no lockouts. The standard's key stretching, 2,048 rounds of PBKDF2, slows them only a little. So think of this as a master password that needs to resist an offline attack.
Use random words
The method is the same as for a strong master password:
- Use five dice, with a word list such as the EFF long list.
- Roll for each word.
- Stop at six words, or more for very large holdings.
| Words | Strength | Our view |
|---|---|---|
| 1 to 3 | Under 40 bits | Too weak |
| 4 to 5 | 52 to 65 bits | Weak to fair |
| 6 | 78 bits | Good |
| 7 to 8 | 90 to 103 bits | Very strong |
Our guide to passphrases versus passwords explains why this works.
What to avoid
- Quotes, lyrics and famous lines.
- Names, dates and addresses.
- A single dictionary word.
- A word from your own seed phrase.
- Anything that you have used as a password.
Keep the format boring
The strength comes from randomness, not from unusual characters. Unusual formats are what cause lock-outs:
- Use lowercase, and single spaces between words.
- Avoid symbols that are hard to find on a hardware wallet's screen.
- Avoid accents and non-English characters, because different devices encode them in different ways.
- Avoid capitals unless you have a clear rule.
See passphrase typos, case and spaces.
Make it stick
- Enter it on your hardware wallet.
- Note the first address of the wallet that it opens.
- Clear the device, enter it again, and compare the addresses.
- Send a small amount, and test again the next week.
Back it up
Never rely on memory alone. Write it down and store it away from your phrase. Our guide on how to back up a passphrase explains how. If you forget it, nothing can bring it back.