Passphrase

A seed phrase rebuilds a wallet’s keys on any device. A passphrase goes into that calculation too: the words and the passphrase together make the keys, so the same words with a different passphrase give a different wallet, with different addresses and a different balance. It isn’t a device’s PIN or an app’s password. It’s part of the key itself, no company keeps a copy, and no one can reset it.

BIP39, the 2013 standard behind seed phrases, sets the rule that makes this work: every passphrase opens a valid wallet, and only the right one opens yours. So a mistyped passphrase doesn’t produce an error. It opens another wallet, empty, and one capital letter is enough to do it.

What it’s for

• Found words aren’t enough. Whoever finds the seed phrase still needs the passphrase, so the two are kept apart.
• A wallet you can show. Because every passphrase opens a wallet, the standard says it offers “plausible deniability”: the words alone can open a wallet with a little in it, while the one that matters needs the passphrase. Under threat, though, your safety comes before any trick, as the guide on physical attacks explains.

What it costs

A second secret, with the same dilemma as the first: write it down and it can be found; don’t, and it can be forgotten. Forget it, and the words alone open a different wallet, and no one can bring the money back. Keep the two in separate places, and make sure whoever should inherit the wallet can reach both; the guide on inheritance explains how.