25th word / pasphrase security length

Question

I'm curious on how long this 25th word / passphrase should be to provide roughly the same level of security as the seed phrase.

Since the seedphrase is BIP 39 there's a set amount of words you can pick from whereas with a passphrase it's a bit different.

Basically, let's assume somebody got ahold of a seed phrase but not the passphrase. What would be a good password length to ensure they couldn't brute force it within reasonable amount of time

Answer 1

If you're using a 24-word BIP39 seed phrase, that encodes 256 bits of entropy. A passphrase also needs 256 bits of entropy to contribute the same level of security.

Estimating the entropy of a password is not straightforward, since it's not always obvious what the search space is, but if we limit ourselves to passwords generated in a well-defined way, some example password lengths can be given (rounded to nearest integer):

• 77 random digits from 0 to 9
• 54 random lowercase English characters
• 45 random lowercase and uppercase English characters
• 19 random words from a 10,000 long wordlist

If that sounds like a lot, it's because 256-bit security is very likely an overkill (see this answer on Information Security StackExchange). You might want to use a 12-word seed phrase, in which case you can cut the numbers above in half.