Quote from: DireWolfM14 on October 08, 2019, 09:12:34 PM

I'm not sure about Ledger, but Trezor can accommodate 35 characters for a passphrase. That might be the standard for Bip39, so I imagine it's the same with any hardware wallet that supports Bip39 passphrases.

The max passphrase length on a Trezor is actually 50 characters (https://wiki.trezor.io/Passphrase), whilst on a Ledger it is 100 characters (https://support.ledger.com/hc/en-us/articles/115005214529-Advanced-passphrase-security).

There is theoretically no limit to how long a passphrase can be. The passphrase is simply used as a salt for the PBKDF2 function, which turns mnemonic phrase in to seed. (You can read more here: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki) As a quick test, I just created and then recovered an Electrum wallet with a passphrase of 20,000 characters.

Quote from: Nunuface on October 09, 2019, 12:09:10 PM

My question is: what if I publicly post my 24-word seed, how safe would my funds behind the passphrase then be?

Ok. Well first of all, obviously don't do that. Tongue

The answer to your question depends on a couple of things. If the attacker knows nothing about your passphrase, and is going to simply have to brute force every combination of 48 characters (6 words * 8 characters each), drawing from a full 95 character ASCII set, this would give 95^48, which is approximately equivalent to 315 bits of entropy. This will never be hacked, and is significantly more entropy than a 24 word mnemonic phrase (256 bits).

If the attacker knows that you have joined 6 dictionary words together, assuming an English word list of around 200,000 words, you are now looking at 200,000^6 combinations, which is around 105 bits of entropy, which although much less, is almost certainly still going to be plenty.

If the attacker knows that you have joined 6 names together, then the number of combinations is far, far smaller. However, if the attack doesn't know any of that, and is just going to blindly bruteforce, then the chances are they would still start with a straightforward dictionary attack.