The way BIP 39 wallets are created is to combine the passphrase along with the seed phrase (12/24 words) to generate a master key, which is then used to derive a hierarchical series of private and public keys. In Bitcoin, you are allowed to do all of what you are asking including:
- Large number of characters (theoretically limitless)
- Special Characters
- Sequence of spaces (2 or more spaces back to back)
Although Bitcoin itself does not have constraints as to what you can include as a passphrase, many implementations of BIP 39 wallets do. You may encounter passphrase length limits based on the wallet implementation, such as Trezor allowing around 50 characters and some other wallets permitting more or fewer characters. It’s recommended to check the documentation of the specific wallet you’re using before creating your passphrase. Generally, software wallets like Sparrow and Electrum will have fewer constraints than hardware wallets like Trezor and Ledger. Also, given that most hardware wallets do not store the passphrase, it may be tedious to re-enter a 250+ character passphrase every time you want to access your bitcoin.
Passphrases provide an extra layer of security that is compelling to anyone serious about keeping their bitcoin safe. That’s because without the passphrase, an attacker cannot access your funds even if they have the seed phrase. It’s important to make sure that your passphrase is secure and memorable and/or safely stored, because losing it means permanent loss of access to the funds tied to the wallet derived with that passphrase. The seed phrase alone will not be enough to recover the wallet. As previously mentioned, it’s the combination of the seed and pass phrases that creates the wallet, so the seed phrase alone (or the seed phrase along with a different pass phrase) will create a new (most likely empty) wallet with a different master seed and different addresses, etc.
