- BIP39 passphrase does not encrypt, it is an extension of the seed phrase. All BIP39 seed phrases have an empty ("") passphrase by default, and you can choose to extend that.
I think that technically, BIP39 uses the word "mnemonic" as the default salt.
Then we can add an optional passphrase which will be appended to the word "mnemonic".
So, not adding a passphrase will lead to parsing 12 words (seedphrase) + "mnemonic" (salt) from the PBKDF2 algorithm.
If we added a passphrase, it would lead to parsing 12 words (seedphrase) + "mnemonic" + "secret passphrase" from the PBKDF2 algorithm.