Derive a recovery key from a passphrase and salt using PBKDF2.
The passphrase to derive the key from
The salt to use in the derivation
The number of iterations to use in the derivation
The number of bits to derive
https://spec.matrix.org/v1.11/client-server-api/#deriving-keys-from-passphrases
Derive a recovery key from a passphrase and salt using PBKDF2.