def _round_function(i: int, passphrase: bytes, e: int, salt: bytes, r: bytes) -> bytes:
    """The round function used internally by the Feistel cipher."""
    return pbkdf2_hmac(
        "sha256",
        bytes([i]) + passphrase,
        salt + r,
        (_BASE_ITERATION_COUNT << e) // _ROUND_COUNT,
        dklen=len(r),
    )
The round function used internally by the Feistel cipher.
electrum/slip39.py