Implement RSA OAEP Padding in Python

import os
import hashlib


def mgf1(seed, length, hash_func=hashlib.sha256):
    """MGF1 mask generation function."""
    output = b""
    counter = 0
    while len(output) < length:
        c = counter.to_bytes(4, "big")
        output += hash_func(seed + c).digest()
        counter += 1
    return output[:length]


def oaep_pad(message, k=128):
    """Simple OAEP-style padding (mock) for RSA encryption."""
    h_len = 32
    if len(message) > k - 2 * h_len - 2:
        raise ValueError("Message too long")
    label_hash = hashlib.sha256(b"").digest()
    ps = b"\x00" * (k - len(message) - 2 * h_len - 2)
    db = label_hash + ps + b"\x01" + message
    seed = os.urandom(h_len)
    db_mask = mgf1(seed, k - h_len - 1)
    masked_db = bytes(a ^ b for a, b in zip(db, db_mask))
    seed_mask = mgf1(masked_db, h_len)
    masked_seed = bytes(a ^ b for a, b in zip(seed, seed_mask))
    return b"\x00" + masked_seed + masked_db


if __name__ == "__main__":
    data = b"small secret"
    padded = oaep_pad(data, k=128)
    print(f"Padded length: {len(padded)} bytes")
    print(f"Padded (hex): {padded.hex()}")