Simple number obfuscator

obfuscate.py

from secrets import randbelow
from typing import List


def decode(s: str) -> int:
    """
    Grab the last 2 bits of each char in the string and turn
    them into a number.
    """
    res = 0
    acc = 1
    for c in s:
        res += acc * (ord(c) & 3)  # take the last two bits of the char
        acc *= 4
    return res


def encode(n: int, min_length=10) -> str:
    """
    Take a number and encode it in a string, 2 bits per char.
    The string is meant to be printable; chars are generated in the
    0x24 ("$") to 0x7b ("{") range.

    The generated string will be at least `min_length` chars long.
    """
    res: List[str] = []

    while n > 0 or len(res) < min_length:
        base = randbelow(0x80 - 0x24) + 0x24  # 0x24 to 0x7b inclusive
        base &= ~3  # make sure the last two bits are 0
        res.append(chr(base + (n & 3)))  # add the last two bits of n to the new char
        n //= 4
    return "".join(res)


secrets = [0, 1, 42, 2**20 - 1, 2**20]
secrets.extend(randbelow(2**20) for _ in range(10))
for secret in secrets:
    encoded = encode(secret)
    print(secret, encoded)
    assert secret == decode(encoded)