sha256_stream_cipher.py

"""Copyright (c) 2026 Jonathan Voss (k98kurz)

Permission to use, copy, modify, and/or distribute this software
for any purpose with or without fee is hereby granted, provided
that the above copyleft notice and this permission notice appear in
all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR
CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
"""

from hashlib import sha256
from struct import pack, unpack
from os import urandom

IV_SIZE = 16

def xor(b1: bytes, b2: bytes) -> bytes:
    """XOR two equal-length byte strings together."""
    b3 = bytearray()
    for i in range(len(b1)):
        b3.append(b1[i] ^ b2[i])
    return bytes(b3)

def derive_key(*parts: bytes) -> bytes:
    """Derives a one-time key from parts."""
    pad = b''.join([sha256(p).digest() for p in parts])
    return sha256(pad).digest()

def keystream(key: bytes, iv: bytes, length: int, start: int = 0) -> bytes:
    """Get a keystream of bytes. If start is specified, it will skip
      that many bytes; if it is a multiple of 32, it will skip those
      hashes.
    """
    key = derive_key(key, iv, b'enc')
    data = b''
    counter = 0
    if start // 32 > 0:
        counter = start // 32
        start -= 32 * counter
    while len(data) < length + start:
        data += sha256(key+counter.to_bytes(4, 'big')).digest()
        counter += 1
    data = data[start:]
    return data[:length]

def symcrypt(key: bytes, iv: bytes, data: bytes) -> bytes:
    """Get a keystream of bytes equal in length to the data bytes,
      then XOR the data with the keystream.
    """
    pad = keystream(key, iv, len(data))
    return xor(data, pad)

def encrypt(key: bytes, data: bytes, iv: bytes | None = None) -> tuple[bytes, bytes]:
    """Encrypt the plaintext, returning the IV and ciphertext together."""
    iv = iv or urandom(IV_SIZE)
    return (iv, symcrypt(key, iv, data))

def decrypt(key: bytes, iv: bytes, ct: bytes) -> bytes:
    """Decrypt the iv+ciphertext. Return the plaintext."""
    return symcrypt(key, iv, ct)

def hmac(key: bytes, iv: bytes, message: bytes) -> bytes:
    """Create an hmac according to rfc 2104 specifications."""
    # set up variables
    B = 136
    ipad_byte = 0x36
    opad_byte = 0x5c
    null_byte = 0x00
    ipad = bytes([ipad_byte] * B)
    opad = bytes([opad_byte] * B)

    key = derive_key(key, iv, b'mac')

    # pad key with null bytes
    key = key + bytes([null_byte] * (B - len(key)))

    # compute and return the hmac
    partial = sha256(xor(key, ipad) + message).digest()
    return sha256(xor(key, opad) + partial).digest()

def check_hmac(key: bytes, iv: bytes, message: bytes, mac: bytes) -> bool:
    """Check an hmac."""
    # first compute the proper hmac
    computed = hmac(key, iv, message)

    # if it is the wrong length, reject
    if len(mac) != len(computed):
        return False

    # compute difference without revealing anything through timing attack
    diff = 0
    for i in range(len(mac)):
        diff += mac[i] ^ computed[i]

    return diff == 0

def seal(key: bytes, plaintext: bytes) -> bytes:
    """Generate an iv, encrypt a message, and create an hmac all in one."""
    iv, ct = encrypt(key, plaintext)
    return pack(
        f'{IV_SIZE}s32s{len(ct)}s',
        iv,
        hmac(key, iv, ct),
        ct
    )

def unseal(key: bytes, ciphergram: bytes) -> bytes:
    """Checks hmac, then decrypts the message."""
    iv, ac, ct = unpack(f'{IV_SIZE}s32s{len(ciphergram)-32-IV_SIZE}s', ciphergram)

    if not check_hmac(key, iv, ct, ac):
        raise Exception('HMAC authentication failed')

    return decrypt(key, iv, ct)