Function preprocess

hashes/md5.py:90–131

Preprocesses the message string: - Convert message to bit string - Pad bit string to a multiple of 512 chars: - Append a 1 - Append 0's until length = 448 (mod 512) - Append length of original message (64 chars) Example: Suppose the input is the following:

(message: bytes)

def preprocess(message: bytes) -> bytes:
    """
    Preprocesses the message string:
    - Convert message to bit string
    - Pad bit string to a multiple of 512 chars:
        - Append a 1
        - Append 0's until length = 448 (mod 512)
        - Append length of original message (64 chars)

    Example: Suppose the input is the following:
        message = "a"

        The message bit string is "01100001", which is 8 bits long. Thus, the
        bit string needs 439 bits of padding so that
        (bit_string + "1" + padding) = 448 (mod 512).
        The message length is "000010000...0" in 64-bit little-endian binary.
        The combined bit string is then 512 bits long.

    Arguments:
        message {[string]} -- [message string]

    Returns:
        processed bit string padded to a multiple of 512 chars

    >>> preprocess(b"a") == (b"01100001" + b"1" +
    ...                     (b"0" * 439) + b"00001000" + (b"0" * 56))
    True
    >>> preprocess(b"") == b"1" + (b"0" * 447) + (b"0" * 64)
    True
    """
    bit_string = b""
    for char in message:
        bit_string += format(char, "08b").encode("utf-8")
    start_len = format(len(bit_string), "064b").encode("utf-8")

    # Pad bit_string to a multiple of 512 chars
    bit_string += b"1"
    while len(bit_string) % 512 != 448:
        bit_string += b"0"
    bit_string += to_little_endian(start_len[32:]) + to_little_endian(start_len[:32])

    return bit_string


def get_block_words(bit_string: bytes) -> Generator[list[int]]: