import base64


def decode_base64(input_string):
    """Decode a base64 string to bytes inferring padding from the length of the
    string."""

    input_bytes = input_string.encode("ascii")
    input_len = len(input_bytes)
    padding = b"=" * (3 - ((input_len + 3) % 4))
    output_len = 3 * ((input_len + 2) // 4) + (input_len + 2) % 4 - 2
    output_bytes = base64.b64decode(input_bytes + padding)
    return output_bytes[:output_len]