Tips and tricks to help you with module 02 of Information Security Lab

ISL-module-02-tips.py

# Encoding and parsing integers
# https://docs.python.org/3/library/stdtypes.html#int.to_bytes
# https://docs.python.org/3/library/stdtypes.html#int.from_bytes
bytes = (42).to_bytes(2, 'big')     # 16-bit integer, big-endian
num = int.from_bytes(bytes, 'big')  # Notice: length is implicit

# Encoding and parsing strings
# https://www.programiz.com/python-programming/methods/string/encode
bytes = "Hello, World!".encode()
string = b"Hello, World!".decode()

# Reading RFC8446:
# Make sure to read section 3: Presentation Language
# https://www.rfc-editor.org/rfc/rfc8446.html#section-3

# Example message:
# struct {
#     Version version = 0x00000001;
#     char name<0..512>;
#     opaque data[16];
# } Example;

# Encoding the message:
def encode(name, data):
    # struct {
    #     Version version = 0x00000001;
    msg = 0x00000001.to_bytes(4, 'big')
    #     char name<0..512>;
    msg += len(name).to_bytes(2, 'big')
    msg += name.encode()
    #     opaque data[16];
    if len(data) != 16:
        raise TypeError("data too short")
    msg += data
    # } Example;
    return msg

# Decode the message:
def decode(msg):
    pos = 0
    # struct {
    #     Version version = 0x00000001;
    version = int.from_bytes(msg[pos:pos+4], 'big')
    pos += 4
    #     char name<0..512>;
    name_len = int.from_bytes(msg[pos:pos+2], 'big')
    pos += 2
    name = msg[pos:pos+name_len].decode()
    pos += name_len
    #     opaque data[16];
    data = msg[pos:pos+16]
    # } Example;
    return version, name, data