Beginner Crypto Toolkit

by Koosha

When it comes to CTF challenges and cybersecurity, having a proper toolkit becomes an essential part; enabling one to use the underlying knowledge with ease. Personally I rather centralize my practice of cryptography in Python.

An inseparable part of cryptography in the internet environment is encoding; Integer, hexadecimal, binary, Ascii and so on.
In this post I’m gonna provide a set of commands and a few tricks, to convert one encoding to another and perform the desired transformations, resulting in a handy platform to deal with in practice cryptography. Enjoy :)

Basic Encodings

In each section we’ll see the commands converting other encodings to the entitled one.

Integer

• Numbers

To Convert a number to an integer.

Examples:

>> int("0a", 16)
Output: 10

>> int("110", 2)
Output: 6

>> int("123")
Output: 123

• Character

To return the Unicode code point for a one-character string.

Examples:

>> ord("A")
Output: 65
>> ord("\n")
Output: 10

• Bytes

Using the bytes object as a list, we can access each byte as an integer value.

Examples:

>> d = b'\x90\x90Hi'
>> print(d[0])
Output: 144
>> print(d[1])
Output: 144
>> print(d[2])
Output: 72
>> print(d[3])
Output: 105

Hex

• Integer

To return the hexadecimal representation of an integer.

Examples:

>> hex(100)
Output: '0x64'

Bytes

• Hex

To create a bytes object from a string of hexadecimal numbers.

Examples:

>> bytes.fromhex("0a")
Output: b'\n'

>> bytes.fromhex("41 0a")
Output: b'A\n'

• Integer

To convert an iterable of ints to bytes.

Examples:

>> i = [72, 101, 108, 108, 111]
>> bytes(i)
Output: b'Hello'

Ascii

• Bytes

To decode the bytes using the codec registered for encoding.

Examples:

>> d = bytes.fromhex("41")
>> d.decode("ascii")
Output: 'A'
>> d.decode("utf-8")
Output: 'A'
>> d.decode()
Output: 'A'

• Integer

To return a Unicode string of one character.

Examples:

>> chr(110)
Output: 'n'
>> chr(123)
Output: '{'

One for All

There is a library in python named codecs. We can use the decode and encode functions to perform various text and binary transformations.

The command format of encode is similar.

List of useful encodings:

You can see the full list here.

Examples:

>> from codecs import encode
>> d = b'Hello'
>> encode(d, "base64")
Output: b'SGVsbG8=\n'
>> encode(d, "zip")
Output: b'x\x9c\xf3H\xcd\xc9\xc9\x07\x00\x05\x8c\x01\xf5'
>> encode(d, "hex")
Output: b'48656c6c6f'

>> from codecs import decode
>> d = b'Hello'
>> decode(d, "ascii")
Output: 'Hello'
>> decode(b'48656c6c6f', "hex")
Output: b'Hello'

A few Tricks

zip

Using the zip python command we can construct tuples out of our inputs, suitable to perform binary operation upon them.

Examples:

>> list(zip('abcdefg', range(3), range(4)))
Output: [('a', 0, 0), ('b', 1, 1), ('c', 2, 2)]
>> list(zip('AAA', 'BBBB'))
Output: [('A', 'B'), ('A', 'B'), ('A', 'B')]
>> list(zip(b'AAA', b'BBBB'))
Output: [(65, 66), (65, 66), (65, 66)]

Application:

Using zip to implement a byte-by-byte xor to a ciphertext with a key.

>> key = b'happy'
>> cipher =  b'hellothereiamheretotestthiscommand'
>> bytes([c ^ k for c, k in zip(cipher, key)])
Output: b'\x00\x04\x1c\x1c\x16'

Resources

• Python Documents