Beginner Crypto Toolkit

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

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

Integer

• Numbers

To Convert a number to an integer.

Command	Format	Parameters	Return Type
int	int(x, base)	x: str base: int	int

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.

Command	Format	Parameters	Return Type
ord	ord(c)	c: one-character str	int

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.

Command	Format	Parameters	Return Type
hex	hex(x)	x: int	str

Examples:

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

Bytes

• Hex

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

Command	Format	Parameters	Return Type
bytes.fromhex	bytes.fromhex(s)	s: str	bytes object

Examples:

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

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

• Integer

To convert an iterable of ints to bytes.

Command	Format	Parameters	Return Type
bytes	bytes(x)	x: an iterable of ints	bytes object

Examples:

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

Ascii

• Bytes

To decode the bytes using the codec registered for encoding.

Command	Format	Parameters	Return Type
bytes.decode	bytes.decode(self, encoding)	encoding: str	str

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.

Command	Format	Parameters	Return Type
chr	chr(i)	i: int	str

Examples:

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

One for All

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

Command	Format	Parameters	Return Type
decode	decode(obj, encoding)	obj: depending on the encoding encoding: str	depending on the encoding

The command format of encode is similar.

List of useful encodings:

Codec	Meaning
ascii	Convert the operand to ascii.
base64	Convert the operand to multiline MIME base64 (the result always includes a trailing ‘\n’).
bz2	Compress the operand using bz2.
hex	Convert the operand to hexadecimal representation, with two digits per byte.
zip	Compress the operand using gzip.
rot13	Return the Caesar-cypher encryption of the operand.

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

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zip

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

Command	Format	Parameters	Return Type
zip	zip(iterables)	iterables	A zip object yielding tuples until an input is exhausted.

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

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• Python Documents