crytool

A collection of basic functions for crypto related work in python, based on the assignments from Coursea's Cryptography I.

Basic byte level operation

This is such a common operation in crypto related coding so here is the basic cheat sheet.

For a simple ascii string:

msg = 'hello world'

For the byte string representation of the msg:

bmsg = msg.encode() # or just b'hello world'

To examine the byte string in hex format:

bmsgArr = [crytool.byte2hex(b) for b in bmsg]

To read a hex string into a byte array:

bstr = bytes.fromhex('32510ba9babebbbefd001547a810e67149caee11d945cd7fc81a05e9f85aac650e9052ba6a8cd8257bf14d13e6f0a803b54fde9e77472dbff89d71b57bddef121336cb85ccb8f3315f4b52e301d16e9f52f904')

To generate a random byte array using urandom:

r16bytes = crytool.random(16)

or with the pyCrypto library,

from Crypto import Random
r16bytes = Random.new().read(AES.block_size) 

To xor two byte arrays of the same length:

pt = b'Sixteen byte key' # or 'Sixteen byte key'.encode()
key = crytool.random( len(pt) )
ct = crytool.xor(pt,key) # stream cipher

To output the byte string in hex representation:

ct.hex() # or "".join([crytool.byte2hex(b) for b in ct])
# e.g. '77020C898AA1E8EBA5019CD916D03962'

To convert an integer to byte(s):

n = 0xAE # some integer
n.to_bytes(1,byteorder='big')     # convert 0xAE to a byte
n.to_bytes(5,byteorder='big')     # convert 0xAE into a byte string of 5 bytes representing 0xAE
n.to_bytes(1,byteorder='big')*5   # output a byte string of byte 0xAE repeating 5 times

To combine (i.e. reduce) a list of bytes to one long byte string:

import functools
a = [n.to_bytes(1,byteorder='big') for n in range(0,5)] # output: [b'\x00', b'\x01', b'\x02', b'\x03', b'\x04']
functools.reduce(lambda x, y: x + y, a) # output: b'\x00\x01\x02\x03\x04'

more to come later