Implementing-cryptography-using-python

Chapter 2 ■ Cryptographic Protocols and Perfect Secrecy 57
>>> msg = "helloworldthisistheonetimepad"
>>> key = ''
>>> for i in range(len(msg)):
>>> key += secrets.choice('ABCDEFGHIJKLMNOPQRSTUVWZYZ')
>>> print (key)
CDHHCYIINAQHKMVOVAAYDPELIRNRU
In some ciphers, you can use language detectors. When the OTP is constructed
properly, there may be a condition that the wrong key will produce English text,
so there is no guarantee that using a language detector will offer the original
message. The next example demonstrates that two possible messages may occur:
Attack at Midnight! or Retreat Do Not Attack.
If you use the correct key, you will see the real message, while using the decoy
key will present the other. If an attacker is trying to brute-force this scheme, it
is possible they will find the wrong message.
The following code recipe introduces using from ... import. To refer to
items from a module within your program’s namespace, you can use the
from ... import statement. When you import modules this way, you can
refer to the functions by name rather than through dot notation. Using the
from ... import construction allows us to reference the defined elements of
a module within our program’s namespace, letting us avoid dot notation. In
this case, we use hexlify and unhexlify:
from binascii import hexlify, unhexlify
def otpSuperMsg(msg1, msg2):
hex1 = hexlify(msg1)
hex2 = hexlify(msg2)
cipher1 = int(hex1, 16)
cipher2 = int(hex2, 16)
msg = cipher1 ^ cipher2
return msg
def otpEnc(msg, key):
superKey = int(msg, 16) ^ key
return superKey
def otpDec(msg, key):
xorMsgKey = msg ^ key
back2hex = format(xorMsgKey, 'x')
evenpad = ('0' * (len(back2hex) % 2)) + back2hex
plaintext = unhexlify(evenpad)
return plaintext
realMessage = b"attackthematmidnight!"
decoyMessage= b"retreatanddonotattack"
msg = otpSuperMsg(realMessage, decoyMessage)