Implementing-cryptography-using-python

Chapter 2 ■ Cryptographic Protocols and Perfect Secrecy 53
>>> print (ord('A'))
65
>>> print (bin(ord('A')))
0b1000001
You will find that the use of XOR is critical to cryptography. The power of
XOR is that it is self-decrypting. If you encrypt a value using XOR, you can
easily reverse it with the same operation. We will show this with a simple
integer solution first by typing the following into the shell. The following code
takes two integers (240, 115) and gets their XOR value (131). You can then XOR
131 with either 240 or 115 and get the other value:
>>> x = 240
>>> y = 115
>>> z = (x ^ y) #131
>>> print (z ^ 115)
240
>>> print (z ^ 240)
115
To XOR a whole string with another string, we should convert plaintext into
an integer, then XOR the integer and reverse that. The print() function will
work in either scope. One of the biggest differences between the two code samples
is the way we encode and decode hexadecimal values.
The following example shows how the encode method is used on the string:
def text2int(msg):
print (msg)
# convert string to hex
hexstr = msg.encode('hex')
print (hexstr)
# convert hex to integer
integer_m = int(hexstr, 16)
print (integer_m)
# convert integer back to hex
back2hex = format(integer_m, 'x')
print (back2hex)
# convert back to string
evenpad = ('0' * (len(back2hex) % 2)) + back2hex
plaintext = evenpad.decode('hex')
print (plaintext)
text2int("Hello World")