One time pad

Introduction

One time pad (abbreviated OTP) is an algorithm to encrypt. It encrypts plaintext using a random key. OTP is unbreakable encryption.[1] The reason is because the key is random and when it have been used, it is deleted.[2]

OTP follows next steps:

1. Creates keys. First it is necessary to create random keys. In this case the keys has specific length.
2. Encrypts plaintext. With key and plaintext, encrypts the text with the XOR operator.
3. Decrypts ciphertext. With the same operator and same key is possible to find  the plaintext.

Functions

• Function createKeysAndSaveInFile(numberKeys, padLength)

Function receives the number of keys that has the file. Pad length is the length of each key. All keys are generated in this function and save in Alice's and Bob's files.

def createKeysAndSaveInFile(numberKeys, padLength):
    fileAlice = open('Alice.txt','a+')
    fileBob = open('Bob.txt','a+')
    for j in range(int(numberKeys)):
        key = ''
        for i in range(int(padLength)):
            aux = random.randint(34,127)
            while aux == 92:
                aux = random.randint(34,127)
                if aux != 92:# ignore character92 '\\'
                    key += chr(random.randint(34,127))
            key += chr(aux)
            fileAlice.write(key)
            fileAlice.write('\n')
            fileBob.write(key)
            fileBob.write('\n')
    fileAlice.close()
    fileBob.close()

• Function deleteFiles()

In this function I ensure keys don't save in hard disk when program is finished.

def deleteFiles():
        os.remove('Alice.txt')
        os.remove('Bob.txt')

• Function cipherMessage(message, key) [3]

messageCipher += chr(ord(message[i]) ^ ord(key[j]))

Before line is the most important, because it is the line that encrypts or decrypts.

To work with XOR operator is necessary to have in binary the cipher/plain text. Python has some functions help to do this.

For example Python has the function bin(number)[2:], it returns given number in binary. To do the one time pad algorithm follows next steps:

1. Get ASCII number of each character (text and key)
2. Change number to binary
3. Apply XOR operator. Remeber XOR operator works:

A		0		0		1		1
B		0		1		0		1
A^B		0		1		1		0

If A and B are the same return false. If A and B are different return true.

The code does the same but more efficient. I want to say that I didn't use directly bin function, because exist another function helps to do more fast.

It works in this way:
>>> bin(4)[2:]
100

>>> bin(5)[2:]
101
>>> 100 ^ 101
1
>>>chr(100 ^ 101) # an specific character
'\x01'
>>> ord('4')
52
>>> ord('5')
53
>>> ord('4') ^ ord('5')
1
>>>chr(ord('4') ^ ord('5')) # an specific character
'\x01'

So, with before lines we can see:
>>>chr(100 ^ 101) # an specific character
'\x01'
and
>>>chr(ord('4') ^ ord('5')) # an specific character
'\x01'
is the same.

def cipherMessage(message, key):
        messageCipher = ''
        j = 0
        for i in range(len(message)):
                messageCipher += chr(ord(message[i]) ^ ord(key[j]))
                j +=1
                if j >= len(key):
                        j = 0
        return messageCipher

• Function getKeyFile(document)

Function return key. First gets the first key in Alice's or Bob's file, then deletes it.

                              
def getKeyFile(document):
        with open(document,'r') as f:
                key = f.readline()
                data = f.read().splitlines(True)

        f = open(document,'r')
        lines = f.readlines()
        f.close()

        f = open(document,'w')
        for line in lines:
                if line != key:
                        f.write(line)
        f.close()
        return key

Complete code

Implementation

Keys to generate -> 3
Length of keys -> 15
Who is turn? (Write a for Alice or b for Bob) ->b
Bob: hello Alice, are you okay?
Cipher text:  @ R?
}4!37 
      u] Q&E2 ,#k
Bob says:  hello Alice, are you okay?

Who is turn? (Write a for Alice or b for Bob) ->A
Alice: NO, IM NOT OK, I was finished my homework but I overwrite it, and I need to do again
Cipher text:  v#G k!Rs+}=gm C  
G 
 Z HaIs   YG  Oz?Xam*W FU  I z4 aEd\L" L  YD.2RH. k_
Alice says:  NO, IM NOT OK, I was finished my homework but I overwrite it, and I need to do again

Who is turn? (Write a for Alice or b for Bob) ->b
Bob: Sorry Alice, ganbatte. oh! we don't have more keys, bye :/
R  o9F  .JW O    O wn FV \ d"  I y OA 
Bob says:  Sorry Alice, ganbatte. oh! we don't have more keys, bye :/

No more keys :(

Figure 1. Example of keys in files.

Figure 2. You can see that Alice's file has only one key, and Bob's file has two, the reason is because Alice sent a message and use that key. Then Bob will use his key for decrypt Alice's ciphertext.

Fig. 3 An example of implementation code. 

Bibliography

1. OTP reading
2. More OTP reading
3. Carlos García He explained me how works in python XOR operator with the function chr() and ord().
4. How works chr and ord function in Python
5. How works XOR operator Python