[SC] 2. One-time-pad Encryption

In cryptography, the one-time pad (OTP) is a type of encryption which has been proven to be impossible to crack if used correctly. Each bit or character from the plaintext is encrypted by a modular addition with a bit or character from a secret random key (or pad) of the same length as the plaintext, resulting in a ciphertext. If the key is truly random, as large as or greater than the plaintext, never reused in whole or part, and kept secret, the ciphertext will be impossible to decrypt or break without knowing the key.
The "pad" part of the name comes from early implementations where the key material was distributed as a pad of paper, so the top sheet could be easily torn off and destroyed after use. For easy concealment, the pad was sometimes reduced to such a small size that a powerful magnifying glass was required to use it. Photos show captured KGB pads that fit in the palm of one's hand,[7] or in a walnut shell.[8] To increase security, one-time pads were sometimes printed onto sheets of highly flammable nitrocellulose.

http://en.wikipedia.org/wiki/One-time_pad

For the first activity of this class, we have to write a python script implementing one time pad encryption.
For my script I used the technique of modular addition to cipher and decipher. The steps of my script are:

1. Generate the pads: Execute the file oneTimePad.py -G to generate the pads. The cipher pad name and the decipher pad name are requested before, also the number of keys to generate and the length of each key. Whole process happens silently and very fast.
   
   
   
   
   Basically, my pad generation function opens/creates the pad file, then, initialize a list of the same size that the lenght of a key, the list is filled with random integer numbers between 0 and 126 (the basic ascii code), that's means that my alphabet lenght is 127.
   When the list is full, I use the python join() function to convert it to a string and write it in the pad file. This is repeated until the specified amount of keys are created.
   Finally, I use the module shutil and import the function shutil.copy() to create a copy of the pad. One is the cipher pad and the other one is the decipher pad.
   
   
   
   
   Original message:
   
   
   
   
   



2. Encrypt a message: Execute the file oneTimePad.py -C to cipher a message file. The name of the original message file, the name of the output file (encrypted message) and the name of the cipher pad file are requested.
   
   
   
   
   This function opens the input file (original message file), reads all the lines and merges them into a single and long line, then, converts the line in a list, now, each character is a element of the list. Opens the cipher pad file and read the first line to get the cipher key, also, converts the line into a list. Then, checks if the length of the message is less than or equal to the length of the key, if is lesser, punctuation marks are appended to the message list, if is equal nothing is done, if is greater, an error is raised and the script finish the execution. After the verification, the message is encypted character by character according the formula:
   
   (int(key[a]) + ord(message[a])) % alphabetLength
   
   Where a corresponds to the index of an element in the lists.
   The characters are converted first in their corresponding ascii codes, then, the corresponding value of the key is added, then the module of the alphabet length is applied. The result is appended in a output list. When all the characters were converted, the output list are converted in a string using the function join() and is written in the specified output file. Finally, the cipher pad file is opened and the first line is removed.
   
   
   
   
   Encrypted message:
   
   
   
   
   



3. Decrypt a message: Execute the file oneTimePad.py -D to decrypt an encrypted message file. The name of the encrypted message file, the name of the output file (decrypted message) and the name of the decipher pad file are requested.
   
   
   
   
   This function opens the input file (encrypted message file), reads all the lines and merges them into a single and long line, then, converts the line in a list, now, each character is a element of the list. Opens the decipher pad file and read the first line, also, converts the line into a list. Then, checks if the length of the message is equal to the length of the key, if is lesser or greater an error is raised and the script finish the execution. After the verification, the message is decrypted character by character according the formula:
   
   (alphabetLength + int(message[a]) - int(key[a])) % alphabetLength
   
   Where a corresponds to the index of an element in the lists.
   The characters are converted first in their corresponding ascii codes, then, the alphabet length is added and the key value is substracted, then the module of the alphabet length is applied. The result is appended in a output list. When all the characters were converted, the output list are converted in a string using the function join() and is written in the specified output file. Finally, the decipher pad file is opened and the first line is removed.
   
   
   
   
   Decrypted message:
   
   
   

We can see how punctuation marks was added at the final of the file. This was because the key length was greater than the length of the message.

Code

References

• http://en.wikipedia.org/wiki/One-time_pad