Cryptography ( Due 12 Sep 2022 )

Cryptography is an ancient study of secret writing. There is a wealth of literature in this field. An extremely readable book on this subject is The Code Book by Simon Singh. This is a field of study that is of particular relevance in Computer Science. Given the widespread use of computers, one of the things people are interested in is making transactions over the internet more secure.

Here is a simple and clever way to encrypt plain text. Assume that the message contains only upper case letters, lower case letters and digits. Let L be the length of the original message, and M the smallest square number greater than or equal to L. Add (M-L) asterisks to the message, giving a padded message with length M. Use the padded message to fill a table of size K x K, where K2 = M. Fill the table in row-major order (left to right in each column, top to bottom for each row).

Now to encrypt, rotate the table 90° clockwise. The encrypted message comes from reading the message in row-major order from the rotated table, omitting any asterisks and maintaining the case of each character from the original message.

Let us say the original message is gonewiththewind. The message length L = 15 and so M = 16. The padded message is gonewiththewind*. Here are two tables showing the padded message and the padded message after rotation.

Original Padded Message
g o n e
w i t h
t h e w
i n d *

Rotated Padded Message
i t w g
n h i o
d e t n
* w h e

So the encrypted message (ignoring the asterisks) is itwgnhiodetnwhe.

Decrypting a message would just be the reverse process of encrypting. Let us consider the encrypted message osotvtnheitersec.

Original Encrypted Message
o s o t
v t n h
e i t e
r s e c

Rotated Message 90° Counter Clockwise
t h e c
o n t e
s t i s
o v e r

So the decrypted message is thecontestisover.

Input: You will read from standard input. The first line is a string P (1 ≤ length ( P ) ≤ 100) that you will have to encrypt according to the following scheme. The second line is a string Q (1 ≤ length (Q) ≤ 100) that you will have to decrypt. Assume that both strings have only upper case letters, lower case letters, and digits. Here is the format of your input cipher.in: 

gonewiththewind
osotvtnheitersec
You will read your input from stdin like so: 
Mac:
python3 Cipher.py < cipher.in

Windows:
python Cipher.py < cipher.in

Output: You will print your output to standard out. The first line will be the encryption of string P and the second line will be the decryption of the string Q. This is the format of your output cipher.out. 

itwgnhiodetnwhe
thecontestisover

The file (Cipher.py) that you will be submitting will have the following structure. You will follow the standard coding conventions in Python. 

# Input: strng is a string of 100 or less of upper case, lower case, 
#        and digits
# Output: function returns an encrypted string 
def encrypt ( strng ):

# Input: strng is a string of 100 or less of upper case, lower case, 
#        and digits
# Output: function returns an encrypted string 
def decrypt ( strng ):

def main():
  # read the two strings P and Q from standard imput

  # encrypt the string P

  # decrypt the string Q

  # print the encrypted string of P and the 
  # decrypted string of Q to standard out

if __name__ == "__main__":
  main()
You may not change the names of the functions listed. They must have the functionality as given in the specifications. You can always add more functions than those listed.

For this assignment you may work with a partner. Both of you must read the paper on Pair Programming and abide by the ground rules as stated in that paper. If you are working with a partner then only one of you will submit the code. Make sure that in the header you have your name and UT EID and your partner's name and UT EID. If you are working alone then you will just have your name and your UT EID.

Use Gradescope on Canvas to submit your Cipher.py file. We should receive your work by 11 PM on Monday, 12 Sep 2022. There will be substantial penalties if you do not adhere to the guidelines.

References