Question

Write a Python program which implements the following two classical cryptosystem which we covered n class: a) Affine Cipher b) Vigenere Cipher Your program should consist of at least five functions: a) Two functions named encrypt, one for each of the two algorithms which accepts a lowercase alphabetical plaintext string and key as input and outputs a corresponding cipher text string. b) Two functions named decrypt, one for each of the two algorithms which accepts a lowercase alphabetical ciphertext string and a key as input and outputs a corresponding plaintext string. A main function which will offer users the option to select a cryptosystem, mode (encryption or decryption), and specify plaintext/ciphertext and key inputs using either the command line or a simple GUI. The output of each cryptosystem function should be display to stdout c) Note that the format of your key will vary depending on which ciphers you choose to implement.

Answer 1

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Answer 2

#Python program to illustate Vigenere Cipher and Affine Cipher
import sys
def gcd(a, b):
   # Everything divides 0
   if (a == 0 or b == 0):
       return 0
   # base case
   if (a == b):
       return a
   # a is greater
   if (a > b):
       return gcd(a - b, b)
   return gcd(a, b - a)

# Function to check and sys.stdout.write if
# two numbers are co-prime or not
def coprime(a, b):
if (gcd(a, b) == 1):
return True
else:
return False
def affine_encrypt(msg,a,b):
   msg = msg.upper()
   #/Cipher Text initially empty
   cipher = ""
   for i in range(len(msg)):
       char = ord(msg[i])
       # Avoid space to be encrypted
       if(msg[i]!=' '):
           ''' applying encryption formula ( a x + b ) mod m
           '''
           cipher = cipher + chr(((((a * (char - 65) ) + b) % 26) + 65))
       else:
           #else simply append space character
           cipher += msg[i]
   return cipher.lower()

def affine_decrypt(enc, a, b):
   enc = enc.upper()
   msg = ""
   a_inv = 0
   flag = 0
  
   #Find a^-1 (the multiplicative inverse of a in the group of integers modulo m.)
   for i in range(26):
       flag = (a * i) % 26
       #Check if (a*i)%26 == 1 ,then i will be the multiplicative inverse of a
       if (flag == 1):
           a_inv = i
   for i in range(len(enc)):
       char = ord(enc[i])
       if(enc[i]!=' '):
           '''Applying decryption formula a^-1 ( x - b ) mod m
           '''
           msg = msg + chr((((a_inv * ((char + 65 - b)) % 26)) + 65))
      
       else:
           #else simply append space characte
           msg += enc[i]
  
   return msg.lower()

#function that generats key from the passed string
def generate_key(inp, key):
   key = list(key)
   inp_size = len(inp)
   key_size = len(key)
   if(inp_size == key_size):
       return key
   else:
       size = inp_size - key_size
       for i in range(size):
           key.append(key[i % key_size])
   return("" . join(key))

# This function returns the
# encrypted text generated
# with the help of the key
def vigenere_encrypt(msg, key):
   msg = msg.upper()
   enc_text = []
   for i in range(len(msg)):
       key_ord = ord(key[i])
       msg_ord = ord(msg[i])

       x = ( key_ord + msg_ord) % 26
       x += 65
       enc_text.append(chr(x))
   return("" . join(enc_text)).lower()

# This function decrypts the
# encrypted text and returns
# the original text
def vigenere_decrypt(enc, key):
   enc = enc.upper()
   msg_text = []
   for i in range(len(enc)):
       enc_ord = ord(enc[i])
       key_ord = ord(key[i])
       x = ((enc_ord - key_ord) + 26) % 26
       x += 65
       msg_text.append(chr(x))
   return("" . join(msg_text)).lower()

def menu():
   sys.stdout.write("1. Affine Cipher\n")
   sys.stdout.write("2. Vigenere Cipher\n")
   sys.stdout.write("0.Exit\n")
   sys.stdout.write("Enter your choice: ")
def sub_menu():
   sys.stdout.write("\t1.Encryption\n")
   sys.stdout.write("\t2.Decryption\n")
   sys.stdout.write("\tEnter your choice: ")
#main function
def main():
   choice = 1
   msg = ""
   enc = ""
   while(choice):
       sys.stdout.write("\nCurrent Message: "+msg+" Encrypted Message: "+enc+"\n\n")
       menu()
       choice = int(input().strip())
       if(choice == 1):
           sub_menu()
           sub_ch = int(input().strip())
           if(sub_ch == 1):
               msg = input("Enter message to encrypt: ")
               a,b = map(int,input("Enter key values a and b to encrypt: ").strip().split(" "))
               if(coprime(a,26)):
                   enc = affine_encrypt(msg, a,b)
                   sys.stdout.write("\nEncrypted Message: "+enc+"\n")
               else:
                   sys.stdout.write("\nSorry the value of a must be co prime of 26"+"\n")
           elif(sub_ch == 2):
               a,b = map(int,input("Enter key values a and b to decrypt: ").strip().split(" "))
               if(enc != ""):
                   sys.stdout.write("\nDecrypted Message : "+affine_decrypt(enc, a,b)+"\n")
               else:
                   sys.stdout.write("\nSorry No Encrypted Message to decrypt")
           else:
               sys.stdout.write("\nInvalid Choice")
       elif(choice == 2):
           sub_menu()
           sub_ch = int(input().strip())
           if(sub_ch == 1):
               msg = input("Enter message to encrypt: ")
               key = input("Enter Key to encrypt: ")
               key = generate_key(msg, key)
              
               enc = vigenere_encrypt(msg, key)
               sys.stdout.write("\nEncrypted Message: "+enc)
           elif(sub_ch == 2):
               key = input("Enter Key to Decrypt: ")
               key = generate_key(msg, key)
               if(enc != ""):
                   sys.stdout.write("\nDecrypted Message : "+vigenere_decrypt(enc, key)+"\n")
               else:
                   sys.stdout.write("\nSorry No Encrypted Message to decrypt"+"\n")
           else:
               sys.stdout.write("\nInvalid Choice"+"\n")
       elif(choice == 0):
           sys.stdout.write("Exiting...")
       else:
           sys.stdout.write("\nInvalid Choice"+"\n")
main()


File Edit earch View Document Project Build Iools Help New Open ave Save All Revert CloseBack Forward Compile BuildExecute Color Chooser Find Jump to Quit ciphers.pyX 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 main 168 a,bmap (int, input ("Enter key values a and b to encrypt) Strip) split(")) if (coprime (a, 26)): enc affine_encrypt msg, a,b) sys.stdout.vrite(nEncrypted Message: "+enc+" else: sys.stdout.write(nsozry the value of a must be co prime of 26") elif(sub ch-2): a,bmap (int, input ("Enter key values a and b to decrypt: ) strip) split(")) if (enc: gys.stdout.write(nDecrypted Messageaffine_decrypt (en, a,b)+"") else: sys.stdout.vrite(nSozry No Encrypted Message to decrypt") else: sys.stdout.vrite(nInvalid Choice" elif (choice2) sub menu ) gub ch -int (input ) .strip) ir (subch1) mag input ( Enter message to encrypt:) key input ("Enter Rey to encrypt: m) key generate key (msg, key) enc vigenere encrypE (mg,key) sys.stdout.vrite(nEncrypted Message: "+enc) elir sub_ch 2): keyinput ("Enter Rey to Decrypt: ") key generate key (msg, key) if (enc: gys.stdout.write(nDecrypted Message"+vigenere decrypt (enc, key)+" else: sys.stdout.write(nsozry No Encrypted Message to decrypt"+" else: sys.stdout.vrite(nInvalid Choice"+) elif (choice0) sys.stdout.write ("Exiting..") else: sys.stdout.vrite (nInvalid Choice"+) line: 14/168 col:0 sel:0 INS TAB mode: CRLF encoding: UTF-8 filetype: Python scope: unknown