The code to decrypt Caeser Cipher.

[lazerwolf101]

This is what I have done so far. I really need help on decryption. When I choose the decryption option I do not want it ask for a key from me. I want the code to figure it out by itself. I was taught that I could achieve this through finding the common characters in the encrypted text. The encryption works perfectly. But decryption is just really hard to figure out. Please help!!!

import collections
print("Caesar Cipher")
alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz "
#alphabet2 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
extra = ".,?'" 
selection = input("Do you want to encrypt [E] or decrypt [D] a message? ")

###########################################################################################################
def encrypt():
        global encrypted_message
        e_message = input("[E]Enter your message: ")
        e_key = input("[E]Enter your shift: ")
        e_key = int(e_key)
        print(e_message)
        print(e_key)

        seperate_message = [a for a in e_message]
        print(seperate_message)

        ascii_message = [ord(a) for a in e_message]
        print(ascii_message)

        shifted_message = [ord(a)+e_key for a in e_message]
        print(shifted_message)

        encrypted_message = " "


      #  for A in e_message:
                #if A in alphabet2:
                      #  encrypted_message += alphabet2[(alphabet2.index(A)+e_key)%(len(alphabet2))]

        for a in e_message:
                if a in alphabet:
                        encrypted_message += alphabet[(alphabet.index(a)+e_key)%(len(alphabet))]

        for a in e_message:
                if a in extra:
                        encrypted_message += extra[(extra.index(a)+e_key)%(len(extra))]



        print("Your encrypted message is: " + encrypted_message)
###########################################################################################################
def decrypt():
        global decrypted_message

        d_message = input("[D]Enter you message: ")
        d_key = input("[D]Enter your shift: ")
        d_key = int(d_key)
        #print(d_message)
        #uprint(d_key)
        
        decrypted_message = " "

       # for A in d_message:
                #if A in alphabet2:
                       # decrypted_message += alphabet2[(alphabet2.index(A)-d_key)%(len(alphabet2))] + ""
        for a in d_message:
                if a in alphabet:
                        decrypted_message += alphabet[(alphabet.index(a)-d_key)%(len(alphabet))] + ""

        for a in d_message:
                if a in extra:
                        decrypted_message += extra[(extra.index(a)-d_key)%(len(extra))]
                        
        print("Your decrypted message is:" + decrypted_message)

        seperate_message = [a for a in decrypted_message]
        print(seperate_message)

        ascii_message = [ord(a) for a in decrypted_message]
        print(ascii_message)

        shifted_message = [ord(a)-d_key for a in decrypted_message]
        print(shifted_message)

        common_letter = collections.Counter(decrypted_message).most_common(1)[0]
        print(common_letter)
###########################################################################################################
if selection == "E":
        encrypt()

if selection == "D":
        decrypt()

[DPaul]

I don't know who taught you that computers will figure out the decypher method by themselves :-)
Sounds like AI.
Maybe what was said involves the most common letter list in a particular language: e.g. E,N,R,I,S,T,O,D,A
Making a frequency table of the cyphered text, might get you a long way with a Cesar cypher.
Paul

[DT2000]

Decrypting the message would require either knowing the shift pattern used or use a character frequency table as mentioned above.

Encrypting the input:

def encrypt(text,s):
result = ""
   # transverse the plain text
   for i in range(len(text)):
      char = text[i]
      # Encrypt uppercase characters in plain text
      
      if (char.isupper()):
         result += chr((ord(char) + s-65) % 26 + 65)
      # Encrypt lowercase characters in plain text
      else:
         result += chr((ord(char) + s - 97) % 26 + 97)
      return result
#check the above function
text = "CEASER CIPHER DEMO"
s = 4

print "Plain Text : " + text
print "Shift pattern : " + str(s)
print "Cipher: " + encrypt(text,s)

Decrypting the message:

message = 'GIEWIVrGMTLIVrHIQS' #encrypted message
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

for key in range(len(LETTERS)):
   translated = ''
   for symbol in message:
      if symbol in LETTERS:
         num = LETTERS.find(symbol)
         num = num - key
         if num < 0:
            num = num + len(LETTERS)
         translated = translated + LETTERS[num]
      else:
         translated = translated + symbol
print('Hacking key #%s: %s' % (key, translated))

(Code is not my work)