Question

Write a python program that implements a Brute Force attack on Shift Cipher. In this program there is only one input - ciphertext - is a sequence of UPPER CASE letters. To make it easy, the program will be interactive and will output all possible plaintexts and ask user which plaintext makes sense. As soon as user will decide YES, the program will stop searching and print the desired plaintext and the found SHIFT KEY.

Answer 1

Here is the completed code for this problem. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

#code

#method to shift alphabets of a cipher text to one place left
def shiftBack(text):
    #creating a string to store shifted text
    shifted=''
    #looping through each character in text
    for i in text:
        #checking if i is an alphabet
        if i.isalpha():
            c=''
            #if i is 'a', setting c to 'z' (wrapping around)
            if i=='a':
                c='z'
            #doing the same for capital 'A' (setting c to 'Z')
            elif i=='A':
                c='Z'
            else:
                #ord returns the order-index of a character, chr() takes an index and
                #returns the corresponding character, so this statement will assign c
                #a value that's before i, for example if i was 'b', then c will be 'a'
                #if i was 'j', c will be 'i'
                c=chr(ord(i)-1)
            #appending c to shifted
            shifted+=c
        else:
            #all other characters are unchanged, simply appending to shifted
            shifted+=i
    #returning shifted text
    return shifted


def main():
    #reading a cipher text (shifted/caesar text)
    cipher=input('Enter a line of cipher text: ')
    ch='n' #loop controller
    shift_key=0 #shift key
    #looping as long as user is not satisfied
    while ch!='y':
        #updating shift key
        shift_key+=1
        #shifting back cipher text once
        cipher=shiftBack(cipher)
        #printing shifted text
        print('\n'+cipher)
        #asking user if this makes sense
        ch=input('Does this make sense? (y/n) ').lower()
    #after exiting loop, displaying shift_key
    print('The shift key is',shift_key)

#invoking main()
main()

#output

Enter a line of cipher text: olssv dvysk

nkrru cuxrj

Does this make sense? (y/n) n

mjqqt btwqi

Does this make sense? (y/n) n

lipps asvph

Does this make sense? (y/n) n

khoor zruog

Does this make sense? (y/n) n

jgnnq yqtnf

Does this make sense? (y/n) n

ifmmp xpsme

Does this make sense? (y/n) n

hello world

Does this make sense? (y/n) y

The shift key is 7