我正在为我的学校项目构建一个凯撒密码，并且想知道解密部分将如何工作

Question

I am writing in Python 3.4 and have so far programmed this:

plaintext = ""
print ("Do you want to encrypt or decrypt a phrase?")
answer = input("Type e for encrypt or d for decrypt and hit 'Enter'.").lower()
if answer == ("e") or answer == ("encrypt"):
    plaintext = input("Enter the phrase you would like to encrypt:")
else:
    print("I don't understand")

shift = int(input("How many shifts would you like to make?")

alphabet = "abcdefghijklmnopqrstuvwxyz"
shiftedAlphabet = "jklmnopqrstuvwxyzabcdefghi"
ciphertext = ""


for eachletter in plaintext:
    position = alphabet.index(eachletter)
    shiftedLetter = shiftedAlphabet[position]
    ciphertext = ciphertext + shiftedLetter


print(ciphertext)


if answer == ("d") or answer == ("decrypt")
    ciphertext = input("Enter the phrase you would like to decrypt:"
else:
    print("mmmm okay")

shiftedAlphabet -  

I don't how to program the decryption part (please note: I would like the code relatively simple and would like it to be similar to that above).

Answer 1

Caesar cyphers are symmetric - you just need to apply the same logic backwards - find the index in the ciphered text and get that index from the plain alphabet:

plain = ''
for eachletter in cipertext:
    position = shiftedAlphabet.index(eachletter)
    shiftedLetter = alphabet[position]
    plain = plain + shiftedLetter

Answer 2

So there are a couple issues with the code you've posted already. There are some minor syntax errors I can overlook, but there are also some logical errors.

Your second prompt to the user asks how many shifts they would like to make, yet the shifted alphabet you use for your program always shifts from 'a' to 'j' . What happens if the use enters a shift that's not 9 ?

On another note, what happens when the input plaintext has a character that is not in the alphabet? When you do .index(eachletter), it will return -1. How will that cause your cipher to behave?

Finally, it appears your instructor would like to be able to press 'e' or 'd' at any time to encrypt or decrypt, however right now if a user tries to decrypt at the beginning, I don't understand is printed. Same problem with decrypt at the end. Also, you will probably need to ask for a shift when you decrypt as well.

My last piece of advice is to think about what a Caesar cipher is. You merely rotate letters. If the input is a cat and the shift is 2 , you get c ecv . To decrypt, you simply need to rotate each letter backwards the same amount, 2 .