Substitution Cipher in Python 3

Question

I am fairly new to Python 3, and I was challenged to make a substitution cipher. I came up with a very bad way to do it, but I can't think of a better way to do it.

import string
# Characters used
alpha = list(string.ascii_letters)

#Message to encrypt
message = list(input('message: '))

newMessage = []

for letter in message:
    if letter == 'a':
        letter = 'n'
        newMessage.append(letter)
    elif letter == 'b':
        letter = 'z'
        newMessage.append(letter)
    elif letter == 'c':
        letter = 'g'
        newMessage.append(letter)
    elif letter == 'd':
        letter = 'h'
        newMessage.append(letter)
    elif letter == 'e':
        letter = 'q'
        newMessage.append(letter)
    elif letter == 'f':
        letter = 'k'
        newMessage.append(letter)
    elif letter == 'i':
        letter = 'm'
        newMessage.append(letter)
    elif letter == 'j':
        letter = 'y'
        newMessage.append(letter)
    elif letter == 'l':
        letter = 'o'
        newMessage.append(letter)
    elif letter == 'p':
        letter = 'x'
        newMessage.append(letter)
    elif letter == 'r':
        letter = 'v'
        newMessage.append(letter)
    elif letter == 's':
        letter = 't'
        newMessage.append(letter)
    elif letter == 'u':
        letter = 'w'
        newMessage.append(letter)
    elif letter == 'n':
        letter = 'a'
        newMessage.append(letter)
    elif letter == 'z':
        letter = 'b'
        newMessage.append(letter)
    elif letter == 'g':
        letter = 'c'
        newMessage.append(letter)
    elif letter == 'h':
        letter = 'd'
        newMessage.append(letter)
    elif letter == 'q':
        letter = 'e'
        newMessage.append(letter)
    elif letter == 'k':
        letter = 'f'
        newMessage.append(letter)
    elif letter == 'm':
        letter = 'i'
        newMessage.append(letter)
    elif letter == 'y':
        letter = 'j'
        newMessage.append(letter)
    elif letter == 'o':
        letter = 'l'
        newMessage.append(letter)
    elif letter == 'x':
        letter = 'p'
        newMessage.append(letter)
    elif letter == 'v':
        letter = 'r'
        newMessage.append(letter)
    elif letter == 't':
        letter = 's'
        newMessage.append(letter)
    elif letter == 'w':
        letter = 'u'
        newMessage.append(letter)
    elif letter == 'A':
        letter = 'N'
        newMessage.append(letter)
    elif letter == 'B':
        letter = 'Z'
        newMessage.append(letter)
    elif letter == 'C':
        letter = 'G'
        newMessage.append(letter)
    elif letter == 'D':
        letter = 'H'
        newMessage.append(letter)
    elif letter == 'E':
        letter = 'Q'
        newMessage.append(letter)
    elif letter == 'F':
        letter = 'K'
        newMessage.append(letter)
    elif letter == 'I':
        letter = 'M'
        newMessage.append(letter)
    elif letter == 'J':
        letter = 'Y'
        newMessage.append(letter)
    elif letter == 'L':
        letter = 'O'
        newMessage.append(letter)
    elif letter == 'P':
        letter = 'X'
        newMessage.append(letter)
    elif letter == 'R':
        letter = 'V'
        newMessage.append(letter)
    elif letter == 'S':
        letter = 'T'
        newMessage.append(letter)
    elif letter == 'U':
        letter = 'W'
        newMessage.append(letter)
    elif letter == 'N':
        letter = 'A'
        newMessage.append(letter)
    elif letter == 'Z':
        letter = 'B'
        newMessage.append(letter)
    elif letter == 'G':
        letter = 'C'
        newMessage.append(letter)
    elif letter == 'H':
        letter = 'D'
        newMessage.append(letter)
    elif letter == 'Q':
        letter = 'E'
        newMessage.append(letter)
    elif letter == 'K':
        letter = 'F'
        newMessage.append(letter)
    elif letter == 'M':
        letter = 'I'
        newMessage.append(letter)
    elif letter == 'Y':
        letter = 'J'
        newMessage.append(letter)
    elif letter == 'O':
        letter = 'L'
        newMessage.append(letter)
    elif letter == 'X':
        letter = 'P'
        newMessage.append(letter)
    elif letter == 'V':
        letter = 'R'
        newMessage.append(letter)
    elif letter == 'T':
        letter = 'S'
        newMessage.append(letter)
    elif letter == 'W':
        letter = 'U'
        newMessage.append(letter)
    else:
        newmessage.append(letter)

print(''.join(newMessage))

Answer 1

ascii letters returns a string, containing lower and uppercase letters. You can iterate over a string directly, so there's no need to convert to a list, for both message and ascii_letters.

You can put the letters in order that you want in another string, following the format of ascii_letters. The, you can use the translate function, as explained here.

from string import ascii_letters

cipher_letters = 'nzghqkcdmyfoialxevtswrupjbNZGHQKCDMYFOIALXEVTSWRUPJB'
trans = str.maketrans(ascii_letters, cipher_letters)

text_to_cipher = input('Text to cipher: ')

ciphered = text_to_cipher.translate(trans)
print(f'Ciphered text: {ciphered}')

You can transfer that into a function too, so you can reuse it.

from string import ascii_letters

cipher_letters = 'nzghqkcdmyfoialxevtswrupjbNZGHQKCDMYFOIALXEVTSWRUPJB'


def ryan_cipher(text):
    trans = str.maketrans(ascii_letters, cipher_letters)
    return text.translate(trans)

if __name__ == '__main__':
    text_to_cipher = input('Text to cipher: ')
    ciphered = ryan_cipher(text_to_cipher)

    print(f'Ciphered text: {ciphered}')

Result:

Text to cipher: Hello, world!
Ciphered text: Dqool, ulvoh!

If you wish to decipher Dqool, ulvoh!, you just need to reverse the direction of the translation table, as such:

def decipher(text):
    trans = str.maketrans(cipher_letters, ascii_letters)
    return text.translate(trans)

Result:

Text to decipher: Dqool, ulvoh!
Deciphered text: Hello, world!

Answer 2

I have made an improved version. it can code and decode. and it requires no long lists

def run():
    x=input("code or decode? ")
    if x == "code":
        a=list("")
        import string
        alpha = str("abcdefghijklmnopqrstuvwxyz")
        message=input("What is your message to encode? ")
        x=len(message)
        y=0
        z=0
        for counter in range(x):
            y=y+1
            letter = alpha.find(message[z:y])
            z=z+1
            letter=letter+13
            if letter > 24:
                letter=letter-24
            letter=alpha[letter:letter+1]
            if counter != x-1:
                print(letter,end="")
            else:
                print(letter)
        x=input("again? ")
        if x == "yes":
            run()
        else:
            input()

    else:
        a=list("")
        import string
        alpha = str("abcdefghijklmnopqrstuvwxyz")
        message=input("What is your message to decode? ")
        x=len(message)
        y=0
        z=0
        for counter in range(x):
            y=y+1
            letter = alpha.find(message[z:y])
            z=z+1
            letter=letter-13
            if letter < 0:
                letter=24+letter
            letter=alpha[letter:letter+1]
            if counter != x-1:
                print(letter,end="")
            else:
                print(letter)
        x=input("again? ")
        if x == "yes":
            run()
        else:
            input()

run()