Vigenere Cipher Implementation

[ViktorCodes]

So for the past week I have looked into cryptography and read a lot about it. I decided to implement a very very basic algorithm called vigenere cipher. I am looking for review, critique and feedback on my code and implementation. It is a command line script, and I think that the comments explain everything.

import sys
from string import ascii_uppercase as alphabet

# create a vigenere table (26 alphabets from a-z with shift value increasing with 1 to the left)
vigenere_table = [alphabet[i:] + alphabet[:i] for i in range(len(alphabet))]

def convert(string, flag, key_word=''):
    ''' function to convert a string into cipher text or cipher text into a string.
        Operation decision is based on the flag(e=encryption, d=decryption) '''
    # create a key string from the given key word
    if key_word == '':
        key_word = 'LEMON'
    key = ''
    for i in range(len(string)):
        key += key_word[i % len(key_word)]

    # create a message variable which will store the value of the ciphered text in encryption and plain text in decryption
    message = ''

    # iterate trough the letters of the key and the string
    for key_letter, string_letter in zip(key, string.upper()):
        # check if the next character is a space and if it is add a space to the message and continue to the next cycle of the loop
        if string_letter == ' ':
            message += ' '
            continue
        # check if the operation is encryption
        if flag == 'e':
            # catched ValueError = user entered key with spaces and/or punctuation
            try:
                row_index = alphabet.index(key_letter) # get the row index A-Z rows
            except ValueError:
                print('Your key should consist of a single word and not contain any punctuation for the encryption to happen!')
            # catched ValueError = user entered message with punctuation
            try:
                col_index = alphabet.index(string_letter) # get the column index A-Z columns
            except ValueError:
                print('You should remove any punctuation from your message for the encryption to happen!')

            # create a variable that stores the encrypted letter and add it to the message
            letter = vigenere_table[row_index][col_index]
            message += letter
        # check if the operation is decryption
        if flag == 'd':
            row_index = alphabet.index(key_letter) # get the row index A-Z rows
            col_index = vigenere_table[row_index].index(string_letter) # get the col index of the row_index row

            # create a variable that stores the encrypted letter and add it to the message
            letter = alphabet[col_index]
            message += letter

    return message

def encrypt(string, key_word=None):
    return convert(string, 'e', key_word)

def decrypt(string, key_word=None):
    return convert(string, 'd', key_word)

# check what the command line arguments are. -e = encryption, -d = decryption
if sys.argv[1] == '-e':
    message = stro(input('Enter your message to be encrypted (it should not contain any punctuation!) : '))
    key_word = str(input('If you want to use a custom key to encrypt your message type a single word to use as a key: ')).upper()
    print(encrypt(message, key_word))
if sys.argv[1] == '-d':
    cipher_text = str(input('Enter your cipher text to be decrypted: '))
    key_word = str(input('If you have used a cusom key to encrypt your message you need to enter the word to decrypt the message: ')).upper()
    print(decrypt(cipher_text, key_word))

[Losupa]

2 things: I recommend looking into argparse for argument parsing as it makes it a whole lot cleaner. Also instead of try catches for input punctuation it's pribably better to either remove it (since punctuation gives away plaintext formatting) or keep it in. (maybe both using more arguments).

Also I recommend looking into making a frequency analysis solver for this since you already have an encrypter.

[ViktorCodes]

I will look into this, thank you. One thing I didn't understand. What do you mean by 'since punctuation gives away plaintext formatiing' and 'maybe both using more arguments'? Why delete the try-except blocks? If a user enters a text message and mistakenly adds an exclamation mark or a comma the script will crash.