I have converted a piece of plaintext to cipher text using an 8 letter keyword. After attempting to solve my vigenere cipher, I realise it is very easily solveable. So my question is, how can I create a more difficult encryption using the vigenere cipher? (without using a one time pad) I have looked at using a key k1 and then another key K2, however the length of this key will just be lcm(k1,k2), thus it does not really make my ciphertext and harder to break.
Can i use the vigenere cipher, and then with the resulting ciphertext, perform a caeser shift on it? With what I understand, this will be slightly more difficult to decrypt than just a vigenere cipher. However I am unsure if in cryptography, it is permissable to combine these two such ciphers?