Can I make a cipher (ex: Vigenère) harder to break? The Vigenère cipher can relatively easy be broken when the key size is small compared to the size of the message. One first finds the length of the key, and then uses frequency analysis to actually find the key.


How can one "add" something non-complicated to the Vigenère cipher to make it harder to break? Would it for example help to first apply a permutation the message in some way?
Are there any general strategies for this when using other ciphers?
For Vigenère specifically, you can make it harder to break by increasing the size of the key and by making the key truly random.  If the key is truly random, longer than the plaintext and never reused, then Vigenère becomes equivalent to a One Time Pad.

Even without going that far, it is possible to strengthen many, though not all, cyphers by lengthening and/or randomising the key.