Is the Vigenère Cryptosystem malleable? Is the Vigenère Cryptosystem malleable, and how can we prove that it is (or it is not) malleable?
Vigenere Cryptosystem is as follow: 

You chose a key $(K_0,...,K_{m-1})$ consisting of elements in $Z_{26}$. 
Then a ciphertext for the message $(M_0,...,M_{n-1})$ is $$(M_i+K_{i\mod m}\mod 26)_{i \in [0..n-1]}$$

It is easy to see that you can generate a ciphertext for the message $(M_0+1,...,M_{n-1}+1)$ by adding 1 to each letter. It is therefore by definition malleable.