Considering a stream cipher that produces a ciphertext $c$ from a message $m$ and a key $k$ is it possible to apply operations (multiplication and/or addition) directly to $c$ without knowing the key ?
Example : $m$ > encrypt > $c$ > $c+5$ > decrypt > $m+5$
I can not manage to get a valid $m$ at the end. I tried to work with hex values, binary data… no success so far. Could you please confirm if this kind of operation is possible on AES-ctr / Xsalsa20 (this one uses nonces) and how to properly do it? Thanks !