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 !