I have an encryption scheme set up with a client and server that utilizes AES 256. What I am curious about is whether or not all information sent from the server to the client can be verified as legitimate using just a single digital signature (ECDSA, secp256k1 specifically) on the key that is sent to the client.
Given a key (k1, k2), and material (m1, m2), I have heard that the following behaviors of a block cipher are possible:
Now I am curious, since I am positive that the key is verified as being legitimate, can I just use the client-sided decryption using AES256 with said key and not need any more digital signatures? Would it be possible say, for someone to get the client to decrypt two different messages using the same key in a malicious manner?
EDIT: Just intuitively, it would not seem to me that I would need a signature for every single message I want verified. I feel as though for some reason one single digital signature would be enough to verify legitimacy for all later messages.