Is this authentication system insecure or not So I want to create a simple, fast and yet secure authentication in two steps for a PC - microcontroller communication.

The first step is a seed request, the second step is an authentication request.

The encryption that is being used is AES-128.

There should be a fixed key k with a length of 16 bytes.

Whenever a seed request is coming a random 16 bytes authentication message m should be generated and then be encrypted with k and sent back to the client as a seed s.

(NOTE: E(k, m) will always output the same s)

The seed s then should be decrypted with k by the client and then the original authentication message m should be sent to the server as an authentication request.

The server compares the auth message m with the message of the request, and grants access if they are equal or rejects the request otherwise.

Also assume that an adversary can collect as many m and s pairs as he wants.

Based on the information above is it feasible to easily break this system or not?
It is vulnerable to replay attacks since the attacker knows the plaintext and its corresponding ciphertext. To mitigate against this you might want to include some information in the message like a timestamp so that assuming time keeping isn't compromised for the server and client, outdated messages, i.e messages marked by a timestamp earlier than a certain timeframe can be invalidated.