I came across an exercise "custom" block cipher which looks like a counter mode implementation, but it is not exactly it. I want to check if this scheme is IND-CPA secure.
Here is a picture of the scheme:
I have already ruled out the following attacks:
1) Using the raw CBC attack, due to that it behaves like a counter mode
2) Attacking the IV, assuming that it increases by a known number every time, because only one of the two plaintext messages will be returned to the adversary. So this makes impossible the connection to one of the two original messages (like the answer on this question suggests)
Any help or suggestions would be very helpful.