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:

enter image description here

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.

  • $\begingroup$ This question asked before and got the answer/hints on the comments. $\endgroup$ – kelalaka Feb 15 '20 at 19:48
  • $\begingroup$ crypto.stackexchange.com/q/77530/18298 $\endgroup$ – kelalaka Feb 15 '20 at 20:43
  • $\begingroup$ I did not find it. Can you post a link to it so I can see it? $\endgroup$ – George Sp Feb 15 '20 at 21:53
  • $\begingroup$ Oh, I'm sorry. I am from a mobile device and your comment did not load without the refresh. Thank you for the link, I will check it out. $\endgroup$ – George Sp Feb 15 '20 at 21:56
  • $\begingroup$ Interesting, exact same picture different size, between two different questions. I wonder how you managed that. Some greek professor was copying pictures into an online curriculum or something? $\endgroup$ – Maarten Bodewes Feb 15 '20 at 22:07