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I get a request from a client who encrypts the message with the wrong AES key. Is there a way for me to know what AES key the client actually used? I understand I may get the decrypted msg which is likely to be wrong, but from what I understand, the key itself cannot be retrieved from the request sent by the client.

In other words, is there a way for me to do something like this -

if (client.aeskey != server.aeskey), print Error.

Please confirm if my understanding is correct.

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AES is just a block cipher. That's a very low level primitive. The verification you are asking about belongs at a much higher level in the protocol design. This is concerning because it sounds like you don't know about the layers which would go in between. –  kasperd Jun 15 at 16:40

1 Answer 1

It is suspicious that the client have send to the server something with a different key than the symmetric one they had agree before, isn't it?

A way how one side could known that the other has used a different key is when the decrypted message doesn't have sense (like doesn't pass a validation system you prepare).

Your server can detect if the key it has, works, but it is cryptanalysis to request which unknown key was used.

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