Given that A wants to send a message M to B,

I have 2 scenarios as shown:

enter image description here

The difference between the 2 is that for case 1, the digital signature is appended to the message before being encrypted using the symmetric key. For case 2, the digital signature is not encrypted using the symmetric key.

May I know which case is better? What are the security concerns of each of the case?

  • $\begingroup$ Im rather new to this topic and the explanation was a little too complex for me. I do not understand the example of the attack on how Eve can strip off the signature and add Eve's own signature. What is the point of that since Eve would not know the message? Does this mean that Case 1 is better? $\endgroup$ Apr 22, 2021 at 11:48
  • $\begingroup$ There are 5 answers, there. Did you read all of them? $\endgroup$
    – kelalaka
    Apr 22, 2021 at 11:49
  • $\begingroup$ yes, but i still do not understand what is the weakness of case 2 $\endgroup$ Apr 22, 2021 at 11:52
  • $\begingroup$ The example on D.W,'s answer should be clear if you draw some figures. It can cause impersonization. $\endgroup$
    – kelalaka
    Apr 22, 2021 at 12:03
  • 1
    $\begingroup$ Beware that encryption of a hash is a terrible way to indicate signature generation. You can see my thoughts on it here. $\endgroup$
    – Maarten Bodewes
    Apr 22, 2021 at 12:24