I've been asked to solve this security problem, as an exercise, but I have some difficulties.

Some notation:

  • C1 means Computer1
  • S1 means Server1,
  • TA means Trusted Autentication system,
  • X -> Y | Z means X sends message Z to Y,
  • K_X,Y(m) means that m is ciphered with the symmetric key shared by X and Y.

Suppose you have C1 who wants to be identified by S1 using a TA, the protocol works as follows:

  • C1 -> S1 | C1
  • S1 -> C1 | nonce1
  • C1 -> S1 | K_C1,TA(nonce1)
  • S1 -> TA | K_S1,TA(C1, K_C1,TA(nonce1))
  • TA -> S1 | K_S1,TA(nonce1)

Why is this not secure? Maybe because C1 sends it's identity as a plaintext to S1, and Eve could try to fake her identity using C1's, using the replay attack? Or what else?

  • Did you mean the 'replay' attack?
    – Daisetsu
    Apr 24, 2016 at 17:06
  • What's the scenario here. Could I assume Eve is a MITM?
    – Daisetsu
    Apr 24, 2016 at 17:24
  • Yes replay attack sorry! @Daisetsu
    – Holyclaus
    Apr 24, 2016 at 21:41
  • What does that "authentication protocol" attempt to authenticate? ​ (It looks like it just attempts to be an identification protocol.) ​ ​ ​ ​
    – user49075
    Apr 24, 2016 at 22:00
  • 1
    Please don't blank out questions - they should be available for later users to read and learn from.
    – Matthew
    Apr 26, 2016 at 10:37

1 Answer 1


This is vulnerable to a MITM. All Eve has to do is sit in the middle and forward traffic from C1 to S1, and from S1 to C1. This entire handshake could occur, and Eve could still be communicating with S1.

  • C1 should also be able to choose a nonce. Then he would have sent K_C1,TA(nonce1+nonce2). Then when it gets decrypted in the final step, C1 and S1 could generate a session key using nonce1+nonce2.
    – Daisetsu
    Apr 24, 2016 at 17:49
  • Also, it's not a good idea in general to have anyone simply encrypting only content sent from another user, they should always append their own nonce or time stamp, or something, to prevent against someone potentially decrypting something encrypted since symmetric encryption uses the same key for both operations.
    – Daisetsu
    Apr 24, 2016 at 17:51

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