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I have read digital signature with Big Brother but don't understand the sequence.

One approach to digital signatures is to have a central authority that knows everything and whom everyone trusts, say Big Brother $(BB).$Each user then chooses a secret key and carries it by hand to $BB$'s office. Thus, only Alice and $BB$ know Alice's secret key, $K_A$, and so on.

When Alice wants to send a signed plaintext message, $P$, to her banker, Bob, she generates $K_A(B, R_A, t, P)$, where $B$ is Bob's identity, $R_A$ is a random number chosen by Alice, $t$ is a timestamp to ensure freshness, and $K_A(B, R_A, t, P)$ is the message encrypted with her key, $K_A.$

After then I don't understand how the following sequence works because there is nothing explained. Can anyone please explain in detail?

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    $\begingroup$ When quoting from a source, please cite the source. $\endgroup$
    – schroeder
    Commented Sep 27, 2021 at 17:20

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Alice - knowing $K_A$ - sends $K_A(B, R_A, t, P)$ to $BB$.

$BB$ - knowing $K_A$ - decrypts $K_A(B, R_A, t, P)$, obtaining $(B, R_A, t, P)$.

$BB$ - knowing $K_B$ and $K_{BB}$ - sends $K_B(A, R_A, t, P, K_{BB}(A, t, P))$ to Bob.

Bob - knowing $K_B$ - decrypts $K_B(A, R_A, t, P, K_{BB}(A, t, P))$, obtaining $(A, R_A, t, P, K_{BB}(A, t, P))$.

At this point you can go on reading.

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  • $\begingroup$ could you explain little, I don't understand what is happening here.. $\endgroup$
    – S. M.
    Commented Sep 27, 2021 at 18:57
  • $\begingroup$ Maybe you don't understand the notation: $K_A(M)$ means encrypting the message $M$ using the key $K_A$. I usually write it as $Enc(M,K_A)$. $\endgroup$
    – Riccardo
    Commented Sep 27, 2021 at 19:00
  • $\begingroup$ all things are understood. But in last paragraph "Trudy replaying either message "-- what does mean by replaying message? $\endgroup$
    – S. M.
    Commented Sep 27, 2021 at 19:39
  • $\begingroup$ It means trying to impersonate Alice by sending to Bob copies of messages sent by Alice to Bob. $\endgroup$
    – Riccardo
    Commented Sep 28, 2021 at 6:47
  • $\begingroup$ @Riccardo here middle in attack is impossible because everything is encrypted? $\endgroup$
    – S. M.
    Commented Sep 28, 2021 at 8:32

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