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?