What is the essence of the cipher with two-faced Janus?

Question

Encryption

I found a small description in textbooks, author: Chmora, but I didn’t fully figure it out, because I’m not special in cryptography, I’m interested in the principle itself with Alice and Bob and Alsa, Bob and Charlie, and what’s the difference?

Answer 1

Alice - the name begins with A, Bob - the name begins with B, Charlie - the name begins with C.
A, B, C are the first letters of the English alphabet.
So these are just convenient labels for the parties to exchange information.