In general, a session/thread ephemeral key pair is exchanged with the participants, and only each users private origin signing validates each sender sections. Since the signatures are passively checked, it does not need redone after each append operation.

Thus, participants are pooled under 1 time constrained key exchange, participants sections origins are signed/time-stamped, and the ciphertext expires on a schedule.

Why? so at some future time side-channel key exfiltration can't be used to data mine the entire history of past tapped conversations/ciphertext. Even if a private signing key is stolen, it is difficult to rebuild the lost ephemeral key session.

Have a nice day. =3

My eyes glazed over reading that. I feel like 30 years of internet has proven one thing: the simplest solution always wins. PGP is many things, but simple and obvious are not among them.