Thanks for your work. I enjoyed watching the story.

If you had had more time, I would have liked to have heard from Ed Scheidt, the "Kryptos code designer". You can see his influence at the end of K2, with the LAYERTWO / IDBYROWS "duress cipher" illustration.

Back in June 1999, Jim Gillogly wrote "Given Scheidt's interest in key escrow, I'd expect the key to be hidden in there for somebody who knows what to look for."

Scheidt also founded a company, Tecsec, which had a patent, 5369702. A quote from that is ...

"Multi-level security is achieved because encrypted objects may be nested within other objects which are also encrypted, possibly within other objects, resulting in multiple layers of encryption."

This fits in thematically with the "layers" and "onion" concepts Sanborn and Scheidt have talked about, and also the "more than one stage" quote of Scheidt from 2015 about K4.

IP lawyers might be interested in reading about the long-running case around "multi-level encryption" by Scott Graham from June 2019. In the end Adobe didn't have to pay any damages.

excerpts...

TecSec  Inc.,  a  company  founded  by  a  former  director of the CIA’s Cryptographic Center, has been pursuing Adobe and several other technology giants in the Eastern District of Virginia since 2010, accusing  them  of  infringing  its  patents  on  multilayered  encryption.  TecSec  argued  at  trial  that  Boeing  had  paid  $10  million  for  a  license  and  Microsoft  paid  $16.5  million  to  settle  previous  litigation,  but  that  Adobe had been trying to “wait out” TecSec’s three founders, who are in their 70s and 80s.

...

Latham  partner  Grant  said  in  openings  that  Landwehr  had  simply  “demonstrated  the  obvious”—that  users  could  put  an  encrypted  or  a  non-encrypted  file  in  an  envelope  and  then  encrypt the envelope.