Steve Bellovin’s “Prehistory of Public Key Cryptography” (08-01-16) says the technique was developed several years earlier than thought—and, according to former NSA Director Bobby (Ray) Inman, "a decade earlier than Diffie and Hellman," possibly inspired by “a World War II–era paper by an unknown person at Bell Labs.”
In a talk “The Early Days in Nuclear Command and Control”, [ex-NSA high-up Jim Frazer] spoke of National Security Action Memorandum 160 (from June 6, 1962), “Permissive Links for Nuclear Weapons in NATO”. Frazer claimed that this memo—signed by President Kennedy and endorsing a memo from his science advisor, Jerome Weisner—was the basis for the invention of public key cryptography by NSA....
Weisner’s memorandum says that “this equipment...would certainly deter unauthorized use by military forces holding the weapons during periods of high tension or military combat”. In other words, non-repudiation—a classic use for public key cryptography—was important; if a bomb is used, they (or their heirs, or civilization’s heirs...) want to know who ordered it. Pending declassification of the rest of the memo, I suspect that this is the crucial seed that led to the invention of public key cryptography at NSA....
[T]he first PALs (Permissive Action Links) deployed were 5-digit mechanical combination locks. The latest versions, the Categories D and F PALs, feature 6- or 12-digit input, and an automatic “limited try” feature which disables the warhead after too many incorrect tries.
Bellovin on PALs is interesting, but Bruce G. Blair writes:
The Strategic Air Command (SAC) in Omaha quietly decided to set the “locks” to all zeros in order to circumvent this safeguard. During the early to mid-1970s, during my stint as a Minuteman launch officer, they still had not been changed. Our launch checklist in fact instructed us, the firing crew, to double-check the locking panel in our underground launch bunker to ensure that no digits other than zero had been inadvertently dialed into the panel. SAC remained far less concerned about unauthorized launches than about the potential of these safeguards to interfere with the implementation of wartime launch orders. And so the “secret unlock code” during the height of the nuclear crises of the Cold War remained constant at OOOOOOOO.
(But those are the letter O, not zeroes. So maybe PALs predate the distinction between “O” and “0” and “l” and “1” on a keyboard?)
More: a very interesting resource maintained by (this?) Bill Stewart, one of the more genial voices on the Cypherpunks list of yore.