Saltzer and Schroeder's Principles
Least privilege, fail-safe defaults, economy of mechanism; plus the confused deputy as the canonical violation.
Security
Reflections on Trusting Trust
A compiler can hide a backdoor in itself with no trace in any source. The limit of source-code review, and what reproducible builds do about it.
Kerckhoffs's Principle and Perfect Secrecy
Security lives in the key; and Shannon's proof that the one-time pad is unbreakable and impractical for the same reason.
The Byzantine Generals Problem
You need 3f+1 nodes to tolerate f liars, and that bound is tight.
Rice's Theorem
Every non-trivial semantic property of programs is undecidable.