Block ciphers are the workhorses of modern encryption. They trade the mathematical perfection of the One-Time Pad for practical invariants that can actually be maintained at scale.

The One-Time Pad is the only encryption scheme with a mathematical proof of perfect secrecy. Its invariants are simple but brutally strict — violate any one of them and the guarantee vanishes entirely.

Cryptography is the discipline of preserving invariants over untrusted channels. Confidentiality, integrity, authenticity — these are guarantees that must hold no matter what.

On the one principle that separates resilient systems from fragile ones.