About
A register, not a manual
Each entry states the promise the system made and the specific thing that broke it — usually procedure rather than mathematics.
This site keeps a register of historical cipher systems. Each entry sets out what a system promised its users, how it was actually operated, and the specific thing that ended it. The entries are grouped into five sections — substitution, polyalphabetic, machines, the pad, and the ways systems were broken — because those are the five distinct answers the subject gives to one question: what happens when a secret has to travel.
The recurring finding is stated plainly on every page. Almost none of these systems fell to mathematics. They fell to procedure: a key sent twice, a stock phrase at the head of every message, a pad page printed a second time under pressure, a codebook that left the room. The mathematics usually held for as long as anyone needed it to. The paperwork did not.
What this site does not do
There is no operational advice here, and there will not be. Nothing on this site tells anyone how to secure anything, what to configure, what to buy or how to manage a key. Nothing here describes an attack against a system in current use. The subject is historical systems and published standards, described factually, with nothing invented.
People long dead are named where the record is clear. No living person is profiled.
Photographs
Photographs are credited beneath their captions. Captions carry information the picture cannot; the credit sits under the caption as a separate, quieter line.