02_Classical_Ciphers — Classical ciphers — the ones you can break by hand

Level: 101 → 201 · for anyone starting from zero

These ciphers are not here for nostalgia. They are here because you can break them, and breaking one teaches what no description of a strong cipher can: what a key has to be, why a published rule is not a secret, and what a pattern in the output costs you.

The sibling library covers the ones whose rule is public — Rotation is not encryption ↗ shows ROT13 as arithmetic on the ASCII table, with a keyspace of one, and explicitly hands the rest over to this chapter. This is where a substitution table stops being a re-labelling of a character set and starts being a cipher with a secret in it.

The pages

#	Page	The question it answers	Status
1	Breaking a cipher by counting	26! keys and it still falls in an afternoon — how?	written, 2026-09-07
2	The one-time pad	The only unbreakable cipher, and why nobody uses it	planned

Every page above is a plan until its row says written; see ROADMAP.md.

The through-line

A large keyspace is not security. The general substitution cipher has more keys than there are atoms in a small mountain, and it is broken by counting letters. What matters is whether the ciphertext carries structure — and every classical cipher does.