01_What_Cryptography_Is — What cryptography is — and what it is for

Level: 101 · for anyone starting from zero

Before any mechanism: the words, and the question every mechanism is an answer to. What are you protecting, from whom, and what would it cost them? A cipher chosen without those three answers is a decoration.

Three properties get bundled under the word "encrypted" in ordinary speech, and keeping them apart is checkpoint 2 of this library:

Property	The question it answers	The tool
Confidentiality	Can anybody else read it?	encryption
Integrity	Was it changed on the way?	a hash, in the right construction
Authenticity	Is it from who it claims?	a MAC, or a signature

Encryption alone gives you the first and — this surprises people — neither of the other two. A ciphertext an attacker cannot read is one they can very often still modify, predictably. That is why 06_Integrity_and_MACs exists as its own chapter, and why modern practice ships the two together as AEAD.

The pages

#	Page	The question it answers	Status
1	What a key is	What is a key, actually — and what makes one good?	planned
2	Kerckhoffs's principle	Why must the design be public and only the key secret?	planned

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

The through-line

The secret has to live somewhere, and the whole field is about choosing where. Every later chapter is a different answer to that.