Cryptography terminology

Hiding a message is not the same as scrambling it at random: the recipient must still be able to read it. The whole field of cryptography hinges on this tension.

The four key terms

Term	Meaning
Plaintext	the readable text, before encryption
Encryption	the process that renders it unreadable
Ciphertext	the result, which is incomprehensible without the key
Key	the shared secret that enables the reverse operation

We say encrypt and decrypt. ‘Crypter’ does not exist in proper French, and ‘décrypter’ has a specific meaning: breaking a message without having the key. This is the attacker’s job, not the recipient’s.

Kerckhoffs’ principle

Formulated in 1883, it can be summed up in a single sentence:

The security of a system must not rest on the secrecy of the method, but solely on the secrecy of the key.

In other words: we assume that the adversary knows your algorithm exactly. If the only thing they lack is the key, the system is sound.

This seems counter-intuitive — why reveal your method? Because a secret method always ends up being leaked: an employee leaves, a device is dismantled, the code is discovered. A key, on the other hand, can be changed in a second.

That is why the algorithms used today (AES, RSA) are public, described in minute detail, and studied by thousands of researchers trying to break them. Their strength lies there, not in secrecy.