# Crypto

### Table of Contents

## About

Cryptography is the science of cryptographic algorithms (known as Cipher) that implements the following functionalities:

- Data Integrity - the message (data) was not altered during transit
- Authenticity - the owner of the message is who he claimed to be

## Usage

### Encryption/Decryption

Cryptographic algorithms transforms:

- and vice versa (decryption (backward))

### Hashing (Data Integrity and Signature)

Cryptographic algorithms transforms transforms a plaintext into a unique hash:

- without secret
- with secret (known as signature)

## Anonymity Security : Kerckhoffs Principle

^{1)} A cryptosystem should be secure even if everything about the system, except the secret (private key), is public knowledge.

In designing security systems, it is wise to assume that the details of the cryptographic algorithm are already available to the attacker. This is known as:

- Kerckhoffs' principle only secrecy of the key provides security,
- or, reformulated as Shannon's maxim, the enemy knows the system.

Because the algorithms are well known. it's the only the parameter of cipher algorithm that protect the anonymity. This parameter is known as the secret (private key).

An attacker who obtains the secret (key) can :

- recover the original message from the encrypted data,
- and issue signatures.

## Characters

Alice and Bob are fictional characters commonly used as placeholder names in cryptology

^{1)}