This document introduces the fundamental concepts of cryptography.

—

When systems communicate over a network, there are three main security goals:

* Confidentiality * Integrity * Authenticity

Cryptography helps achieve these goals.

—

Only authorized parties can read the data.

Example:

Alice sends a password to Bob.

Alice ---- Internet ---- Bob
^
Attacker 

Without protection, the attacker can read the password.

Solution:

Encryption

—

Data must not be modified during transmission.

Example:

Original:
Transfer $100

Modified:
Transfer $10000 

Solution:

Digital Signatures

—

Verify who actually sent the data.

Example:

Someone claims to be your bank.

How do you know it is really your bank?

Solution:

Digital Signatures
Certificates

—

Cryptography is the practice of protecting information.

Main categories:

Cryptography
│
├── Encryption
│
└── Digital Signatures

—

A key is a secret value used by cryptographic algorithms.

Think of it like a house key:

With key    -> Open the door
Without key -> Cannot open the door

—

Encryption protects:

Confidentiality

Goal:

Prevent unauthorized parties from reading data.

Process:

Plain Text
    ↓
Encrypt
    ↓
Cipher Text
    ↓
Decrypt
    ↓
Plain Text

Example:

Hello World
    ↓
A83D91F22C...

Only someone with the correct key can recover the original message.

—

Encryption can be implemented using:

* Symmetric Cryptography * Asymmetric Cryptography

—

Uses:

ONE SECRET KEY

for both encryption and decryption.

Workflow:

Secret Key
    ↓
Encrypt
    ↓
Cipher Text
    ↓
Decrypt
    ↓
Plain Text

Example:

Encrypt("Hello", secret_key)

Decrypt(ciphertext, secret_key) 

Advantages:

* Fast * Efficient * Easy to implement

Disadvantages:

* Secret key must be shared securely * If the key leaks, all data can be decrypted

Common algorithms:

* AES * ChaCha20 * DES (legacy)

—

Uses:

TWO KEYS

Public Key
Private Key 

The keys are mathematically related.

Public Key:

* Can be shared publicly

Private Key:

* Must remain secret

—

Purpose:

Confidentiality

Workflow:

Public Key  -> Encrypt

Private Key -> Decrypt 

Example:

Alice owns:

Public Key
Private Key

Bob wants to send a secret message:

Encrypt(message, Alice Public Key)

Alice decrypts:

Decrypt(ciphertext, Alice Private Key)

Result:

Anyone can encrypt.

Only Alice can decrypt. 

—

Digital signatures provide:

* Integrity * Authenticity

Workflow:

Private Key -> Sign

Public Key  -> Verify 

Example:

Document
    ↓
Sign with Private Key
    ↓
Signed Document

Verification:

Signed Document
    ↓
Verify with Public Key
    ↓
Valid / Invalid

Result:

Only the owner can sign.

Everyone can verify. 

—

Goal:

Hide data

Question answered:

Can someone read this?

Can use:

Symmetric Cryptography
Asymmetric Cryptography

Workflow:

Public Key  -> Encrypt
Private Key -> Decrypt

or

Secret Key -> Encrypt
Secret Key -> Decrypt

—

Goal:

Verify authenticity
Detect tampering

Questions answered:

Who sent this?

Was this modified? 

Uses:

Asymmetric Cryptography

Workflow:

Private Key -> Sign
Public Key  -> Verify

—

—

Encryption
=
Hide data
=
Confidentiality

# Digital Signature

Verify sender
+
Detect tampering
================

Authenticity + Integrity

# Symmetric

1 Secret Key

# Asymmetric

Public Key + Private Key

# Encryption

Symmetric OR Asymmetric

# Digital Signature

Asymmetric