Course 17 - Computer Network Security Protocols And Techniques | Episode 4: Asymmetric Cryptography: RSA, Diffie-Hellman | CyberCode Academy

In this lesson, you’ll learn about:

What asymmetric (public key) cryptography is and why it is needed
How the RSA algorithm works and where it is used in practice
How Diffie-Hellman enables secure key exchange over public networks
Why asymmetric cryptography is vulnerable without authentication

Introduction This lesson provides an in-depth explanation of asymmetric key cryptography, focusing on RSA and Diffie-Hellman. These algorithms solve a fundamental problem in network security: how to communicate securely over an insecure channel, such as the internet, without sharing secrets in advance. Asymmetric Cryptography Overview Asymmetric cryptography uses two mathematically related keys:

Public key: Shared with everyone
Private key: Kept secret by the owner

What is encrypted with one key can only be decrypted with the other. This model enables secure communication, authentication, and key exchange at scale. 1. RSA (Rivest–Shamir–Adleman) RSA is a general-purpose asymmetric encryption algorithm based on the computational difficulty of factoring very large numbers. Key Generation

Two large prime numbers are selected: P and Q
These are multiplied to produce n = P × Q
A public key is created: (n, e)
A private key is created: (n, d)
Knowing n does not make it feasible to derive d without factoring n

Encryption and Decryption

The sender converts the message into a number M
Encryption is performed using the public key:
- C = M^e mod n
The receiver decrypts using the private key:
- M = C^d mod n

Only the private key holder can reverse the operation. Practical Use of RSA

RSA operations are slow and computationally expensive
It is not used to encrypt large data
Instead, RSA is commonly used to:
- Securely exchange a symmetric session key
- Authenticate servers and users
The exchanged symmetric key is then used with fast algorithms like AES

2. Diffie-Hellman Key Exchange Diffie-Hellman is not an encryption algorithm; it is a key exchange protocol. Purpose

Allows two parties to generate a shared symmetric key
No prior secret is required
The shared key is never transmitted over the network

How It Works

Two public values are agreed upon:
- A large prime number P
- A generator G
Each party chooses a private value:
- Alice chooses X
- Bob chooses Y
Public values are exchanged:
- Alice sends G^X mod P
- Bob sends G^Y mod P
Both compute the same shared secret:
- G^(XY) mod P

Even though all exchanged values are public, the shared secret remains secure. Key Properties

Secure against passive eavesdropping
Enables perfect forward secrecy when used correctly
Widely used in secure protocols such as TLS

3. Man-in-the-Middle (MITM) Vulnerability Both RSA and Diffie-Hellman are mathematically secure, but they are vulnerable at the protocol level if identities are not verified. The Attack

An attacker intercepts the key exchange
Establishes one secret key with Alice
Establishes a different secret key with Bob
Relays messages between both sides while decrypting and re-encrypting them

Both parties believe they are communicating securely, but the attacker sees everything. The Solution

Authentication is mandatory
Identity verification must occur before or during key exchange
Common solutions include:
- Digital certificates
- Trusted certificate authorities
- Signed public keys

Without authentication, encryption alone does not guarantee security. Key Takeaways

Asymmetric cryptography solves the secure key distribution problem
RSA relies on the difficulty of factoring large numbers
RSA is mainly used for key exchange and authentication, not bulk data encryption
Diffie-Hellman enables secure key exchange without sharing secrets
Both systems are vulnerable to MITM attacks without authentication
Secure systems always combine encryption + authentication

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Course 17 - Computer Network Security Protocols And Techniques | Episode 4: Asymmetric Cryptography: RSA, Diffie-Hellman

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