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

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

Published 4 months, 2 weeks ago
Description
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
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