Chapter 1: Introduction to Cryptography

What is Cryptography?

• Cryptography secures communication by converting data into an unintelligible format (encryption).
• Main goals include confidentiality, integrity, authenticity, and non-repudiation.
• Cryptology encompasses both cryptography (encryption) and cryptanalysis (breaking encryption).

Cryptography vs. Cryptanalysis

• Cryptography focuses on developing secure communication methods through encryption.
• Cryptanalysis involves breaking cryptographic algorithms to find weaknesses without the decryption key.

Steganography vs. Cryptography

• Steganography conceals the existence of a message within other data, while cryptography scrambles the message content.
• Cryptography makes intercepted messages unreadable, whereas steganography hides the message itself.
• Combining both enhances overall security.

Goals of Cryptography

• Confidentiality: Only the intended recipient can read the message.
• Integrity: The message remains unaltered during transit.
• Authentication: Verifies the sender's identity.
• Non-repudiation: The sender cannot deny having sent the message.

History of Cryptography

• Early Cryptography: Uses ancient methods like Egyptian hieroglyphs and the Caesar Cipher.
• Classical Cryptography: Developed monoalphabetic and polyalphabetic ciphers, including the Vigenère cipher.
• Modern Cryptography: Emergence of public-key cryptography in 1976 (e.g., Diffie-Hellman, RSA) and the rise of computer-aided cryptography affecting internet security, e-commerce, and blockchain.

Basic Terminology in Cryptography

• Plaintext: Original message before encryption.
• Ciphertext: Encrypted message.
• Encryption: Process of converting plaintext to ciphertext.
• Decryption: Process of converting ciphertext back to plaintext.
• Key: Information used to encrypt and decrypt data.
• Algorithm: Mathematical procedure for encryption or decryption.

Types of Cryptography

• Symmetric Cryptography: Same key for both encryption and decryption (e.g., AES, DES).
• Asymmetric Cryptography: Uses a pair of keys (public and private) for encryption and decryption (e.g., RSA, ECC).

The Shift Cipher (Caesar Cipher)

• One of the simplest and oldest encryption methods named after Julius Caesar.
• Each letter in plaintext is shifted a fixed number down the alphabet.
• Example with a shift of 3: HELLO becomes KHOOR.

Strengths and Weaknesses of the Shift Cipher

• Strengths: Easy to implement; historically significant; suitable for low-security applications.
• Weaknesses: Vulnerable to frequency analysis; small key space (only 25 possible shifts).

The Substitution Cipher

• Replaces each letter of plaintext with another from a fixed substitution alphabet.
• More complex than the shift cipher: e.g., HELLO becomes XUBBE based on a defined substitution.

Types of Substitution Ciphers

• Monoalphabetic Substitution Cipher: Fixed unique letter replacements for ciphertext.
• Polyalphabetic Substitution Cipher: Uses multiple cipher alphabets, increasing resistance to frequency analysis (e.g., Vigenère Cipher).

Strengths and Weaknesses of the Substitution Cipher

• Strengths: Provides a higher level of security than the Caesar cipher; manageable implementation for low-risk communications.
• Weaknesses: Vulnerable to frequency analysis; key management can become complicated as the key grows in complexity.

Exercises

• Practical exercises include encrypting messages with chosen shift values and creating a monoalphabetic substitution cipher from a shuffled alphabet.

This quiz explores the fundamental concepts of cryptography as introduced in Chapter 1. Learn about the definitions and purposes of cryptography, including its crucial roles in securing communication. Understand the relationship between cryptography and cryptology, and how encryption and cryptanalysis work together.