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.