Chapter_1 Introduction to Cryptography.pdf

Chapter 1: Introduction to cryptography Dr. Labed Abdeldjalil What is Cryptography? ▪ Definition: Cryptology Cryptography is the science of securing communication by converting data into a form that is unintelligible without special knowledg...

Chapter 1: Introduction to cryptography Dr. Labed Abdeldjalil What is Cryptography? ▪ Definition: Cryptology Cryptography is the science of securing communication by converting data into a form that is unintelligible without special knowledge Cryptography (encryption). It ensures confidentiality, integrity, authenticity, and non-repudiation. ▪ Relation to Cryptology: Cryptanalysis Cryptology is the broader field that includes both Cryptography (encryption) and Cryptanalysis (breaking encryption). 2 Cryptography vs. Cryptanalysis o Focuses on creating secure communication methods. Cryptography: o Involves encryption (encoding information) The art of breaking cryptographic algorithms and finding Cryptanalysis: weaknesses in encryption without access to the decryption key. 3 Steganography vs Cryptography Steganography: Conceals the existence of a message. The data is hidden inside other data (e.g., hiding text within an image). Cryptography: Scrambles the message so even if intercepted, it is unreadable. Comparison: Steganography hides the existence, while cryptography hides the content. Both can be combined for enhanced security. 4 Goals of Cryptography Confidentiality: Integrity: Authentication: Non-repudiation: Ensuring that only the Ensuring the message Verifying the identity of Ensuring the sender intended recipient can hasn’t been altered during the sender. cannot deny having sent read the message. transit. the message. 5 History of Cryptography - Part 1 ▪ Early Cryptography: Ancient Egyptian hieroglyphs, Spartan Scytale, Caesar Cipher (shift cipher). ▪ Classical Cryptography: Monoalphabetic and polyalphabetic ciphers (Vigenère cipher).Symmetric cryptography (same key for encryption and decryption). Modern Cryptography: Public-key cryptography introduced in 1976 (Diffie-Hellman, RSA). Importance of computers in cryptographic calculations. Cryptography in the digital age: Internet, E-commerce, Blockchain. 6 History of Cryptography - Part 2 ▪ https://youtu.be/RTDYRl1uaMI?si=CwEeD2rJFEKywH9p 7 Basic Terminology in Cryptography Plaintext: Ciphertext: Encryption: Decryption: Key: Algorithm: The original The Process of Process of A piece of The message or encrypted converting converting information mathematical data before message. plaintext to ciphertext that procedure encryption. ciphertext. back to determines used for plaintext. the output of encryption the and cryptographic decryption. algorithm (public or private). 8 Types of Cryptography ▪ Symmetric Cryptography: Same key for encryption and decryption (e.g., AES, DES). ▪ Asymmetric Cryptography: Public and private key pair, where one key encrypts and the other decrypts (e.g., RSA, ECC). 9 The Shift Cipher (Caesar Cipher) One of the oldest and simplest forms of encryption. Also known as the Caesar Cipher, named after Julius Caesar, who used it to protect his military communications. How It Works: Each letter in the plaintext is shifted by a fixed number of positions down or up the alphabet. For example, with a shift of 3: Plaintext: HELLO Ciphertext: KHOOR (Each letter shifted by 3). 10 Strengths and Weaknesses of the Shift Cipher ▪ Strengths: Easy to implement and understand. Suitable for low-security applications and historical relevance. ▪ Weaknesses: Vulnerability to Frequency Analysis: The cipher preserves the frequency of letters, making it easy to break by analyzing the most common letters in the ciphertext. Key Space is Small: Only 25 possible shifts for the English alphabet (shift of 0 means no encryption). ▪ Historical Relevance: It’s an important stepping stone in the development of more complex ciphers. 11 Exercises ▪ Exercise 1: Choose a shift value between 1 and 25 and encrypt the following plaintext message using the chosen shift value: Plaintext: "CYBERSECURITY IS ESSENTIAL“ ▪ Exercise 2: You have intercepted the following ciphertext: Ciphertext: "FRZDUGV GLUHY“ Decrypt the ciphertext without knowing the key (perform a brute-force attack by trying all possible shifts from 1 to 25). 12 The Substitution Cipher A substitution cipher replaces each letter of the plaintext with another letter from the alphabet. It uses a fixed substitution for each letter in the alphabet, unlike the shift cipher which uses a systematic shift. Example: Plaintext: HELLO Ciphertext: XUBBE (where each letter is substituted by a pre-defined letter). 13 Types of Substitution Ciphers: Monoalphabetic Substitution Cipher: Each letter in the plaintext is replaced by a unique corresponding letter in the ciphertext alphabet. Polyalphabetic Substitution Cipher: Uses multiple cipher alphabets to encrypt the message, making it more resistant to frequency analysis (e.g., Vigenère Cipher). 14 Strengths and Weaknesses of the Substitution Cipher ▪ Strengths: Provides more security than the Caesar cipher as each letter is substituted independently. Easy to implement but offers moderate security for low-risk communications. ▪ Weaknesses: Vulnerability to Frequency Analysis: Since the substitution is consistent, the frequency of letters remains the same, making it vulnerable to attacks based on analyzing letter frequencies. Key Management: Requires securely sharing and managing the substitution table (key), which becomes problematic as the key grows in complexity. ▪ Historical Use: Widely used in historical cryptography, such as the Atbash Cipher and Playfair Cipher. 15 Exercises ▪ Exercise 1: In this exercise create and apply a monoalphabetic substitution cipher. First, randomly generate your own substitution alphabet by shuffling the letters of the standard alphabet. Once you have created your substitution alphabet, use it to encrypt the following plaintext: "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG." After you have completed the encryption, provide both the substitution alphabet and the resulting ciphertext. ▪ Exercise 2: Use the substitution mapping below to encrypt the message “March 12 0300” ▪ Original: ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 ▪ Maps to: 2BQF5WRTD8IJ6HLCOSUVK3A0X9YZN1G4ME7P Use the substitution mapping to decrypt the message C2SVX2VP 16 Exercises ▪ Exercise 3: The chart to the right shows the frequency of different characters in some encrypted text. What can you deduce about the mapping? 17