Lecture Notes: Substitution Ciphers in Computer Security What is a Substitution Cipher? A substitution cipher is a type of cryptographic technique that involves replacing each plaintext character with a different character, according to a predetermined substitution scheme. This substitution can be based on a simple alphabet shift (as in the Caesar cipher) or a more complex substitution table. Types of Substitution Ciphers ● Caesar Cipher: A simple substitution cipher that shifts each letter in the plaintext by a fixed number of positions in the alphabet. ● Monoalphabetic Substitution: Each plaintext letter is replaced by a single ciphertext letter according to a fixed substitution table. ● Polyalphabetic Substitution: Each plaintext letter can be replaced by multiple ciphertext letters, depending on its position in the message. Advantages of Substitution Ciphers ● Simple to Implement: Substitution ciphers are relatively easy to implement and understand. ● Can Be Effective: When used with strong keys and proper implementation, substitution ciphers can be difficult to break. Disadvantages of Substitution Ciphers ● Vulnerable to Frequency Analysis: The frequency distribution of letters in the ciphertext can provide clues to the plaintext. ● Limited Key Space: The number of possible keys for a substitution cipher is relatively small, making it easier to brute-force attack. Breaking Substitution Ciphers ● Frequency Analysis: Analyzing the frequency distribution of letters in the ciphertext to identify patterns. ● Ciphertext Only Attacks: Trying different substitution tables to see if a meaningful message emerges. ● Known Plaintext Attacks: If a portion of the plaintext is known, it can be used to deduce the substitution table. Modern Applications of Substitution Ciphers ● Stream Ciphers: Some stream ciphers use substitution techniques to encrypt individual bits of data. ● Block Ciphers: Certain block ciphers, such as the Data Encryption Standard (DES), incorporate substitution elements. Conclusion While substitution ciphers were widely used in the past, they are now considered relatively weak due to their susceptibility to frequency analysis. Modern cryptographic algorithms rely on more complex techniques, such as public-key cryptography, to provide stronger security. However, understanding substitution ciphers can be helpful for understanding the principles of cryptography and the evolution of encryption techniques.