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Cryptography II Content Symmetric Encryption Block Ciphers SP-Networks The advanced Encryption Standard Modes of Operation Hash Functions The Birthday Paradox Message Authentication Codes Symmetric Ciphers Symmetric ciphers are divided into two categories: Stream cipher: each plaintext corresponds to one ciphertext Block cipher Block Ciphers Block ciphers use a key to encrypt a fixed-size block of plaintext into a fixed-size block of ciphertext If you’re careful, you can convert between block and stream ciphers using modes of operation SP-Networks Claude Shannon suggested that all that was required for a strong cipher was repeated substitution and permutation SP-Networks combine a substitution process with a permutation into a single round Rounds are then repeated enough times to ensure the algorithm is secure What does it mean by “permutation”? It means that the ciphertext can be translated back to plaintext Substitution Boxes - Add confusion by re

Cryptography I Content What is encryption? Primitive types Historic ciphers The One Time Pad Stream Ciphers Cryptanalysis(密码分析学，密码翻译法) Encryption Encryption: We encode a message such that only authorised users may read it Cipher: takes a string of plaintext, and converts it into a string of ciphertext Encryption can provide: Confidentiality Integrity Authenticity Notation A cipher converts a plaintext message M into a ciphertext C under the control of a key K C is not a secret, but without knowledge of the key, it should be impossible to reconstruct M Comes in two forms: Symmetric- same key for encryption/decryption Asymmetric- separate keys The Caesar Cipher An early substitution cipher , we replace each letter of plaintext with a shifted letter further down the alphabet Vulnerable to frequency analysis Frequency Analysis The frequency of occurrences of each character are very consistent across the same language The longer the ciphertext, the e

Computer Security - Foundations of Security Content What is Computer Security Managing security Security vs Ease of use Data vs Information Principles of Computer Security Design What is Computer Security Security is about the protection of assets Prevention: Preventing access and damage to assets Detection: Steps to detect the access or damage of assets Recovery: Measures allowing us to recover from asset damage Assets could be physical or simply information Historic Computer Security Historically systems have been built to serve single users Often only a few highly trusted users were permitted to access a system Mistakes still a concern Current multi-user systems have totally different security concerns Modern Computer Security Possibly thousands of users Distributed over wide networks Not all users are inherently trustworthy More and more things are moving electronic Require protocols to manage them Managing Security Within organisations, manage