Lecture "On safety and security of information systems: Symmetric ciphers advanced encryption standard" provide students with knowledge about: AES Origins; AES Structure; AES Key Expansion; An AES Example; . Please refer to the detailed content of the lecture! | SYMMETRIC CIPHERS ADVANCED ENCRYPTION STANDARD Contents 1 AES Origins 2 AES Structure 3 AES Key Expansion 4 An AES Example 5 AES Decryption 1. AES Origins AES Origins Clear a replacement for DES was needed have theoretical attacks that can break it have demonstrated exhaustive key search attacks Can use Triple-DES but slow has small blocks The AES Cipher - Rijndael Designed by Rijmen-Daemen in Belgium has 128 192 256 bit keys 128 bit data Designed to have resistance against known attacks speed and code compactness on many CPUs design simplicity AES Encryption Process 2. AES Structure AES Structure Plaintext block size 128 bits. Key length 16 24 or 32 bytes 128 192 or 256 bits . The algorithm is referred to as AES-128 AES-192 or AES-256 depending on the key length Detailed Structure 128-bit block as consisting of a 4 4 matrix of bytes arranged as follows The 4 4 matrix of bytes shown above is referred to as the state array in AES Detailed Structure Detailed Structure The key is depicted as a square matrix of bytes This key is then expanded into an array of key schedule words Each word is four bytes and the total key schedule is 44 words for the 128-bit key Detailed Structure Detailed Structure Four different stages are used one of permutation and three of substitution Substitute bytes Uses an S-box to perform a byte-by-byte substitution of the block Shift Rows A simple permutation Mix Columns A substitution that makes use of arithmetic over Add Round Key A simple bitwise XOR of the current block with a portion of the expanded key Detailed Structure For both encryption and decryption the cipher begins with an Add Round Key stage followed by nine rounds that each includes all four stages followed by a tenth round of three stages. Only the Add Round Key stage makes use of the key a. Substitute Bytes Transformation Each individual byte of Stateis mapped into a new byte in S-Box The leftmost 4 bits of the byte are used as a row value and the rightmost 4 bits are used as
