FIT1047 Lecture Notes - Lecture 7: Block Cipher Mode Of Operation, Message Authentication Code, Public-Key Cryptography

• School: Monash University
• Department: Information Technology
• Course: FIT1047
• Professor: Carsten Rudolf

Document Summary

A cryptographic key is shared between two (or more) principals. Has been used for more than 3000 years. Early example: alphabetic substitution (we will try this in the tutorial/lab). Main idea: use the shared secret to scramble a message in a way that it cannot be understood without knowledge of the secret. In addition to substitutions (s-boxes), the order of message parts is changed. Aes uses 14 cycles in the 256-bit version and each round looks like this (picture shows 128-bit) Integrity is not straightforward (e. g. change order of blocks, change bits etc. ) Start with an initialization vector and then combine each encrypted block with the next block. Thus, blocks in wrong order cannot be decrypted and a changed block will disable decryption of next bock. Result is one block that can be used to check integrity of the complete message: m1 m2 . mx.