Block Ciphers Cryptography Lecture 2 Chantilly Academy Poorvi
Block Ciphers Cryptography Lecture 2: Chantilly Academy Poorvi Vora Department of Computer Science George Washington University
Most generally A cipher is a substitution. For example, substitute one byte with another: 2/15/2022 0 1 2 3 4 5 6 7 5 3 7 4 0 1 2 6 Chantilly Academy Crypto Lecture 2: Spring 07 2
However • It’s easy to break if you substitute one letter for another, or one set of 8 bits for another • So you take many, many bits and substitute them with others • Today’s block ciphers take 128 bits at a time, and substitute them with another 128 bits. • Can we just use a table like the on the previous slide? 2/15/2022 Chantilly Academy Crypto Lecture 2: Spring 07 3
Previous Block Cipher Standard: Data Encryption Standard (DES)
One round of DES: Feistel Cipher Ri-1 Li-1 ff Li = Ri-1 2/15/2022 Equal length Ki Ri = Li-1 f. Ki(Ri-1) Chantilly Academy Crypto Lecture 2: Spring 07 5
Diagram cut from FIPS standard 2/15/2022 Chantilly Academy Crypto Lecture 2: Spring 07 6
Attacks on DES was broken in the late 90 s with special purpose hardware built by the Electronic Frontier Foundation 2/15/2022 Chantilly Academy Crypto Lecture 2: Spring 07 7
Current Block Cipher Standard Advanced Encryption Standard (AES)
Standardization • While looking for a replacement for DES, NIST requested proposals from all over the world • Several workshops were held to discuss the proposals and their potential flaws • Researchers worked to try to break the proposals • From final shortlist with similar security properties, NIST chose one by two Flemish cryptographers, that was slightly more efficient, then known as Rijndael 2/15/2022 Chantilly Academy Crypto Lecture 2: Spring 07 9
AES • Chosen for security, efficiency, implementation • Key lengths: – 128 bits (10 rounds) – 192 bits (12 rounds) – 256 bits (14 rounds) • Consists of: XOR with key, S-box substitution, permutation, mixcolumns 2/15/2022 Chantilly Academy Crypto Lecture 2: Spring 07 10
High-level AES (all byte operations, 1 round shown) XOR with key S-box is an algebraic operation Shift Rows Mix Columns 2/15/2022 Chantilly Academy Crypto Lecture 2: Spring 07 11
- Slides: 11