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Introduction to Block Ciphers in Cryptography - Chantilly Academy Lecture 2

In this lecture, delve into the world of block ciphers, their functionality, and importance in securing data. Understand the basics, evolution from DES to AES, and the intricate workings of advanced encryption standards.

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Introduction to Block Ciphers in Cryptography - Chantilly Academy Lecture 2

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  1. Block Ciphers Cryptography Lecture 2: Chantilly Academy Poorvi Vora Department of Computer Science George Washington University

  2. Most generally A cipher is a substitution. For example, substitute one byte with another: Chantilly Academy Crypto Lecture 2: Spring 07

  3. 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 one on the previous slide? Chantilly Academy Crypto Lecture 2: Spring 07

  4. Previous Block Cipher Standard: Data Encryption Standard (DES)

  5. One round of DES: Feistel Cipher Equal length Ri-1 Li-1 f f Ki Ri = Li-1fKi(Ri-1) Li = Ri-1 Chantilly Academy Crypto Lecture 2: Spring 07

  6. Diagram cut from FIPS standard Chantilly Academy Crypto Lecture 2: Spring 07

  7. Attacks on DES DES was broken in the late 90s with special purpose hardware built by the Electronic Frontier Foundation Chantilly Academy Crypto Lecture 2: Spring 07

  8. Current Block Cipher StandardAdvanced Encryption Standard (AES)

  9. 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 Chantilly Academy Crypto Lecture 2: Spring 07

  10. 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 Chantilly Academy Crypto Lecture 2: Spring 07

  11. High-level AES (all byte operations, 1 round shown) XOR with key S-box is an algebraic operation S-box Shift Rows Mix Columns Chantilly Academy Crypto Lecture 2: Spring 07

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