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Block Ciphers

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:. However.

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Block Ciphers

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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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