Cryptography Lecture 2: Chantilly Academy

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

3
However

• Its 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
• Todays 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?

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Previous Block Cipher Standard Data Encryption
Standard (DES)
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One round of DES Feistel Cipher
Equal length
Ri-1
Li-1
f
f
Ki
Ri Li-1?fKi(Ri-1)
Li Ri-1
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Diagram cut from FIPS standard
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Attacks on DES

• DES was broken in the late 90s with special
  purpose hardware built by the Electronic Frontier
  Foundation

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Current Block Cipher StandardAdvanced Encryption
Standard (AES)
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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

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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

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High-level AES (all byte operations, 1 round
shown)
XOR with key
S-box is an algebraic operation
S-box
Shift Rows
Mix Columns