Block Ciphers After the DES

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Block CiphersAfter the DES

• CS 470
• Introduction to Applied Cryptography
• Instructor Ali Aydin Selcuk

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After the DES

• DES was designed for h/w slow in s/w.
• It was also suspect, due to the secret design
  process
• By late 80s, need for an independently
  developed, fast-in-s/w cipher was clear.

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FEAL(Shimizu Miyaguchi, 1987)

• Idea More complicated f functionbut fast in
  s/wwith fewer rounds hence higher speed.
• Feistel cipher, originally 4 rounds. (later
  increased to 6, 8, 16, 32)
• Broken repeatedly
• 4 rounds with 5 known plaintexts
• 8 rounds with 12 chosen plaintexts
• 16 rounds with 228 chosen plaintexts
• 32 rounds may be adequately secure.

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IDEA(Lai Massey, 1992)

• 64-bit block size, 128-bit key
• 8.5 rounds of
• ? bitwise XOR
• addition mod 216
• ? multpln mod 216 1
• decryption same as encryption, with inverse
  keys
• very secure, a bit slow (DES)

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RC5(Rivest, 1994)

• Extremely simple flexible
• Variable block size (w), key size (b), no. of
  rounds (r) specified as RC5-w/r/b.
• Encryption algorithm
• L1 L0 K0
• R1 R0 K1
• for i 2 to 2r1 do
• Li Ri-1
• Ri ((Li-1 ? Ri-1) ltltlt Ri-1) Ki
• For 64-bit block size (w32), 24 rounds (r12) is
  secure

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Blowfish(Schneier, 1994)

• 16-round Feistel cipher with key-dependent,
  large, variable s-boxes
• variable s-boxes are not fixed targets for
  analysis
• with large (8x32) s-boxes, chances of a weak
  combination is negligible
• Simple, very fast f function. (s-box, , ?)
• x (x1,x2,x3,x4), where each xi is 8-bit.
• f(x) ((S1(x1) S2(x2)) ? S3(x3)) S4(x4)
• Slow key schedule.

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Advanced Encryption Standard (AES)

• Successful public design process
• NISTs request for proposals (1997)
• 15 submissions (1998)
• 5 finalists (1999)
• Mars (IBM)
• RC6 (RSA)
• Twofish (Schneier et al.)
• Serpent (Anderson et al.)
• Rijndael (Daemen Rijmen)
• Winner Rijndael (2000)

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Rijndael

• An SP cipher with algebraically designed s-boxes
  (optimal against LC DC)
• 128, 192, or 256-bit block size128, 192, or
  256-bit key.
• 10-14 rounds of
• ByteSub
• ShiftRow
• MixColumn
• AddRoundKey
• Decryption is similar to encryption (by design)

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Rijndael Round Function

• Input is divided into 8-bit cells organized in 4
  rows.Eg. for 128-bit block size
• ByteSub Each cell goes through the s-box.
• ShiftRow Each of the 4 rows is shifted by a
  different amount (diffusion)
• MixColumn Every column, taken as a 4-entry
  vector over GF(28), is multiplied by a special
  4x4 matrix over GF(28).
• AddRoundKey All cells are XORed by 8-bit keys.

X1,1 X1,2 X1,3 X1,4
X2,1 X2,2 X2,3 X2,4
X3,1 X3,2 X3,3 X3,4
X4,1 X4,2 X4,3 X4,4