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Secured Communications

AES

History

  • DES security was seen inadequate due the size of used key.
    • Brute force attacks with modern computers were too effective
  • NIST arranged open competition in 1997 for next symmetric encryption standard
    • 15 proposals were accepted in first round
      • submission were public and everyone was allowed to evaluate them
    • 5 were left for second round
    • Rijndael, Submission of Vincent Rijmen and Joan Daemen was selected November 2001

AES

  • Key can be 128,192 or 256 bits
  • Block size 128 bits
    • Rijndael allowed also 192 and 256 bit block

AES encryption

  • add roundkey to plaindata (XOR)
  • run round functions (10 rounds)
  • after 10 rounds get ciphertext as result

AES round

  • 128 bits entering the round is handled as 16 bytes and entered in the 4*4 matrix
    • matrix is filled columnwise
  1. substitution
    • Add resistance to to differential and linear cryptoanalysis
  2. shiftrow
    • Diffusion of bits over multiple rounds
  3. mix columns
    • Diffusion of bits over multiple rounds
  4. add roundkey (XOR to result)
  • each byte in matrix is substituted with another byte from S-box (16×16)
    • first 4 bits state row, last first bit state column
  • shiftrow
    • bytes are shifted row wise in matrix
    • first row is shifted 0 left (no shift), 2nd 1 left, 3rd 2 left and 4th 3 left
  • column transformation
    • the matrix is multiplied in GF(2^{8}) with matrix
      \begin{array}{cccc}
02 & 03 & 01 & 01\\
01 & 02 & 03 & 01\\
01 & 01 & 02 & 03\\
03 & 01 & 01 & 02\end{array}
  • XOR round key (in 4×4 matrix) to result

Roundkey

  • key divided in four columns W(0), W(1), W(2), W(3)
  • create forty more columns with formula W(i)=W (i-4) XOR W(i-1)
    • If i is multiple of 4 then W(i)=W (i-4) XOR T(W(i-1))
    • T is a transformation function where substitution and shifting is used
  • Round key for j:th round is W(4*j), W(4*j+1),W(4*j+2),W(4*j+3)
Last modified: 2013/07/01 14:42