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Advanced Encryption Standard. Reference: Stallings, Data and Computer Communications, 7th Edition ,Pearson/P-H, 2004. AES Background. 1997 --National Institute of Standards and Technology (NIST) issues a call for proposals.

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advanced encryption standard

Advanced Encryption Standard

Reference: Stallings, Data and Computer Communications, 7th Edition,Pearson/P-H, 2004

aes background
AES Background
  • 1997 --National Institute of Standards and Technology (NIST) issues a call for proposals.
  • 2001--AES issues as a federal information processing standards (FIPS 197)
aes requirements
AES Requirements
  • Security Strength Equal to or Better than 3DES
  • Significantly More Efficient than 3DES
  • Symmetric Block Cipher
  • Block Length = 128 bits
  • Support for Key Lengths of 128, 192, and 256 bits
evaluation criteria for aes proposals
Evaluation Criteria for AES Proposals
  • Security
  • Computational Efficiency
  • Memory Requirements
  • Hardware and Software Suitability
  • Flexibility
the state and key schedule
The State and KeySchedule
  • Fig. 21.2 shows the AES algorithm structure.
  • Input is a 128 bit block (16 bytes) that is placed in the state array (Fig. 21.3)
  • The key is entered in a block and divided into key schedule words of 4 bytes/word.
  • The key schedule is an expansion of the key—eg, a 128 bit key is expanded into 44 key schedule words.
  • A square matrix of bytes is used by the standard to describe the state.
rounds and transformation stages
Rounds and Transformation Stages
  • The encryption process executes a round function, Nr times, with the number of rounds (Nr) being dependent on key size.
  • The round function consists of four transformation stages.
    • SubBytes()
    • ShiftRows()
    • MixColumns()
    • AddRoundKey()
rounds and transformation stages p 2
Rounds and Transformation Stages (p.2)
  • The cipher begins with an AddRoundKey().
  • All rounds then execute each of the transformations except the last round.
  • The MixColumns( ) transformation is not executed in the final round.
  • For a 128 bit key, there are 10 rounds.
  • 12 and 14 rounds are used with keys of 192 and 256.
subbytes transformation
SubBytes ( ) Transformation
  • The substitute transformation is an S-Box process, that is independent of the key.
  • Each of the bytes of the State is replaced by a different byte, according to a table.
  • The table is fixed and derived from two transformations defined in the standard.
  • The table is an 8 x 8 array, indexed with the State byte.
shiftrows transformation
ShiftRows( ) Transformation
  • The ShiftRows() transformation is a permutation that is performed row by row on the State array, independently of the key.
  • The first row is not shifted.
  • The 2nd row is circularly shifted left 1 byte.
  • The 3rd row is circularly shifted left 2 bytes.
  • The 4th row is circularly shifted left 3 bytes.
mixcolumns transformation
MixColumns() Transformation
  • The MixColumns( ) transformation manipulates each column of the state array.
  • The process can be described as a matrix multiplication of a polynomial and the state array.
  • This process does not depend on the key.
addroundkey transformation
AddRoundKey( ) Transformation
  • The AddRoundKey( ) transformation uses the key schedule word.
  • The process is a bitwise XOR of the columns of the state array, with the key schedule word.
aes decryption
AES Decryption
  • AES decryption is accomplished using inverses of the transformations, in the appropriate order.
  • The AddRoundKey( ) is its own inverse when (since A  B  B = A).