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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 l.jpg

Advanced Encryption Standard

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

Aes background l.jpg
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)

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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

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Evaluation Criteria for AES Proposals

  • Security

  • Computational Efficiency

  • Memory Requirements

  • Hardware and Software Suitability

  • Flexibility

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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.

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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()

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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.

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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.

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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.

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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.

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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.

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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).