The Advanced Encryption Standard (AES) is a symmetric block cipher chosen by the U.S. government to replace the Data Encryption Standard (DES) in 2001. AES supports 128-bit blocks and key lengths of 128, 192, and 256 bits. It performs encryption and decryption through a series of transformations including substitution, shifting, mixing, and adding round keys derived from the original key. The number of transformation rounds varies based on the key length, from 10 rounds for 128-bit keys up to 14 rounds for 256-bit keys. AES provides improved security over DES with higher computational efficiency.

1. Advanced Encryption Standard
Reference: Stallings, Data and
Computer Communications, 7th
Edition,Pearson/P-H, 2004

2. 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)

3. 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

4. Evaluation Criteria for AES
Proposals
• Security
• Computational Efficiency
• Memory Requirements
• Hardware and Software Suitability
• Flexibility

5. The State and Key Schedule
• 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.

6. 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()

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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).