3. 3
AES - introdution
Ø In 2000, the NIST formally adopted the AES
encryption algorithm and published it as a federal
standard under the designation FIPS-197.
Ø It was chosen because of its security,
performance, efficiency, implement ability, and low
memory requirements.
• Rijndael was selected as the AES
– Designed by Vincent Rijmen and Joan Daemen
in Belgium
4. 4
AES - introdution
Ø The AES Cipher - Rijndael
• An iterative
– processes data as block of 4 columns of 4 bytes
(128 bits)
– operates on entire data block in every round
• Rijndael design:
– simplicity
– has 128/192/256 bit keys, 128 bits data
– resistant against known attacks
– speed and code compactness on many CPUs
• The MixColumn function in the AES algorithm is
an important property of the cipher
5. 5
AES - introdution
Ø The MixColumn function in the AES algorithm
is an important property of the cipher
ü It provides strength against differential and linear
attacks due to the complexity of its mathematical
operations.
ü Require computational resources in software
implementation.
ü Replacing the MixColumn function, the speed
performance of the AES algorithm will be
improved.
ü Propose for a modified AES algorithm using
multiple S-Boxes.
12. AES -128-bit values
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• Data block viewed as 4-by-4 table of bytes
• Represented as 4 by 4 matrix of 8-bit
bytes.
• Key is expanded to array of 32 bits words
1 byte
17. AES- SubBytes: Byte Substitution
• A simple substitution of each byte
– provide a confusion
• Uses one S-box of 16x16 bytes containing a permutation of all 256
8-bit values
• Each byte of state is replaced by byte indexed by row (left 4-bits) &
column (right 4-bits)
– eg. byte {95} is replaced by byte in row 9 column 5
– which has value {2A}
• S-box constructed using defined transformation of values in Galois
Field- GF(28)
23. AES - ShiftRows
• Shifting, which permutes the bytes.
• A circular byte shift in each each
– 1st row is unchanged
– 2nd row does 1 byte circular shift to left
– 3rd row does 2 byte circular shift to left
– 4th row does 3 byte circular shift to left
• In the encryption, the transformation is
called ShiftRows
• In the decryption, the transformation is
called InvShiftRows and the shifting is to
the right
26. AES - MixColumns
• ShiftRows and MixColumns provide diffusion to
the cipher
• Each column is processed separately
• Each byte is replaced by a value dependent on
all 4 bytes in the column
• Effectively a matrix multiplication in GF(28) using
prime poly m(x) =x8+x4+x3+x+1
27. AES -MixClumns Scheme
The MixColumns transformation operates at the column level; it
transforms each column of the state to a new column.
29. AES - AddRoundKey
• XOR state with 128-bits of the round key
• AddRoundKey proceeds one column at a
time.
– adds a round key word with each state
column matrix
– the operation is matrix addition
• Inverse for decryption identical
– since XOR own inverse, with reversed keys
• Designed to be as simple as possible
34. AES -Key Expansion submodule
• RotWord performs a one byte circular left shift on a word
For example:
RotWord[b0,b1,b2,b3] = [b1,b2,b3,b0]
• SubWord performs a byte substitution on each byte of
input word using the S-box
• SubWord(RotWord(temp)) is XORed with RCon[j] – the
round constant
35. AES Security
• AES was designed after DES.
• Most of the known attacks on DES were already tested
on AES.
• Brute-Force Attack
– AES is definitely more secure than DES due to the larger-size
key.
• Statistical Attacks
– Numerous tests have failed to do statistical analysis of the
ciphertext
• Differential and Linear Attacks
– There are no differential and linear attacks on AES as yet.
36. AES ALGORITHM USING MULTIPLE S-BOXES
vPROPOSED MODIFIED 128-AES ALGORITHM
USING MULTIPLE S-BOXES
Ø The MixColumns function is perceive to be
requiring more computational resources in
software implementation as compared to the
other functions
Ø Propose for a modified version of the 128-AES
algorithm using two substitution boxes
ü The first S-Box is the Rijndael S-Box
ü The second S-Box is constructed using XOR
operation and affine transformation
38. AES - CONSTRUCTION OF THE NEW S-BOX
vThe second S-Box is derived from the
original S- Box as designed in the AES
ØExclusive OR Operation
§ each cell in the AES-Rijndael will be
XORed with 7F
AES-Rijndael[x,y] XOR (7F)
§ The Key[i] shall be any hexadecimal value
between 00 to FF (Table 1. AES-
2SboxXOR7F)
40. AES - CONSTRUCTION OF THE NEW S-BOX
vThe second S-Box is derived from the
original S- Box as designed in the AES
ØExclusive OR Operation
ØAffine Transform Operation
ü After creating the initial values of AES-
2SboxXOR
ü Scramble the bits in each byte value, we next
apply the following transformation to each bit bi
as stored in the initial AES-2SboxXOR7F:
41. AES - CONSTRUCTION OF THE NEW S-BOX
ØAffine Transform Operation
ü For the inverse AES-2SboxXOR, the following
transformation to each bit was used for bit
scrambling:
46. CONCLUSION
üModified AES algorithm using multiple
S-boxes.
üWe observed that the speed performance
greatly increased in the modified AES
algorithm using multiple S-Boxes, while
the security side has slightly weakened.
üEasily implemented using cheap
processors and a minimum amount of
memory.
üVery efficient
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47. Referents
• [1] National Institute of Standards and
Technology, Advanced Encryption Standard,
FIPS 197 (2011).
• [2] Paper “Modified AES Algorithm Using
Multiple S-Boxes”
• [3] Wiki & slide & Internet.
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