Slide an toàn và hệ thống bảo mật thông tin chapter 5 cryptography symmetric modern aes

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SYMMETRIC CIPHERS
ADVANCED ENCRYPTION STANDARD

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Contents

AES Origins

2)

AES Structure

3)

AES Key Expansion

4)

An AES Example

5)

AES Decryption

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1)

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1. AES Origins

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AES Origins
 Clear a replacement for DES was needed have theoretical attacks that can

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break it have demonstrated exhaustive key search attacks

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 Can use Triple-DES – but slow, has small blocks

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The AES Cipher - Rijndael

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 Designed by Rijmen-Daemen in Belgium

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 has 128/192/256 bit keys, 128 bit data

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 Designed to have:

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 resistance against known attacks

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 design simplicity

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 speed and code compactness on many CPUs

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AES Encryption Process

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2. AES Structure

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AES Structure

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 Plaintext block size: 128 bits.

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 Key length:16, 24, or 32 bytes (128, 192, or 256 bits).

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depending on the key length

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 The algorithm is referred to as AES-128, AES-192, or AES-256,

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Detailed Structure

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 128-bit block as consisting of a 4 × 4 matrix of bytes, arranged as follows:

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 The 4 × 4 matrix of bytes shown above is referred to as the state array in

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Detailed Structure

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Detailed Structure

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 The key is depicted as a square matrix of bytes

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 This key is then expanded into an array of key schedule words

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 Each word is four bytes, and the total key schedule is 44 words for the

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128-bit key

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Detailed Structure

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Detailed Structure
Four different stages are used, one of permutation and three of substitution:

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 Substitute bytes: Uses an S-box to perform a byte-by-byte substitution of the

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 Shift Rows: A simple permutation

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 Mix Columns: A substitution that makes use of arithmetic over

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 Add Round Key: A simple bitwise XOR of the current block with a portion

of the expanded key

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Detailed Structure
 For both encryption and decryption, the cipher begins with an Add Round

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Key stage, followed by nine rounds that each includes all four stages,

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followed by a tenth round of three stages.

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 Only the Add Round Key stage makes use of the key

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a. Substitute Bytes Transformation

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 Each individual byte of Stateis mapped into a new byte in S-Box

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bits are used as a column value.

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 The leftmost 4 bits of the byte are used as a row value and the rightmost 4

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 These row and column values serve as indexes into the S-box to select a

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unique 8-bit output value

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Substitute Bytes Transformation (cont.)

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S-box

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Substitute Bytes Transformation
 For example, the hexadecimal value {95} references row 9, column 5 of

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the S-box, which contains the value .Accordingly, the value is mapped

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into the value.

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 Here is an example of the SubBytes transformation:

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b. ShiftRows Transformation

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 The forward shift row transformation, called ShiftRows.

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 The first row of Stateis not altered. For the second row, a 1-byte circular

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left shift is performed. For the third row, a 2-byte circular left shift is

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performed. For the fourth row, a 3-byte circular left shift is performed

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ShiftRows Transformation

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c. MixColumns Transformation
 Each byte of a column is mapped into a new value that is a function of all

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four bytes in that column. The transformation can be defined by the

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following matrix multiplication on State

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MixColumns Transformation

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 the individual additions and multiplications are performed in GF(28).

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d. Add Round Key Transformation

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 XOR state with 128-bits of the round key

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