AES(Advanced encryption standard), Lecture notes of Cybercrime, Cybersecurity and Data Privacy

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Advanced

Encryption

Standard

(AES)

1 Slides taken from chapter 7 of “Cryptography and Network Security” by Behrouz Forouzan

Objectives ❏ To review a short history of AES ❏ To define the basic structure of AES ❏ To define the transformations used by AES ❏ To define the key expansion process

Criteria

The criteria defined by NIST for selecting AES fall into three areas:

1. Security
2. Cost
3. Implementation.

Rounds

AES is a non-Feistel cipher that encrypts and decrypts a data block of 128 bits. It uses 10 , 12 , or 14 rounds. The key size, which can be 128 , 192 , or 256 bits, depends on the number of rounds But the round keys are always 128 bits.

Data Units

Data units used in AES

Block-to-state and state-to-block transformation

Structure of Each Round

TRANSFORMATIONS

To provide security, AES uses four types of transformations: substitution, permutation, mixing, and key-adding. Substitution Permutation Mixing Key Adding Topics discussed in this section:

SubBytes transformation

SubBytes

Permutation

Another transformation found in a round is shifting, which permutes the bytes. ShiftRows In the encryption, the transformation is called ShiftRows.

Figure shows how a state is transformed using ShiftRows transformation. The figure also shows that InvShiftRows transformation creates the original state.

Permutation

Mixing

 The multiplication operation is defined as:  Multiplication by 1 means no change,  Multiplication by 2 means shifting to the left, a conditional XOR with 0 x 1 B ( 0001 1011 ) will be performed if the shifted value is larger than 0 xFF or in other words leftmost bit of original value is 1.  Multiplication by 3 means shifting to the left, then performing conditional XOR with 0 x 1 B if required and then performing XOR with the initial unshifted value.