__Advanced Encryption Standard__

AES is a block cipher with a block length of 128 bits. AES allows for three different key lengths: 128, 192, or 256 bits. Encryption consists of 10 rounds of processing for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys. Except for the last round in each case, all other rounds are identical. Each round of processing consists of one single-byte based substitution step, followed? by what is known as a row-wise permutation step, followed by a ?column-based? substitution step, followed by the addition of the round key.

128-bit block as consisting of a 4?4 matrix( referred to as the state array) arranged as follows:

bo |
b4 |
b8 |
b12 |

b1 |
b5 |
b9 |
b13 |

b2 |
b6 |
b10 |
b14 |

b3 |
b7 |
b11 |
b15 |

AES also has the notion of a word. A word consists of four bytes, that is 32 bits. Therefore, each column of the state array is a word, as is each row. Each round of processing works on the input state array and produces an output state array. The output state array produced by the last round is rearranged into a 128-bit output block. The same steps are used in encryption and decryption, the order in which the steps are carried out is different.

**The Encryption Key and Its Expansion**

Assuming a 128-bit key, the key is also arranged in the form of a matrix of 4 ? 4 bytes. As with the input block, the first word from the key fills the first column of the matrix, and so on. ?The four column words of the key matrix are expanded into a schedule of 44 words. Each round consumes four words from the key schedule.

The figure below depicts the arrangement of the encryption key in the form of 4-byte words and the expansion of the key into a key schedule consisting of 44? 4-byte words.

The? input state array is XORed with the first four words of the key schedule. The same thing happens during decryption? except that now we XOR the ciphertext state array with the last four words of the key schedule.

For encryption, each round consists of the following four steps:

- Substitute bytes
- Shift rows
- Mix columns
- Add.

For decryption, each round consists of the following four steps:

- Inverse shift rows
- Inverse substitute bytes
- Add round key
- Inverse mix columns.

The last round for encryption does not involve the ?Mix columns? step. The last round for decryption does not involve the ?Inverse mix columns? step.

**Four steps in each round of processing**

**STEP 1:** It is called SubBytes for byte-by-byte substitution during the forward process. The corresponding substitution step used during decryption is called InvSubBytes.

This step consists of using a 16 ? 16 lookup table to find a replacement byte for a given byte in the input state array. The entries in the lookup table are created by using the notions of multiplicative inverses in GF(28) and bit scrambling to destroy the bit-level correlations inside each byte.

**STEP 2:** It is called ShiftRows for shifting the rows of the state array during the forward process. The corresponding transformation during decryption is denoted InvShiftRows for Inverse Shift-Row Transformation. The goal of this transformation is to scramble the byte order inside? each 128-bit block.

**STEP 3:** It is called MixColumns for mixing up of the bytes in each column separately during the forward process. The corresponding transformation during decryption is denoted InvMixColumns and stands for inverse mix column transformation. The goal here is to further scramble up the 128-bit input block.

The shift rows step along with the mix column step causes each bit of the cipher text to depend on every bit of the plaintext after 10 rounds of processing.

**STEP 4:**? AddRoundKey for adding the round key to the output of the previous step during the forward process. The corresponding step during decryption is denoted? InvAddRound-Key for inverse add round key transformation.

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