Cryptography: THE OVERALL STRUCTURE OF AES

• The number of rounds shown in Figure, 10, is for the case when the encryption key is 128 bit long. (the number of rounds is 12 when the key is 192 bits, and 14 when the key is 256.)
• Before any round-based processing for encryption can begin, 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: 1) Substitute bytes, 2) Shift rows, 3) Mix columns, and 4) Add round key. The last step consists of XORing the output of the previous three steps with four words from the key schedule.
• For decryption, each round consists of the following four steps: 

1) Inverse shift rows,
2) Inverse substitute bytes,
3) Add round key, and
4) Inverse mix columns.
The third step consists of XORing the output of the previous two steps with four words from the key schedule. Note the differences between the order in which substitution and shifting operations are carried out in a decryption round vis-a-vis the order in which similar operations are carried out in an encryption round.

• 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.