InvShiftRows

InvShiftRows is a crucial operation within the Advanced Encryption Standard (AES) symmetric-key encryption algorithm. It serves as the inverse transformation of the ShiftRows operation used in the encryption process. While ShiftRows cyclically shifts the rows of the state matrix to the right, InvShiftRows performs a corresponding cyclic shift to the left. This step is essential for ensuring diffusion, meaning that changes in one part of the plaintext affect multiple parts of the ciphertext.

In AES, the state is represented as a 4x4 matrix of bytes. The InvShiftRows operation is applied to each row of this matrix individually. The first row (Row 0) remains unchanged. The second row (Row 1) is shifted one position to the left. The third row (Row 2) is shifted two positions to the left. Finally, the fourth row (Row 3) is shifted three positions to the left. These leftward shifts are circular, meaning that bytes shifted off one end re-enter the row at the other end.

The purpose of InvShiftRows is to reverse the effect of ShiftRows during the decryption process. By shifting the rows in the opposite direction, it helps to restore the original arrangement of bytes within the state matrix, allowing the subsequent operations of InvSubBytes and InvMixColumns to correctly decrypt the ciphertext back into plaintext. This step, along with the other transformations in AES, contributes to the algorithm's strong security against various cryptanalytic attacks.