How the decryption process of AES is the reverse process of Encryption process of AES? Mean Reverse in what sense mean final round would be conducted first or steps in each round would be conducted first? Please Guide me...
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$\begingroup$ reverse in a sense that every step done in encryption needs to be reversed. For Sbox, there is an Inverse Sbox. For MDS, there is inverse of MDS such the multiplication of MDS and invMDS is an identity Matrix. Since inverse of XOR is xor, so round keys are xored in decryption but in reverse order meaning, round 10 key is xored first, then round 9 and so on... $\endgroup$– cryptCommented May 1, 2018 at 17:16
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1$\begingroup$ @RI8S That is basically an answer rather a comment $\endgroup$– Ella RoseCommented May 1, 2018 at 20:14
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$\begingroup$ @EllaRose posted an answer. $\endgroup$– cryptCommented May 2, 2018 at 8:47
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3 Answers
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Reverse in a sense that every step done in encryption needs to be reversed. For Sbox, there is an Inverse Sbox. For MDS, there is an inverse of MDS(invMDS) such the multiplication of MDS and invMDS is an identity Matrix. Since inverse of XOR is xor, so round keys are xored in decryption but in reverse order meaning, round 10 key is xored first, then round 9 and so on.
If you write down all the encrypting steps done between Plaintext and Ciphertext from Top to Bottom, then while decryption, you need to inverse of each step from Bottom to Top (moving from ciphertext to plaintext)
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In decryption mode, the operations are in reverse order compared to their order in encryption mode. Thus it starts with an initial round, followed by 9 iterations of an inverse normal round and ends with an AddRoundKey. An inverse normal round consists of the following operations in this order: AddRoundKey, InvMixColumns, InvShiftRows, and InvSubBytes. An initial round is an inverse normal round without the InvMixColumns.
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Linke in encryption, you first run the rounds and then the last round. Thus, you don't run the last round first and then the N-1 rounds.
