AES Encryption and Decryption

In this program, we have implemented 128 bit key AES. This project was done as part of the Kludge Project regarding Image Steganography using K means clustering.

AES Encryption:

It mainly consists of two parts: Key expansion and Encryption

Key Expansion:

For 128 bit key AES encryption, there are 11 rounds. For each round, one key is needed. All the keys need to be of 16 bytes. From the 16 byte key given by the user, we generate 10 more keys to use in the following rounds. The procedure for generating the keys is as follows:

1. Create a 176 byte array (here, expandedKey) inorder to store the keys.
2. The first 16 bytes will be used to store the original input key.
3. The last four bytes of the current key will be passed through the g function (explained below).
4. The output from the g function will be XORed with the first four bytes of the current key to generate the first four bytes of the next key The first four bytes of the next key will be XORed with the 5th - 8th bytes of the current key to generate the 5th - 8th bytes of the next key The 5th - 8th bytes of the next key will be XORed with the 9th - 12th bytes of the current key to generate the 9th - 12th bytes of the next key The 9th - 12th bytes of the next key will be XORed with the last four bytes of the current key to generate the last four bytes of the next key
5. This will be continued until the keys are generated

G function:

1. The 4 bytes are shifted one to the left. eg: {B0, B1, B2, B3} becomes {B1, B2, B3, B0}
2. These bytes are substituted using the forward substitution box for aes encryption.
3. The first byte is then XORed with the round constant.

(image from https://www.brainkart.com/article/AES-Key-Expansion_8410/)

Encryption:

We initially pad the message so that the length is a multiple of 16. Round 0:

1. The padded plaintext is then XORed with the first key. This (now called State Matrix) is now passed on to the next step.

Round 1 to Round 9:

2. Sub Bytes: The bytes in state matrix are replaced using the forward S-box

3. Shift Rows: The row elements are shifted among each other. The first row is left unchanged, the second row is shifted to the left by 1, the third row is shifted to the left by 2, the fourth row is shifted to the left by 3.

4. Mix columns: The state matrix is multiplied by a constant matrix to form a new matrix

(image from https://www.youtube.com/watch?v=Z_7aOkS8tOA&t=578s)

5. Add round key: The state matrix is XORed with the key for that round.
6. These 4 steps are done for each round

Round 10: In the last round, we dont perform mix columns. We perform sub bytes, shift rows and add round key. This gives us the cipher text.

AES Decryption:

Decryption is the reverse process of encryption. Thus, we perform the steps to undo the changes made to the plaintext.

Each round of decryption has the following rounds:(round 9 - 1)

1. Add round key: XOR the key for that particular round with the state matrix
2. Inverse MixColumns: Its same as the encryption MixColumns, but the constant matrix multiplied is different

(image from https://www.youtube.com/watch?v=SDrzMyqi2Sc)

3. ShiftRows: The row elements are shifted among each other. The first row is left unchanged, the second row is shifted to the RIGHT by 1, the third row is shifted to the RIGHT by 2, the fourth row is shifted to the RIGHT by 3.
4. Inverse SubByte: The elements in the matrix are substituted using the inverse S-box.

The initial round (round 10) of decryption doesn't have the inverse mix columns step The final round (round 0) consists of just XORing the key (K0) again. Note: The keys are also used in the reverse way. K10 is used in the initial round and K0 is used in the final round.

How to Execute:

Make sure all the files for encryption/decryption are in the same directory Run the main_encrypt/main_decrypt to encrypt or decrypt

NOTE: Make sure the key is of 16 characters. For decryption, make sure the encrypted text is in hex and the length is a multiple of 16

References:

1. https://www.geeksforgeeks.org/advanced-encryption-standard-aes/
2. https://www.youtube.com/watch?v=Z_7aOkS8tOA

Key Expansion:

3. https://www.brainkart.com/article/AES-Key-Expansion_8410/
4. https://www.youtube.com/watch?v=pF0Pj8rGzKI

Testing:

5. https://www.cryptool.org/en/cto/aes-step-by-step