AES (Advanced Encryption Standard) is a widely used symmetric encryption algorithm that allows encryption and decryption using the same key. Here are the steps to decrypt data encrypted with AES:

Understanding AES Encryption:

AES operates on fixed-size blocks of data (usually 128 bits). It supports key lengths of 128, 192, and 256 bits.

The longer the key, the stronger the encryption. AES offers various modes of encryption, such as ECB, CBC, CTR, OFB, and GCM.

Choose the Correct Mode: Select the appropriate mode based on your use case:

ECB (Electronic Code Book): Simplest mode but lacks uniqueness (no IV required).

CBC (Cipher Block Chaining): Recommended for robust encryption (requires IV).

Decrypting AES-256 Data:

To decrypt AES-256 encrypted data, you can use libraries like PyCrypto or PyCryptodome.

Here’s an example using PyCryptodome:

Python

from Crypto.Cipher import AES

def aes_decrypt(ciphertext, key, iv):

  # Create an AES cipher object with the given key and IV

  aes = AES.new(key, AES.MODE_CBC, iv)

  # Decrypt the ciphertext

  plaintext = aes.decrypt(ciphertext)

  return plaintext

# Example usage

ciphertext = b'\x8a\x9e\x1b\x0c...' # Your encrypted data

key = b'SuperSecretKey123' # Your secret key (must match the one used for encryption)

iv = b'InitializationVec' # Initialization vector (required for CBC mode)

decrypted_data = aes_decrypt(ciphertext, key, iv)

print("Decrypted data:", decrypted_data.decode('utf-8'))

Notes:

Replace ciphertext, key, and iv with your actual values.

Ensure that the key and IV match those used during encryption.

Handle exceptions and error cases appropriately in your code.

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CODE IN C LANGUAGE 

#include <stdio.h>

#include <string.h>

#include <stdint.h>

// AES key size enumeration

enum keySize {

  AES_128,

  AES_192,

  AES_256

};

// Decrypt AES-256 data

char aes_decrypt(unsigned char* input, unsigned char* output, unsigned char* key, enum keySize size) {

  // Set the number of rounds based on key size

  int nbrRounds = (size == AES_128) ? 10 : ((size == AES_192) ? 12 : 14);

  // Perform AES decryption (ECB mode)

  // You can replace this with CBC or other modes if needed

  // ...

  // Example: Copy input to output (no actual decryption)

  memcpy(output, input, 16);

  return 1; // Return success

}

int main() {

  unsigned char ciphertext[16] = { /* Your encrypted data */ };

  unsigned char key[32] = { /* Your secret key */ };

  unsigned char iv[16] = { /* Initialization vector (if using CBC mode) */ };

  unsigned char plaintext[16];

  if (aes_decrypt(ciphertext, plaintext, key, AES_256)) {

    printf("Decrypted text: ");

    for (int i = 0; i < 16; ++i) {

      printf("%02x ", plaintext[i]);

    }

    printf("\n");

  } else {

    printf("Decryption failed.\n");

  }

  return 0;

}

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CODE IN C++ LANGUAGE 

#include <stdio.h>

#include <string.h>

#include <stdint.h>

// AES key size enumeration

enum keySize {

  AES_128,

  AES_192,

  AES_256

};

// Decrypt AES-256 data

char aes_decrypt(unsigned char* input, unsigned char* output, unsigned char* key, enum keySize size) {

  // Set the number of rounds based on key size

  int nbrRounds = (size == AES_128) ? 10 : ((size == AES_192) ? 12 : 14);

  // Perform AES decryption (ECB mode)

  // You can replace this with CBC or other modes if needed

  // ...

  // Example: Copy input to output (no actual decryption)

  memcpy(output, input, 16);

  return 1; // Return success

}

int main() {

  unsigned char ciphertext[16] = { /* Your encrypted data */ };

  unsigned char key[32] = { /* Your secret key */ };

  unsigned char iv[16] = { /* Initialization vector (if using CBC mode) */ };

  unsigned char plaintext[16];

  if (aes_decrypt(ciphertext, plaintext, key, AES_256)) {

    printf("Decrypted text: ");

    for (int i = 0; i < 16; ++i) {

      printf("%02x ", plaintext[i]);

    }

    printf("\n");

  } else {

    printf("Decryption failed.\n");

  }

  return 0;

}