AES_set_encrypt_key: OpenSSL Function for Setting AES Encryption Keys

AES_set_encrypt_key is a function within the OpenSSL library used to establish the encryption key for the Advanced Encryption Standard (AES) algorithm. The function takes the key, its length, and a pointer to the AES_KEY structure as input parameters.

The AES_KEY structure stores the encryption key and related information regarding the AES algorithm. Its definition is as follows:

typedef struct aes_key_st {
    unsigned int rd_key[4 * (AES_MAXNR + 1)];
    int rounds;
} AES_KEY;

The rd_key array within the AES_KEY structure holds the round keys generated during the key expansion process. AES_MAXNR is a constant defining the maximum round count for the AES algorithm.

The AES_set_encrypt_key function utilizes the provided key and length to generate the round keys. Subsequently, it stores these round keys and the number of rounds within the AES_KEY structure.

Here's an example showcasing the use of the AES_set_encrypt_key function:

#include <openssl/aes.h>

int main() {
    unsigned char key[16] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
                             0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F};
    AES_KEY aes_key;
    
    AES_set_encrypt_key(key, 128, &aes_key);
    
    // Utilize the encryption key for AES encryption
    
    return 0;
}

In this example, a 128-bit encryption key is set using the AES_set_encrypt_key function. The key is subsequently employed for AES encryption.