GMAC openssl example

gmac.c

#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <openssl/evp.h>

int main()
{
    int ret = 0, unused = 0;

    uint8_t key[] = {
                        0x77, 0xbe, 0x63, 0x70, 0x89, 0x71, 0xc4, 0xe2,
                        0x40, 0xd1, 0xcb, 0x79, 0xe8, 0xd7, 0x7f, 0xeb
                    };
    uint8_t iv[] = { 0xe0, 0xe0, 0x0f, 0x19, 0xfe, 0xd7, 0xba, 0x01,
                     0x36, 0xa7, 0x97, 0xf3 };
    uint8_t aad[] = { 0x7a, 0x43, 0xec, 0x1d, 0x9c, 0x0a, 0x5a, 0x78,
                      0xa0, 0xb1, 0x65, 0x33, 0xa6, 0x21, 0x3c, 0xab };

    uint8_t tag[16] = {0};
    uint8_t exp[] = { 0x20, 0x9f, 0xcc, 0x8d, 0x36, 0x75, 0xed, 0x93,
                      0x8e, 0x9c, 0x71, 0x66, 0x70, 0x9d, 0xd9, 0x46 };

    EVP_CIPHER_CTX *ctx = NULL;

    ctx = EVP_CIPHER_CTX_new();
    if (ctx == NULL) {
        return -1;
    }

    /* EVP GCM but no IV and KEY */
    ret = EVP_EncryptInit_ex(ctx, EVP_aes_128_gcm(), NULL, NULL, NULL);
    if (ret != 1) {
        return -1;
    }

    /* Set IV Length */
    ret = EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, sizeof(iv), NULL);
    if (ret != 1) {
        return -1;
    }

    /* Initialize ctx with key and iv */
    ret = EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv);
    if (ret != 1) {
        return -1;
    }

    /* No output is required .. just calculate the MAC. */
    ret = EVP_EncryptUpdate(ctx, NULL, &unused, aad, sizeof(aad));
    if (ret != 1) {
        return -1;
    }

    /* Finalize the authentication and get the tag */
    ret = EVP_EncryptFinal_ex(ctx, NULL, &unused);
    if (ret != 1) {
        return -1;
    }

    /* Get the output tag */
    ret = EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, sizeof(tag), tag);
    if (ret != 1) {
        return -1;
    }

    printf("tag: ");
    for (int i = 0; i < sizeof(tag); i ++) {
        printf("%02x", tag[i]);
    }
    printf("\n");

    ret = memcmp(tag, exp, sizeof(tag));
    if (ret == 0) {
        printf("Tags match.. GMAC operation ok\n");
    } else {
        printf("Tags dont match.. GMAC operation failure\n");
    }

    return 0;
}