aesgcm.c

/*
 * Copyright 2012-2024 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the Apache License 2.0 (the "License").  You may not use
 * this file except in compliance with the License.  You can obtain a copy
 * in the file LICENSE in the source distribution or at
 * https://www.openssl.org/source/license.html
 */

/*
 * Simple AES GCM authenticated encryption with additional data (AEAD)
 * demonstration program.
 */

#include <stdio.h>
#include <openssl/err.h>
#include <openssl/bio.h>
#include <openssl/evp.h>
#include <openssl/core_names.h>

/* AES-GCM test data obtained from NIST public test vectors */

/* AES key */
static const unsigned char gcm_key[] = {
    0xee, 0xbc, 0x1f, 0x57, 0x48, 0x7f, 0x51, 0x92, 0x1c, 0x04, 0x65, 0x66,
    0x5f, 0x8a, 0xe6, 0xd1, 0x65, 0x8b, 0xb2, 0x6d, 0xe6, 0xf8, 0xa0, 0x69,
    0xa3, 0x52, 0x02, 0x93, 0xa5, 0x72, 0x07, 0x8f
};

/* Unique initialisation vector */
static const unsigned char gcm_iv[] = {
    0x99, 0xaa, 0x3e, 0x68, 0xed, 0x81, 0x73, 0xa0, 0xee, 0xd0, 0x66, 0x84
};

/* Example plaintext to encrypt */
static const unsigned char gcm_pt[] = {
    0xf5, 0x6e, 0x87, 0x05, 0x5b, 0xc3, 0x2d, 0x0e, 0xeb, 0x31, 0xb2, 0xea,
    0xcc, 0x2b, 0xf2, 0xa5
};

/*
 * Example of Additional Authenticated Data (AAD), i.e. unencrypted data
 * which can be authenticated using the generated Tag value.
 */
static const unsigned char gcm_aad[] = {
    0x4d, 0x23, 0xc3, 0xce, 0xc3, 0x34, 0xb4, 0x9b, 0xdb, 0x37, 0x0c, 0x43,
    0x7f, 0xec, 0x78, 0xde
};

/* Expected ciphertext value */
static const unsigned char gcm_ct[] = {
    0xf7, 0x26, 0x44, 0x13, 0xa8, 0x4c, 0x0e, 0x7c, 0xd5, 0x36, 0x86, 0x7e,
    0xb9, 0xf2, 0x17, 0x36
};

/* Expected AEAD Tag value */
static const unsigned char gcm_tag[] = {
    0x67, 0xba, 0x05, 0x10, 0x26, 0x2a, 0xe4, 0x87, 0xd7, 0x37, 0xee, 0x62,
    0x98, 0xf7, 0x7e, 0x0c
};

/*
 * A library context and property query can be used to select & filter
 * algorithm implementations. If they are NULL then the default library
 * context and properties are used.
 */
static OSSL_LIB_CTX *libctx = NULL;
static const char *propq = NULL;

static int aes_gcm_encrypt(void)
{
    int ret = 0;
    EVP_CIPHER_CTX *ctx;
    EVP_CIPHER *cipher = NULL;
    int outlen, tmplen;
    size_t gcm_ivlen = sizeof(gcm_iv);
    unsigned char outbuf[1024];
    unsigned char outtag[16];
    OSSL_PARAM params[2] = {
        OSSL_PARAM_END, OSSL_PARAM_END
    };

    printf("AES GCM Encrypt:\n");
    printf("Plaintext:\n");
    BIO_dump_fp(stdout, gcm_pt, sizeof(gcm_pt));

    /* Create a context for the encrypt operation */
    if ((ctx = EVP_CIPHER_CTX_new()) == NULL)
        goto err;

    /* Fetch the cipher implementation */
    if ((cipher = EVP_CIPHER_fetch(libctx, "AES-256-GCM", propq)) == NULL)
        goto err;

    /* Set IV length if default 96 bits is not appropriate */
    params[0] = OSSL_PARAM_construct_size_t(OSSL_CIPHER_PARAM_AEAD_IVLEN,
                                            &gcm_ivlen);

    /*
     * Initialise an encrypt operation with the cipher/mode, key, IV and
     * IV length parameter.
     * For demonstration purposes the IV is being set here. In a compliant
     * application the IV would be generated internally so the iv passed in
     * would be NULL.
     */
    if (!EVP_EncryptInit_ex2(ctx, cipher, gcm_key, gcm_iv, params))
        goto err;

    /* Zero or more calls to specify any AAD */
    if (!EVP_EncryptUpdate(ctx, NULL, &outlen, gcm_aad, sizeof(gcm_aad)))
        goto err;

    /* Encrypt plaintext */
    if (!EVP_EncryptUpdate(ctx, outbuf, &outlen, gcm_pt, sizeof(gcm_pt)))
        goto err;

    /* Output encrypted block */
    printf("Ciphertext:\n");
    BIO_dump_fp(stdout, outbuf, outlen);

    /* Finalise: note get no output for GCM */
    if (!EVP_EncryptFinal_ex(ctx, outbuf, &tmplen))
        goto err;

    /* Get tag */
    params[0] = OSSL_PARAM_construct_octet_string(OSSL_CIPHER_PARAM_AEAD_TAG,
                                                  outtag, 16);

    if (!EVP_CIPHER_CTX_get_params(ctx, params))
        goto err;

    /* Output tag */
    printf("Tag:\n");
    BIO_dump_fp(stdout, outtag, 16);

    ret = 1;
err:
    if (!ret)
        ERR_print_errors_fp(stderr);

    EVP_CIPHER_free(cipher);
    EVP_CIPHER_CTX_free(ctx);

    return ret;
}

static int aes_gcm_decrypt(void)
{
    int ret = 0;
    EVP_CIPHER_CTX *ctx;
    EVP_CIPHER *cipher = NULL;
    int outlen, rv;
    size_t gcm_ivlen = sizeof(gcm_iv);
    unsigned char outbuf[1024];
    OSSL_PARAM params[2] = {
        OSSL_PARAM_END, OSSL_PARAM_END
    };

    printf("AES GCM Decrypt:\n");
    printf("Ciphertext:\n");
    BIO_dump_fp(stdout, gcm_ct, sizeof(gcm_ct));

    if ((ctx = EVP_CIPHER_CTX_new()) == NULL)
        goto err;

    /* Fetch the cipher implementation */
    if ((cipher = EVP_CIPHER_fetch(libctx, "AES-256-GCM", propq)) == NULL)
        goto err;

    /* Set IV length if default 96 bits is not appropriate */
    params[0] = OSSL_PARAM_construct_size_t(OSSL_CIPHER_PARAM_AEAD_IVLEN,
                                            &gcm_ivlen);

    /*
     * Initialise an encrypt operation with the cipher/mode, key, IV and
     * IV length parameter.
     */
    if (!EVP_DecryptInit_ex2(ctx, cipher, gcm_key, gcm_iv, params))
        goto err;

    /* Zero or more calls to specify any AAD */
    if (!EVP_DecryptUpdate(ctx, NULL, &outlen, gcm_aad, sizeof(gcm_aad)))
        goto err;

    /* Decrypt plaintext */
    if (!EVP_DecryptUpdate(ctx, outbuf, &outlen, gcm_ct, sizeof(gcm_ct)))
        goto err;

    /* Output decrypted block */
    printf("Plaintext:\n");
    BIO_dump_fp(stdout, outbuf, outlen);

    /* Set expected tag value. */
    params[0] = OSSL_PARAM_construct_octet_string(OSSL_CIPHER_PARAM_AEAD_TAG,
                                                  (void*)gcm_tag, sizeof(gcm_tag));

    if (!EVP_CIPHER_CTX_set_params(ctx, params))
        goto err;

    /* Finalise: note get no output for GCM */
    rv = EVP_DecryptFinal_ex(ctx, outbuf, &outlen);
    /*
     * Print out return value. If this is not successful authentication
     * failed and plaintext is not trustworthy.
     */
    printf("Tag Verify %s\n", rv > 0 ? "Successful!" : "Failed!");

    ret = 1;
err:
    if (!ret)
        ERR_print_errors_fp(stderr);

    EVP_CIPHER_free(cipher);
    EVP_CIPHER_CTX_free(ctx);

    return ret;
}

int main(int argc, char **argv)
{
    if (!aes_gcm_encrypt())
        return EXIT_FAILURE;

    if (!aes_gcm_decrypt())
        return EXIT_FAILURE;

    return EXIT_SUCCESS;
}