Hello world example of using the authenticated encryption with mbed TLS. The canonical source for this example lives at https://github.com/ARMmbed/mbed-os-example-tls

mbed TLS Benchmark example on mbed OS

This application performs authenticated encryption and authenticated decryption of a buffer. It serves as a tutorial for the basic authenticated encryption functions of mbed TLS.

Getting started

Building with mbed CLI

If you'd like to use mbed CLI to build this, then you should set up your environment if you have not done so already. For instructions, refer to the main readme. The instructions on this page relate to using the developer.mbed.org Online Compiler

Import the program in to the Online Compiler, select your board from the drop down in the top right hand corner and then compile the application. Once it has built, you can drag and drop the binary onto your device.

Monitoring the application

The output in the terminal window should be similar to this:

terminal output

plaintext message: 536f6d65207468696e67732061726520626574746572206c65667420756e7265616400
ciphertext: c57f7afb94f14c7977d785d08682a2596bd62ee9dcf216b8cccd997afee9b402f5de1739e8e6467aa363749ef39392e5c66622b01c7203ec0a3d14
decrypted: 536f6d65207468696e67732061726520626574746572206c65667420756e7265616400

DONE

authcrypt.cpp

Committer:
mbed_official
Date:
2018-05-10
Revision:
63:5e7be856a68b
Parent:
48:6b6340f5cdc3
Child:
75:33f1c34c8a2d

File content as of revision 63:5e7be856a68b:

/*
 *  Hello world example of using the authenticated encryption with Mbed TLS
 *
 *  Copyright (C) 2017, Arm Limited, All Rights Reserved
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  Licensed under the Apache License, Version 2.0 (the "License"); you may
 *  not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

#include "authcrypt.h"

#include "mbed.h"

#include "mbedtls/cipher.h"
#include "mbedtls/entropy.h"
#include "mbedtls/ctr_drbg.h"
#if DEBUG_LEVEL > 0
#include "mbedtls/debug.h"
#endif

#include <string.h>

const unsigned char Authcrypt::secret_key[16] = {
    0xf4, 0x82, 0xc6, 0x70, 0x3c, 0xc7, 0x61, 0x0a,
    0xb9, 0xa0, 0xb8, 0xe9, 0x87, 0xb8, 0xc1, 0x72,
};

const char Authcrypt::message[] = "Some things are better left unread";

const char Authcrypt::metadata[] = "eg sequence number, routing info";

Authcrypt::Authcrypt(mbedtls_platform_context* platform_ctx)
{
    // The platform context can be used by cryptographic calls which require it.
    // Please refer to https://github.com/ARMmbed/mbedtls/issues/1200 for more information.
    _platform_ctx = platform_ctx;
    memset(ciphertext, 0, sizeof(ciphertext));
    memset(decrypted, 0, sizeof(decrypted));

    mbedtls_entropy_init(&entropy);
    mbedtls_ctr_drbg_init(&drbg);
    mbedtls_cipher_init(&cipher);
}

Authcrypt::~Authcrypt()
{
    memset(ciphertext, 0, sizeof(ciphertext));
    memset(decrypted, 0, sizeof(decrypted));

    mbedtls_cipher_free(&cipher);
    mbedtls_ctr_drbg_free(&drbg);
    mbedtls_entropy_free(&entropy);
}

int Authcrypt::run()
{
    mbedtls_printf("\r\n\r\n");
    print_hex("plaintext message",
              reinterpret_cast<const unsigned char *>(message),
              sizeof(message));

    /*
     * Seed the PRNG using the entropy pool, and throw in our secret key as an
     * additional source of randomness.
     */
    int ret = mbedtls_ctr_drbg_seed(&drbg, mbedtls_entropy_func, &entropy,
                                    secret_key, sizeof(secret_key));
    if (ret != 0) {
        mbedtls_printf("mbedtls_ctr_drbg_seed() returned -0x%04X\r\n", -ret);
        return ret;
    }

    /* Setup AES-CCM contex */
    ret = mbedtls_cipher_setup(&cipher,
                    mbedtls_cipher_info_from_type(MBEDTLS_CIPHER_AES_128_CCM));
    if (ret != 0) {
        mbedtls_printf("mbedtls_cipher_setup() returned -0x%04X\r\n", -ret);
        return ret;
    }

    ret = mbedtls_cipher_setkey(&cipher, secret_key,
                                8 * sizeof(secret_key), MBEDTLS_ENCRYPT);
    if (ret != 0) {
        mbedtls_printf("mbedtls_cipher_setkey() returned -0x%04X\r\n", -ret);
        return ret;
    }

    /*
     * Encrypt-authenticate the message and authenticate additional data
     *
     * First generate a random 8-byte nonce.
     * Put it directly in the output buffer as the recipient will need it.
     *
     * Warning: you must never re-use the same (key, nonce) pair. One of
     * the best ways to ensure this to use a counter for the nonce.
     * However, this means you should save the counter accross rebots, if
     * the key is a long-term one. The alternative we choose here is to
     * generate the nonce randomly. However it only works if you have a
     * good source of randomness.
     */
    const size_t nonce_len = 8;
    mbedtls_ctr_drbg_random(&drbg, ciphertext, nonce_len);

    size_t ciphertext_len = 0;
    /*
     * Go for a conservative 16-byte (128-bit) tag and append it to the
     * ciphertext
     */
    const size_t tag_len = 16;
    ret = mbedtls_cipher_auth_encrypt(&cipher, ciphertext, nonce_len,
                        reinterpret_cast<const unsigned char *>(metadata),
                        sizeof(metadata),
                        reinterpret_cast<const unsigned char *>(message),
                        sizeof(message),
                        ciphertext + nonce_len, &ciphertext_len,
                        ciphertext + nonce_len + sizeof(message),
                        tag_len);
    if (ret != 0) {
        mbedtls_printf("mbedtls_cipher_auth_encrypt() returned -0x%04X\r\n",
                       -ret);
        return ret;
    }
    ciphertext_len += nonce_len + tag_len;

    /*
     * The following information should now be transmitted:
     * - First ciphertext_len bytes of ciphertext buffer
     * - Metadata if not already transmitted elsewhere
     */
    print_hex("ciphertext", ciphertext, ciphertext_len);

    /* Decrypt-authenticate */
    size_t decrypted_len = 0;

    ret = mbedtls_cipher_setkey(&cipher, secret_key, 8 * sizeof(secret_key),
                                MBEDTLS_DECRYPT);
    if (ret != 0) {
        mbedtls_printf("mbedtls_cipher_setkey() returned -0x%04X\r\n", -ret);
        return ret;
    }

    ret = mbedtls_cipher_auth_decrypt(&cipher, ciphertext, nonce_len,
                    reinterpret_cast<const unsigned char *>(metadata),
                    sizeof(metadata), ciphertext + nonce_len,
                    ciphertext_len - nonce_len - tag_len, decrypted,
                    &decrypted_len, ciphertext + ciphertext_len - tag_len,
                    tag_len);
    /* Checking the return code is CRITICAL for security here */
    if (ret == MBEDTLS_ERR_CIPHER_AUTH_FAILED) {
        mbedtls_printf("Something bad is happening! Data is not "
                       "authentic!\r\n");
        return ret;
    } else if (ret != 0) {
        mbedtls_printf("mbedtls_cipher_authdecrypt() returned -0x%04X\r\n",
                       -ret);
        return ret;
    }

    print_hex("decrypted", decrypted, decrypted_len);

    mbedtls_printf("\r\nDONE\r\n");

    return 0;
}

void Authcrypt::print_hex(const char *title,
                          const unsigned char buf[],
                          size_t len)
{
    mbedtls_printf("%s: ", title);

    for (size_t i = 0; i < len; i++)
        mbedtls_printf("%02x", buf[i]);

    mbedtls_printf("\r\n");
}