Transport Layer Security

Ensure all methods of communication are utilizing industry standard encryption configurations for TLS. The use of TLS ensures that all data remains confidential and untampered with while in transit. Utilize free certificate authority services such as Let’s Encrypt if the embedded device utilizes domain names.

Example of how to perform a basic certificate validation against a root certificate authority, using the OpenSSL library functions. :

#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/pem.h>
#include <openssl/x509.h>
#include <openssl/x509_vfy.h>

int main() {

  const char ca_bundlestr[] = "./ca-bundle.pem";
  const char cert_filestr[] = "./cert-file.pem";

  BIO              *certbio = NULL;
  BIO               *outbio = NULL;
  X509          *error_cert = NULL;
  X509                *cert = NULL;
  X509_NAME    *certsubject = NULL;
  X509_STORE         *store = NULL;
  X509_STORE_CTX  *vrfy_ctx = NULL;
  int ret;

  /* ---------------------------------------------------------- *
   * These function calls initialize openssl for correct work.  *
   * ---------------------------------------------------------- */
  OpenSSL_add_all_algorithms();
  ERR_load_BIO_strings();
  ERR_load_crypto_strings();

  /* ---------------------------------------------------------- *
   * Create the Input/Output BIO's.                             *
   * ---------------------------------------------------------- */
  certbio = BIO_new(BIO_s_file());
  outbio  = BIO_new_fp(stdout, BIO_NOCLOSE);

  /* ---------------------------------------------------------- *
   * Initialize the global certificate validation store object. *
   * ---------------------------------------------------------- */
  if (!(store=X509_STORE_new()))
     BIO_printf(outbio, "Error creating X509_STORE_CTX object\n");

  /* ---------------------------------------------------------- *
   * Create the context structure for the validation operation. *
   * ---------------------------------------------------------- */
  vrfy_ctx = X509_STORE_CTX_new();

  /* ---------------------------------------------------------- *
   * Load the certificate and cacert chain from file (PEM).     *
   * ---------------------------------------------------------- */
  ret = BIO_read_filename(certbio, cert_filestr);
  if (! (cert = PEM_read_bio_X509(certbio, NULL, 0, NULL))) {
    BIO_printf(outbio, "Error loading cert into memory\n");
    exit(-1);
  }

  ret = X509_STORE_load_locations(store, ca_bundlestr, NULL);
  if (ret != 1)
    BIO_printf(outbio, "Error loading CA cert or chain file\n");

  /* ---------------------------------------------------------- *
   * Initialize the ctx structure for a verification operation: *
   * Set the trusted cert store, the unvalidated cert, and any  *
   * potential certs that could be needed (here we set it NULL) *
   * ---------------------------------------------------------- */
  X509_STORE_CTX_init(vrfy_ctx, store, cert, NULL);

  /* ---------------------------------------------------------- *
   * Check the complete cert chain can be build and validated.  *
   * Returns 1 on success, 0 on verification failures, and -1   *
   * for trouble with the ctx object (i.e. missing certificate) *
   * ---------------------------------------------------------- */
  ret = X509_verify_cert(vrfy_ctx);
  BIO_printf(outbio, "Verification return code: %d\n", ret);

  if(ret == 0 || ret == 1)
  BIO_printf(outbio, "Verification result text: %s\n",
             X509_verify_cert_error_string(vrfy_ctx->error));

  /* ---------------------------------------------------------- *
   * The error handling below shows how to get failure details  *
   * from the offending certificate.                            *
   * ---------------------------------------------------------- */
  if(ret == 0) {
    /*  get the offending certificate causing the failure */
    error_cert  = X509_STORE_CTX_get_current_cert(vrfy_ctx);
    certsubject = X509_NAME_new();
    certsubject = X509_get_subject_name(error_cert);
    BIO_printf(outbio, "Verification failed cert:\n");
    X509_NAME_print_ex(outbio, certsubject, 0, XN_FLAG_MULTILINE);
    BIO_printf(outbio, "\n");
  }

  /* ---------------------------------------------------------- *
   * Free up all structures                                     *
   * ---------------------------------------------------------- */
  X509_STORE_CTX_free(vrfy_ctx);
  X509_STORE_free(store);
  X509_free(cert);
  BIO_free_all(certbio);
  BIO_free_all(outbio);
  exit(0);
}

Considerations (Disclaimer: The List below is non-exhaustive):

  • Use the latest possible version of TLS for new products (as of writing, this is TLS 1.2)
  • Consider implementing TLS two-way authentication for firmware that accepts TLS connections from a limited group of allowed clients.
  • If possible, consider using mutual-authentication to authenticate both end-points.
  • Validate the certificate public key, hostname, and chain.
  • Ensure certificate and their chains use SHA256 for signing.
  • Disable deprecated SSL and early TLS versions.
  • Disable deprecated, NULL and weak cipher suites.
  • Ensure private key and certificates are stored securely - e.g. Secure Environment or Trusted Execution Environment, or protected using strong cryptography.
  • Keep certificates updated with up to date secure configurations.
  • Ensure proper certificate update features are available upon expiration.
  • Verify TLS configurations utilizing services such as ssllabs.com, nmap using --script ssl-enum-ciphers.nse, TestSSLServer.jar, sslscan and sslyze.

Other Example(s):

To utilize TLS, there are other options besides OpenSSL. A non-exhaustive list is below.

Formerly PolarSSL, a list of projects using mbed TLS can be found at:

Examples of implementation can be found at

Formerly CyaSSL, wolfSSL and a list of projects using wolfSSL can be found at:

Examples of implementation can be found at:

Additional References

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