ESP x509 Certificate Bundle

Overview

The ESP x509 Certificate Bundle API provides an easy way to include a bundle of custom x509 root certificates for TLS server verification.

Note

The bundle is currently not available when using WolfSSL.

The bundle comes with the complete list of root certificates from Mozilla’s NSS root certificate store. Using the gen_crt_bundle.py python utility the certificates’ subject name and public key are stored in a file and embedded in the ESP32-C3 binary.

When generating the bundle you may choose between:

  • The full root certificate bundle from Mozilla, containing more than 130 certificates. The current bundle was updated Tue Oct 5 09:35:35 2021 GMT.

  • A pre-selected filter list of the name of the most commonly used root certificates, reducing the amount of certificates to around 35 while still having around 90 % coverage according to market share statistics.

In addition it is possible to specify a path to a certificate file or a directory containing certificates which then will be added to the generated bundle.

Note

Trusting all root certificates means the list will have to be updated if any of the certificates are retracted. This includes removing them from cacrt_all.pem.

Configuration

Most configuration is done through menuconfig. Make and CMake will generate the bundle according to the configuration and embed it.

To enable the bundle when using ESP-TLS simply pass the function pointer to the bundle attach function:

esp_tls_cfg_t cfg = {
     .crt_bundle_attach = esp_crt_bundle_attach,
};

This is done to avoid embedding the certificate bundle unless activated by the user.

If using mbedTLS directly then the bundle may be activated by directly calling the attach function during the setup process:

mbedtls_ssl_config conf;
mbedtls_ssl_config_init(&conf);

esp_crt_bundle_attach(&conf);

Generating the List of Root Certificates

The list of root certificates comes from Mozilla’s NSS root certificate store, which can be found here The list can be downloaded and created by running the script mk-ca-bundle.pl that is distributed as a part of curl. Another alternative would be to download the finished list directly from the curl website: CA certificates extracted from Mozilla

The common certificates bundle were made by selecting the authorities with a market share of more than 1 % from w3tech’s SSL Survey. These authorities were then used to pick the names of the certificates for the filter list, cmn_crt_authorities.csv, from this list provided by Mozilla.

Updating the Certificate Bundle

The bundle is embedded into the app and can be updated along with the app by an OTA update. If you want to include a more up-to-date bundle than the bundle currently included in IDF, then the certificate list can be downloaded from Mozilla as described in Updating the Certificate Bundle.

Application Example

Simple HTTPS example that uses ESP-TLS to establish a secure socket connection using the certificate bundle with two custom certificates added for verification: protocols/https_x509_bundle.

HTTPS example that uses ESP-TLS and the default bundle: protocols/https_request.

HTTPS example that uses mbedTLS and the default bundle: protocols/https_mbedtls.

API Reference

Functions

esp_err_t esp_crt_bundle_attach(void *conf)

Attach and enable use of a bundle for certificate verification.

Attach and enable use of a bundle for certificate verification through a verification callback. If no specific bundle has been set through esp_crt_bundle_set() it will default to the bundle defined in menuconfig and embedded in the binary.

Return

  • ESP_OK if adding certificates was successful.

  • Other if an error occured or an action must be taken by the calling process.

Parameters

  • [in] conf: The config struct for the SSL connection.

void esp_crt_bundle_detach(mbedtls_ssl_config *conf)

Disable and dealloc the certification bundle.

Removes the certificate verification callback and deallocates used resources

Parameters

  • [in] conf: The config struct for the SSL connection.

void esp_crt_bundle_set(const uint8_t *x509_bundle)

Set the default certificate bundle used for verification.

Overrides the default certificate bundle. In most use cases the bundle should be set through menuconfig. The bundle needs to be sorted by subject name since binary search is used to find certificates.

Parameters

  • [in] x509_bundle: A pointer to the certificate bundle.

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