Using SHA256 certificate hasing

[Tudor B.]

Hello.

After some great struggles, we've managed to enable WiFi and encrypted socket + TLS connection to google.com and various websites.

Since then, we've been looking into having a r1.pem that contains all trusted certificates. Since that entails around 143 certificates, that equates to 223kb so the file is too big. The team lead had a great idea: sha256 fingerprint checking, which would make the file ~4,5kb if we store just the fingerprints for the 143 certificates.

So I've been looking into enabling this instead of the normal certificate verification in mbedtls. Currently I'm dabbling with:

Note: devepment is done on Macbook (so MacOS), and we're using Nordic SDK v3.0.0.

#define MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK

and:

mbedtls_ssl_conf_ca_cb(&conf, cert_verify_callback, NULL);

 

I've succeeded in hacking the SDK itself and overriding the certificate check from inside:

/opt/nordic/ncs/v3.0.0/modules/crypto/mbedtls/library/ssl_tls.c:10045:

ret = mbedtls_x509_crt_verify_restartable(
chain,
ca_chain, ca_crl,
ssl->conf->cert_profile,
hostname,
&ssl->session_negotiate->verify_result,
f_vrfy, p_vrfy, rs_ctx);

with a custom one written by us.

But I doubt I'm doing it correctly given the define and callback mentioned above. Any hints regarding this from anyone?

[Emil Lenngren]

Did you consider that most servers don't include a copy of the root certificate in the certificate chain they send? With only a fingerprint, you won't be able to verify the signature in the last certificate in the sent chain, if I'm not mistaken.

You should instead create a minium binary file containing a sorted list of root certificates having the lookup key set to the subject name (in DER format). A hash of this could maybe also work. It's enough if the value contains the raw public key and type (rsa, secp256r1 or secp384r1).

Espressif uses this approach in their sdk: see docs.espressif.com/.../esp_crt_bundle.html and github.com/.../gen_crt_bundle.py.