X.509 Certificates are accepted as CAs without BC extension

[mradoy99]

X.509 certificates that are missing the BasicConstraints extension are still regarded as CA certificates, if the KeyUsage extension allows Cert Signing.

The BasicConstraints extension must be present and the cA flag set according to RFC 5280, 6.1.4 (k)

https://github.com/libressl/openbsd/blob/libressl-v4.2.0/src/lib/libcrypto/x509/x509_purp.c#L593

(Also, the documentation comment does not seem to completely match the function output. Return value 2 does not exist, 5 is undocumented.)

[botovq]

This is the historic behavior accepted by the OpenSSL "X.509" verifier, which predates both RFC 5280 and its predecessor 3280. As the documentation of the public API counterpart X509_check_ca(3) indicates, the CA here means "allowed to sign another certificate" where one should probably add "at some point in the past".

You still have to opt into the RFC 5280 behavior by setting X509_V_FLAG_X509_STRICT in the verify parameters to ensure the stricter interpretation of CA cert corresponding to the return value 1 of this function. In this case the verifier will also ensure that the version is v3 among other things.

Thanks for pointing out the oudated comment, I will look into synching this up with the public docs.

@bob-beck anything you want to add here? Should we look into making this a bit stricter?