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X509_CHECK_CA(3) Library Functions Manual X509_CHECK_CA(3)

X509_check_ca —
check whether a certificate is a CA certificate

#include <openssl/x509v3.h>

int
X509_check_ca(X509 *cert);

This function checks whether the given certificate is a CA certificate, that is, whether it can be used to sign other certificates.

This functions returns non-zero if cert is a CA certificate or 0 otherwise.

The following return values identify specific kinds of CA certificates:

1
an X.509 v3 CA certificate with basicConstraints extension CA:TRUE
3
a self-signed X.509 v1 certificate
4
a certificate with keyUsage extension with bit keyCertSign set, but without basicConstraints
5
a certificate with an outdated Netscape Certificate Type extension telling that it is a CA certificate

BASIC_CONSTRAINTS_new(3), EXTENDED_KEY_USAGE_new(3), X509_check_issued(3), X509_EXTENSION_new(3), X509_new(3), X509_verify_cert(3)

X509_check_ca() first appeared in OpenSSL 0.9.7f and has been available since OpenBSD 3.8.
June 6, 2019 OpenBSD-current