X509_check_ca - check if given certificate is CA certificate
int X509_check_ca(X509 *cert);
This function checks if given certificate is CA certificate (can be used to sign
Function return 0, if it is not CA certificate, 1 if it is proper X509v3 CA
certificate with basicConstraints extension CA:TRUE, 3, if it is
self-signed X509 v1 certificate, 4, if it is certificate with keyUsage
extension with bit keyCertSign set, but without
basicConstraints, and 5 if it has outdated Netscape Certificate Type
extension telling that it is CA certificate.
Actually, any nonzero value means that this certificate could have
been used to sign other certificates.
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