#include <openssl/x509v3.h> int X509_check_ca(X509 *cert);
This function checks if given certificate is CA certificate (can be used to sign other certificates).
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4, if it is certificate with keyUsage extension with bit keyCertSign set, but without basicConstraints, and 5 if it has outdated Netscape Certificate Type extension telling that it is CA certificate.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.
X509_verify_cert(3), X509_check_issued(3), X509_check_purpose(3)
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