openssl: how to read a .crt file..?

Question

I want to read the certifi.crt file using OpenSSL API (not in commands). I have no idea how to do that. If any one knows, please help me. Thank you.

If you give example code that will be very helpful.

Answer 1

If the ".crt" extension refers to a PEM text file (begins with -----BEGIN CERTIFICATE----- followed by base64), then get started in the OpenSSL docs here.

Here is some code to get you started (link with ssl, e.g. g++ a.c -lssl):

#include <stdio.h>
#include <openssl/x509.h>
#include <openssl/pem.h>

int main(int argc, char** argv)
{
    FILE* f = fopen("certifi.crt", "r");
    if (f != NULL)
    {
        X509* x509 = PEM_read_X509(f, NULL, NULL, NULL);
        if (x509 != NULL)
        {
            char* p = X509_NAME_oneline(X509_get_issuer_name(x509), 0, 0);
            if (p)
            {
                printf("NAME: %s\n", p);
                OPENSSL_free(p);
            }
            X509_free(x509);
        }
    }
    return 0;
}