how to pass PEM certificate as first arg of i2d_X509

问题

I'm creating self-signed certificate by:

openssl req -new -x509 -key privkey.pem -out cert.pem -days 1095

How do I pass cert.pem to i2d_X509? I need something like:

 len = i2d_X509(".\cert.pem", &buf);

but my certificate in a PEM file..

Here is my code: (I used the example in https://www.openssl.org/docs/crypto/d2i_X509.html)

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

int main(void)
{
    int len,i;
    unsigned char *buf;

    buf = NULL;
    len = i2d_X509((X509*)".\cert.pem", &buf);

    if (len < 0){
        printf("error len < 0");
        return -1;
     }

    printf("buf:");
    for (i = 0; i < len ; i++)
        printf("0x%02X", *(buf+i));

    return 0;
}

回答1:

How do I pass cert.pem to i2d_X509? ...

You don't/can't. You need to read the certificate with PEM_read_bio_X509. PEM_read_bio_X509 returns an X509*. Then you can pass it to i2d_X509.

Be sure to call X509_free on the pointer when done with it.

Or, do as Philippe suggests - convert it to ASN.1/DER and then use it with d2i_X509_fp.

Also see the OpenSSL man pages on the PEM Read/Write functions.

来源：https://stackoverflow.com/questions/29970497/how-to-pass-pem-certificate-as-first-arg-of-i2d-x509