Openssl: Convert Distinguished Name from String to DER encoding

Question

I have the issuer of a X.509 certificate as a string representation:

"C=UT/O=foobar/OU=foo/serialNumber=0123/CN=TEST ca"

How can I get the Distinguished Name elements from a String representation to a DER encoded form (with their respective OIDs) with openssl in C.

Answer 1

i2d_X509_NAME is function for converting openSSL X509_NAME* object to DER format.

I don't know is there function to convert whole "C=UT/O=foobar/OU=foo/serialNumber=0123/CN=TEST ca" string to X509_NAME* directly.

But maybe simple (error checkings are missing for simplicity) example helps:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <openssl/pem.h>

int main()
{   
    unsigned char *buffer = NULL;
    X509_NAME *name = X509_NAME_new();

    X509_NAME_add_entry_by_txt(name, "O",  MBSTRING_UTF8, 
                               (unsigned char*)"Organization", -1, -1, 0);
    X509_NAME_add_entry_by_txt(name, "CN",  MBSTRING_UTF8, 
                               (unsigned char*)"MyName", -1, -1, 0 );
    
    int len = i2d_X509_NAME(name, &buffer);     
    fwrite(buffer, len, 1, stdout);
    OPENSSL_free(buffer);
    X509_NAME_free(name);
        
    return EXIT_SUCCESS;
}

Test:

$ gcc x509_name.c -Wall -o x509_name -lcrypto
$ ./x509_name | hexdump -C
00000000  30 28 31 15 30 13 06 03  55 04 0a 0c 0c 4f 72 67  |0(1.0...U....Org|
00000010  61 6e 69 7a 61 74 69 6f  6e 31 0f 30 0d 06 03 55  |anization1.0...U|
00000020  04 03 0c 06 4d 79 4e 61  6d 65                    |....MyName|
0000002a
$