how to get the Keyusage value from the X509 certificate?

Question

I want to retrieve the Key usage value from the X509 structured certificate , i tried the following code

 X509* lcert=NULL;
 lCert=PEM_read(filename); // fun         


        

Answer 1

I think the easiest way is to use a memory BIO:

...
X509 *lcert = NULL;
BUF_MEM *bptr = NULL;
char *buf = NULL;
int loc;

FILE *f = fopen("your cert goes here", "rb");
if( (lcert = PEM_read_X509(f, &lcert, NULL, NULL)) == NULL){
    // error handling...
}

loc = X509_get_ext_by_NID( lcert, NID_key_usage, -1);
X509_EXTENSION *ex = X509_get_ext(lcert, loc);

BIO *bio = BIO_new(BIO_s_mem());
if(!X509V3_EXT_print(bio, ex, 0, 0)){
    // error handling...
}
BIO_flush(bio);
BIO_get_mem_ptr(bio, &bptr);

// now bptr contains the strings of the key_usage, take 
// care that bptr->data is NOT NULL terminated, so
// to print it well, let's do something..
buf = (char *)malloc( (bptr->length + 1)*sizeof(char) );

memcpy(buf, bptr->data, bptr->length);
buf[bptr->length] = '\0';

// Now you can printf it or parse it, the way you want...
printf ("%s\n", buf);

...

In my case, for a teste certificate, it has printed "Digital Signature, Non Repudiation, Key Encipherment"

There are other ways, like using an ASN1_BIT_STRING *. I can show you if the above doesn't fit your needs.

Regards.