How to verify key length of a PEM certificate using openSSL functions - c

Question

How do I verify the key length of a PEM certificate that is generated in this way:
# openssl genrsa -des3 -out server.key 1024
# openssl req -new -key server.key -out server.csr
# cp server.key server.key.org
# openssl rsa -in server.key.org -out server.key
# openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt
What I need is a C function using procedures from OpenSSL, that performs validation on a PEM certificate (I use it for the lighttpd HTTPS server), and returns the length of the key stored in the certificate (in this case, 1024).

Answer 1

After some tweaking, I believe have found the right routines.
The following should get you started with exploring other OpenSSL routines, in case you need to handle other types of certificates (x509, pem).
Also read through your local x509.h and pem.h for structures and functions that will recover other information you're after.
/* Compile with 'gcc -Wall -lcrypto foo.c' or similar...
---------------------------------------------------------
$ ./a.out server.crt
Opened: server.crt
RSA Public Key: (1024 bit)
$ ./a.out server.key
ERROR: could not read x509 data from server.key
*/
#include <stdio.h>
#include <openssl/crypto.h>
#include <openssl/x509.h>
#include <openssl/pem.h>
int main(int argc, char *argv[])
{
FILE *fp = NULL;
X509 *x509 = NULL;
EVP_PKEY *public_key = NULL;
fp = fopen(argv[1], "r");
if (fp) {
PEM_read_X509(fp, &x509, NULL, NULL);
fclose(fp);
if (x509) {
fprintf(stderr, "Opened PEM certificate file: %s\n", argv[1]);
/* do stuff with certificate... */
public_key = X509_get_pubkey(x509);
if (public_key) {
switch (public_key->type) {
case EVP_PKEY_RSA:
fprintf(stdout, "RSA Public Key: (%d bit)\n", BN_num_bits(public_key->pkey.rsa->n));
break;
default:
fprintf(stdout, "Unknown public key type? See OpenSSL documentation\n");
break;
}
EVP_PKEY_free(public_key);
}
X509_free(x509);
}
else {
fprintf(stderr, "ERROR: could not read x509 data from %s\n", argv[1]);
return EXIT_FAILURE;
}
}
else {
fprintf(stderr, "ERROR: could not open file!\n");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}