DSA Signing with OpenSSL - c

Question

I'm tryng to sign using DSA from OpenSSL. I have the files containing public and private keys.
First of all I make an unicast connection and every thing is fine. After that I need a multicast UDP connection and I want to sign the packets. I'm trying to use function PEM_read_DSA_PUBKEY() in order to load my public key from my cert but it doesn't work. It returns always NULL instead of a DSA struct.
Here you have a simplistic version of the code. I compile like this:
gcc -Wall -g -lm prueba.c -o prueba -lcrypto
Any idea? Thank you!
#include <stdio.h>
#include <openssl/dsa.h>
#include <openssl/pem.h>
int main()
{
FILE *DSA_cert_file = fopen("./certs/cert.pem", "r");
if (DSA_cert_file == NULL)
return 1;
printf("Certificate read\n");
DSA *dsa = DSA_new();
if((dsa = PEM_read_DSA_PUBKEY(DSA_cert_file, 0, 0, 0)) == NULL)
return 1;
printf("DSA public key read\n");
return 0;
}

Answer 1

Are you using a password-protected public key?
If yes, you are required to pass a callback function as the third argument to PEM_read_DSA_PUBKEY, so if the provided password matches, it will be able to properly load your key.
Update:
Alternatively, as pointed by Hasturkun, you can pass a null-terminated string as the fourth argument. Quoting the official documentation:
If the cb parameters is set to NULL
and the u parameter is not NULL then
the u parameter is interpreted as a
null terminated string to use as the
passphrase. If both cb and u are NULL
then the default callback routine is
used which will typically prompt for
the passphrase on the current terminal
with echoing turned off.

Answer 2

Does your cert.pem contains a X.509 certificate ? It looks like PEM_read_DSA_PUBKEY expects a PEM-encoded DSA public key without the X.509 container.
Try something like that instead:
X509 *cert;
EVP_PKEY *pk;
DSA *dsa;
cert = PEM_read_X509(DSA_cert_file,NULL,NULL,NULL);
if (!cert) { /* error */ }
pk = X509_get_pubkey(cert);
if (!pk) { /* error */ }
if (pk->type != 116) { /* not a dsa key */ }
dsa = pk->pkey.dsa