#include <stdio.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>
#include <openssl/objects.h>
#include <openssl/x509.h>
#include <openssl/err.h>
#include <openssl/pem.h>
#include <openssl/ssl.h>

int main ()
{
  int err;
  int sig_len;
  unsigned char sig_buf [ 4096 ];
  static char certfile[] = "cert.pem";
  static char keyfile[]  = "key.pem";
  static char data[]     = "str";

  EVP_MD_CTX     md_ctx;
  EVP_PKEY *      pkey;
  FILE *          fp,*fp2;
  X509 *        x509;

  /* Just load the crypto library error strings,
   * SSL_load_error_strings() loads the crypto AND the SSL ones */
  /* SSL_load_error_strings();*/
  ERR_load_crypto_strings();

  /* Read private key */
  fp2= fopen (sign2, "r+");
  fp = fopen (keyfile, "r");
  if (fp == NULL) exit ( 1 );
  pkey = PEM_read_PrivateKey(fp, NULL, NULL, NULL);
  fclose (fp);

  if (pkey == NULL) {
        ERR_print_errors_fp (stderr);
        exit ( 1 );
  }

  /* Do the signature */

  EVP_SignInit   (&md_ctx, EVP_sha1());
  EVP_SignUpdate (&md_ctx, data, strlen(data));
  sig_len = sizeof(sig_buf);
  err = EVP_SignFinal (&md_ctx, sig_buf, &sig_len, pkey);

  if (err !=  1 ) {
        ERR_print_errors_fp(stderr);
        exit ( 1 );
  }

  EVP_PKEY_free (pkey);

  /* Read public key */

  fp = fopen (certfile, "r");
  if (fp == NULL) exit ( 1 );
  x509 = PEM_read_X509(fp, NULL, NULL, NULL);
//x509 = PEM_read_RSAPublicKey(fp, NULL, NULL, NULL);
  fclose (fp);

  if (x509 == NULL) {
        ERR_print_errors_fp (stderr);
        exit ( 1 );
  }

  /* Get public key - eay */
  pkey=X509_get_pubkey(x509);
  if (pkey == NULL) {
        ERR_print_errors_fp (stderr);
        exit ( 1 );
  }

  /* Verify the signature */

  EVP_VerifyInit   (&md_ctx, EVP_sha1());
  EVP_VerifyUpdate (&md_ctx, data, strlen((char*)data));
  err = EVP_VerifyFinal (&md_ctx, sig_buf, sig_len, pkey);
  EVP_PKEY_free (pkey);

  if (err !=  1 ) {
        ERR_print_errors_fp (stderr);
        exit ( 1 );
  }
  printf ("Signature Verified Ok.\n");
  return( 0 );
}

x509 = PEM_read_X509(fp, NULL, NULL, NULL)

 15316 :error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c: 642 :Expecting: RSA PUBLIC KEY