Add openconnect_get_cert_DER() function

This translates a cert into an SSL-library-agnostic form, so that the caller
can then process it using their own choice of tools.

As with the new openconnect_get_cert_details(), this isn't marked as a
public function yet because we anticipate more changes to the API.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

libopenconnect.map.in

@@ -54,5 +54,6 @@ OPENCONNECT_PRIVATE {
 	openconnect_create_useragent;
 	openconnect_report_ssl_errors;
 	openconnect_get_cert_details;
+	openconnect_get_cert_DER;
 	openconnect_sha1;
 };

openconnect.h

@@ -130,6 +130,10 @@ int openconnect_get_cert_sha1(struct openconnect_info *vpninfo,
 			      struct x509_st *cert, char *buf);
 char *openconnect_get_cert_details(struct openconnect_info *vpninfo,
 				   struct x509_st *cert);
+/* Returns the length of the created DER output, in a newly-allocated buffer
+   that will need to be freed by the caller. */
+int openconnect_get_cert_DER(struct openconnect_info *vpninfo,
+			     struct x509_st *cert, unsigned char **buf);
 int openconnect_set_http_proxy(struct openconnect_info *vpninfo, char *proxy);
 int openconnect_passphrase_from_fsid(struct openconnect_info *vpninfo);
 int openconnect_obtain_cookie(struct openconnect_info *vpninfo);

openssl.c

@@ -22,6 +22,8 @@
  *   Boston, MA 02110-1301 USA
  */
 
+#include <errno.h>
+
 #include <openssl/evp.h>
 
 #include "openconnect-internal.h"
@@ -36,3 +38,27 @@ int openconnect_sha1(unsigned char *result, void *data, int len)
 
         return 0;
 }
+
+int openconnect_get_cert_DER(struct openconnect_info *vpninfo,
+			     struct x509_st *cert, unsigned char **buf)
+{
+	BIO *bp = BIO_new(BIO_s_mem());
+	BUF_MEM *certinfo;
+	size_t l;
+
+	if (!i2d_X509_bio(bp, cert)) {
+		BIO_free(bp);
+		return -EIO;
+	}
+
+	BIO_get_mem_ptr(bp, &certinfo);
+	l = certinfo->length;
+	*buf = malloc(l);
+	if (!*buf) {
+		BIO_free(bp);
+		return -ENOMEM;
+	}
+	memcpy(*buf, certinfo->data, l);
+	BIO_free(bp);
+	return l;
+}