The gnutls vtls back-end was previously ignoring any password set via
CURLOPT_KEYPASSWD. Presumably this was because
gnutls_certificate_set_x509_key_file did not support encrypted keys.

gnutls now has a gnutls_certificate_set_x509_key_file2 function that
does support encrypted keys. Let's determine at compile time whether the
available gnutls supports this new function. If it does then use it to
pass the password. If it does not then emit a helpful diagnostic if a
password is set. This is preferable to the previous behaviour of just
failing to read the certificate without giving a reason in that case.

Signed-off-by: Mike Crowe <mac_at_mcrowe.com>

---
 configure.ac    |  1 +
 lib/vtls/gtls.c | 44 +++++++++++++++++++++++++++++++++++---------
 2 files changed, 36 insertions(+), 9 deletions(-)
diff --git a/configure.ac b/configure.ac
index 843a3ba..26d77eb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1836,6 +1836,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
             AC_MSG_NOTICE([Added $gtlslib to LD_LIBRARY_PATH])
           fi
         fi
+        AC_CHECK_FUNCS(gnutls_certificate_set_x509_key_file2)
       fi
 
     fi
diff --git a/lib/vtls/gtls.c b/lib/vtls/gtls.c
index c54dfc1..9f3ea10 100644
--- a/lib/vtls/gtls.c
+++ b/lib/vtls/gtls.c
@@ -656,15 +656,41 @@ gtls_connect_step1(struct connectdata *conn,
 #endif
 
   if(data->set.str[STRING_CERT]) {
-    if(gnutls_certificate_set_x509_key_file(
-         conn->ssl[sockindex].cred,
-         data->set.str[STRING_CERT],
-         data->set.str[STRING_KEY] ?
-         data->set.str[STRING_KEY] : data->set.str[STRING_CERT],
-         do_file_type(data->set.str[STRING_CERT_TYPE]) ) !=
-       GNUTLS_E_SUCCESS) {
-      failf(data, "error reading X.509 key or certificate file");
-      return CURLE_SSL_CONNECT_ERROR;
+    if(data->set.str[STRING_KEY_PASSWD]) {
+#if HAVE_GNUTLS_CERTIFICATE_SET_X509_KEY_FILE2
+      const unsigned int supported_key_encryption_algorithms =
+        GNUTLS_PKCS_USE_PKCS12_3DES | GNUTLS_PKCS_USE_PKCS12_ARCFOUR |
+        GNUTLS_PKCS_USE_PKCS12_RC2_40 | GNUTLS_PKCS_USE_PBES2_3DES |
+        GNUTLS_PKCS_USE_PBES2_AES_128 | GNUTLS_PKCS_USE_PBES2_AES_192 |
+        GNUTLS_PKCS_USE_PBES2_AES_256;
+      if(gnutls_certificate_set_x509_key_file2(
+           conn->ssl[sockindex].cred,
+           data->set.str[STRING_CERT],
+           data->set.str[STRING_KEY] ?
+           data->set.str[STRING_KEY] : data->set.str[STRING_CERT],
+           do_file_type(data->set.str[STRING_CERT_TYPE]),
+           data->set.str[STRING_KEY_PASSWD],
+           supported_key_encryption_algorithms) !=
+         GNUTLS_E_SUCCESS) {
+        failf(data,
+              "error reading X.509 potentially-encrypted key certificate file");
+        return CURLE_SSL_CONNECT_ERROR;
+#else
+        failf(data, "gnutls lacks support for encrypted key files");
+        return CURLE_SSL_CONNECT_ERROR;
+#endif
+      }
+    } else {
+      if(gnutls_certificate_set_x509_key_file(
+           conn->ssl[sockindex].cred,
+           data->set.str[STRING_CERT],
+           data->set.str[STRING_KEY] ?
+           data->set.str[STRING_KEY] : data->set.str[STRING_CERT],
+           do_file_type(data->set.str[STRING_CERT_TYPE]) ) !=
+         GNUTLS_E_SUCCESS) {
+        failf(data, "error reading X.509 key or certificate file");
+        return CURLE_SSL_CONNECT_ERROR;
+      }
     }
   }
 
-- 
2.1.4
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