tls: prevent server from using dhe keys < 768

As part of the fix for logjam, node was upgraded to a
level of openssl which rejects connections to servers that
are using keys smaller than 768 bits. It is still possible,
however, to create a server that uses a smaller key size
and and older client may be able to connect to it.

This PR moves us to a secure by default stance on the
server side as well, preventing the creation of a server
using a dhe key size less than 768. This can be overridden
with the command line option which is also added.

It is derived from

9b35be5

which was landed in later io.js/node versions but makes
the limit 1024.  This PR uses the smaller limit in order
to meet the recomendations for logjam while matching was
was done on the client side in openssl to minimize the
potential impacton users.

The command line option will only be documented in the
release notes and will not be added to the tls
documentation.  The goal is that people who are
upgrading are aware and can use the option if they
run into issues, but otherwise the option is not
visible/used.

Address review comments 1

Address second set of review comments

doc/api/tls.markdown

@@ -203,7 +203,8 @@ automatically set as a listener for the [secureConnection][] event.  The
 
   - `dhparam`: DH parameter file to use for DHE key agreement. Use
     `openssl dhparam` command to create it. If the file is invalid to
-    load, it is silently discarded.
+    load, it is silently discarded. Its key length should be greater
+    than or equal to 768 bits, otherwise an error will be thrown.
 
   - `handshakeTimeout`: Abort the connection if the SSL/TLS handshake does not
     finish in this many milliseconds. The default is 120 seconds.

src/node.cc

@@ -3041,6 +3041,8 @@ static void ParseArgs(int* argc,
 #if HAVE_OPENSSL
       SSL3_ENABLE = true;
 #endif
+    } else if (strcmp(arg, "--allow-insecure-server-dhparam") == 0) {
+      ALLOW_INSECURE_SERVER_DHPARAM = true;
     } else if (strcmp(arg, "--help") == 0 || strcmp(arg, "-h") == 0) {
       PrintHelp();
       exit(0);

src/node_crypto.cc

@@ -77,6 +77,7 @@ namespace node {
 
 bool SSL2_ENABLE = false;
 bool SSL3_ENABLE = false;
+bool ALLOW_INSECURE_SERVER_DHPARAM = false;
 
 namespace crypto {
 
@@ -785,6 +786,13 @@ void SecureContext::SetDHParam(const FunctionCallbackInfo<Value>& args) {
   if (dh == NULL)
     return;
 
+  if (!ALLOW_INSECURE_SERVER_DHPARAM) {
+    const int keylen = BN_num_bits(dh->p);
+    if (keylen < 768) {
+      return env->ThrowError("DH parameter is less than 768 bits");
+    }
+  }
+
   SSL_CTX_set_options(sc->ctx_, SSL_OP_SINGLE_DH_USE);
   int r = SSL_CTX_set_tmp_dh(sc->ctx_, dh);
   DH_free(dh);

src/node_crypto.h

@@ -63,6 +63,7 @@ namespace node {
 
 extern bool SSL2_ENABLE;
 extern bool SSL3_ENABLE;
+extern bool ALLOW_INSECURE_SERVER_DHPARAM;
 
 namespace crypto {
 

test/fixtures/keys/Makefile

@@ -1,4 +1,4 @@
-all: agent1-cert.pem agent2-cert.pem agent3-cert.pem agent4-cert.pem agent5-cert.pem ca2-crl.pem ec-cert.pem dh1024.pem dh2048.pem
+all: agent1-cert.pem agent2-cert.pem agent3-cert.pem agent4-cert.pem agent5-cert.pem ca2-crl.pem ec-cert.pem dh512.pem dh1024.pem dh2048.pem
 
 
 #
@@ -170,6 +170,9 @@ ec-cert.pem: ec-csr.pem ec-key.pem
 		-signkey ec-key.pem \
 		-out ec-cert.pem
 
+dh512.pem:
+	openssl dhparam -out dh512.pem 512
+
 dh1024.pem:
 	openssl dhparam -out dh1024.pem 1024
 

test/fixtures/keys/dh512.pem

@@ -0,0 +1,4 @@
+-----BEGIN DH PARAMETERS-----
+MEYCQQDpl3okBAjG92NSOaQEsIyqzvJRN06yHuGXunxYVIqxg7TnU8DBZW0ZYyiJ
+rJLRA/9b9dCk5DXpq1pFGoAkYLoDAgEC
+-----END DH PARAMETERS-----

test/simple/test-tls-dhe-allow-insecure-server-dhparam.js

@@ -0,0 +1,52 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+//
+// Flags: --allow-insecure-server-dhparam
+
+var common = require('../common');
+
+var assert = require('assert');
+var tls = require('tls');
+var fs = require('fs');
+var key =  fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem');
+var cert = fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem');
+
+function loadDHParam(n) {
+  var path = common.fixturesDir;
+  if (n !== 'error') path += '/keys';
+  return fs.readFileSync(path + '/dh' + n + '.pem');
+}
+
+// validate that the server will accept a key smaller than
+// 768 provided the required command line option is specified
+// the flags statement above ensures that the test harnesss
+// runs the test with the required command line option
+function test512() {
+  var options = {
+    key: key,
+    cert: cert,
+    dhparam: loadDHParam(512)
+  };
+
+  var server = tls.createServer(options)
+}
+
+test512();

test/simple/test-tls-dhe.js

@@ -95,8 +95,25 @@ function testError() {
   ntests++;
 }
 
+// validate that the server will not allow keys less than
+// 768
+function test512() {
+  var options = {
+    key: key,
+    cert: cert,
+    dhparam: loadDHParam(512)
+  };
+
+  assert.throws(function() {var server = tls.createServer(options)},
+                'Should not be able to create server with key smaller than 768');
+}
+
+// test client/server communication with different key sizes
 test1024();
 
+// test server key length enforcement
+test512();
+
 process.on('exit', function() {
   assert.equal(ntests, nsuccess);
 });