crypto: expose ECDH class

For consistency with the rest of the crypto classes, exposes the ECDH
class. Originally, only the createECDH function was exposed, and there
was no real reason to hide the class.

PR-URL: nodejs/node#8188
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/crypto.js

@@ -79,10 +79,6 @@ const {
 } = require('internal/crypto/util');
 const Certificate = require('internal/crypto/certificate');
 
-function createECDH(curve) {
-  return new ECDH(curve);
-}
-
 module.exports = exports = {
   // Methods
   _toBuf: toBuf,
@@ -92,7 +88,7 @@ module.exports = exports = {
   createDecipheriv: Decipheriv,
   createDiffieHellman: DiffieHellman,
   createDiffieHellmanGroup: DiffieHellmanGroup,
-  createECDH,
+  createECDH: ECDH,
   createHash: Hash,
   createHmac: Hmac,
   createSign: Sign,
@@ -124,6 +120,7 @@ module.exports = exports = {
   Decipheriv,
   DiffieHellman,
   DiffieHellmanGroup,
+  ECDH,
   Hash,
   Hmac,
   Sign,

lib/internal/crypto/diffiehellman.js

@@ -168,6 +168,9 @@ DiffieHellman.prototype.setPrivateKey = function setPrivateKey(key, encoding) {
 
 
 function ECDH(curve) {
+  if (!(this instanceof ECDH))
+    return new ECDH(curve);
+
   if (typeof curve !== 'string')
     throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'curve', 'string');
 

test/parallel/test-crypto-classes.js

@@ -18,6 +18,7 @@ const TEST_CASES = {
   'Verify': ['RSA-SHA1'],
   'DiffieHellman': [1024],
   'DiffieHellmanGroup': ['modp5'],
+  'ECDH': ['prime256v1'],
   'Credentials': []
 };