key: error when only either x or y are provided

PR-URL: https://github.com/indutny/elliptic/pull/112
Reviewed-By: Fedor Indutny <[email protected]>

lib/elliptic/ec/key.js

@@ -1,6 +1,9 @@
 'use strict';
 
 var BN = require('bn.js');
+var elliptic = require('../../elliptic');
+var utils = elliptic.utils;
+var assert = utils.assert;
 
 function KeyPair(ec, options) {
   this.ec = ec;
@@ -81,6 +84,15 @@ KeyPair.prototype._importPrivate = function _importPrivate(key, enc) {
 
 KeyPair.prototype._importPublic = function _importPublic(key, enc) {
   if (key.x || key.y) {
+    // Montgomery points only have an `x` coordinate.
+    // Weierstrass/Edwards points on the other hand have both `x` and
+    // `y` coordinates.
+    if (this.ec.curve.type === 'mont') {
+      assert(key.x, 'Need x coordinate');
+    } else if (this.ec.curve.type === 'short' ||
+               this.ec.curve.type === 'edwards') {
+      assert(key.x && key.y, 'Need both x and y coordinate');
+    }
     this.pub = this.ec.curve.point(key.x, key.y);
     return;
   }