ec: fix point decoding for other curves

src/ec.js

@@ -8,7 +8,6 @@ var BigInteger = require('bigi')
 // constants
 var TWO = BigInteger.valueOf(2)
 var THREE = BigInteger.valueOf(3)
-var SEVEN = BigInteger.valueOf(7)
 
 function ECFieldElementFp(q,x) {
     this.x = x;
@@ -340,6 +339,8 @@ ECPointFp.decodeFrom = function (curve, buffer) {
     assert(type === 0x02 || type === 0x03, 'Invalid sequence tag')
 
     var isYEven = (type === 0x02)
+    var a = curve.getA().toBigInteger()
+    var b = curve.getB().toBigInteger()
     var p = curve.getQ()
 
     // We precalculate (p + 1) / 4 where p is the field order
@@ -348,7 +349,7 @@ ECPointFp.decodeFrom = function (curve, buffer) {
     }
 
     // Convert x to point
-    var alpha = x.pow(3).add(SEVEN).mod(p)
+    var alpha = x.pow(3).add(a.multiply(x)).add(b).mod(p)
     var beta = alpha.modPow(curve.P_OVER_FOUR, p)
 
     // If beta is even, but y isn't, or vice versa, then convert it,

test/ec.js

@@ -64,6 +64,21 @@ describe('ec', function() {
           })
         })
       })
+
+      it('supports other curves', function() {
+        var f = fixtures.valid.ECPointFp[1]
+        var ecparams2 = sec('secp256r1')
+        var curve = ecparams2.getCurve()
+
+        var D = BigInteger.ONE
+        var Q = ecparams2.getG().multiply(D)
+
+        var buffer = Q.getEncoded(true)
+        var decoded = ECPointFp.decodeFrom(curve, buffer)
+
+        assert(Q.equals(decoded.Q))
+        assert(decoded.compressed, true)
+      })
     })
   })
 })