--- a/src/crypto/elliptic/elliptic.go
+++ b/src/crypto/elliptic/elliptic.go
@@ -210,8 +210,9 @@
 
 	x3 := new(big.Int).Mul(alpha, alpha)
 	beta8 := new(big.Int).Lsh(beta, 3)
+	beta8.Mod(beta8, curve.P)
 	x3.Sub(x3, beta8)
-	for x3.Sign() == -1 {
+	if x3.Sign() == -1 {
 		x3.Add(x3, curve.P)
 	}
 	x3.Mod(x3, curve.P)