bn256: don't claim a 128-bit security level.
It's no longer true.
Fixes golang/go#19479
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
-// Package bn256 implements a particular bilinear group at the 128-bit security level.
+// Package bn256 implements a particular bilinear group.
//
// Bilinear groups are the basis of many of the new cryptographic protocols
// that have been proposed over the past decade. They consist of a triplet of
// Barreto-Naehrig curve as described in
// http://cryptojedi.org/papers/dclxvi-20100714.pdf. Its output is compatible
// with the implementation described in that paper.
+//
+// (This package previously claimed to operate at a 128-bit security level.
+// However, recent improvements in attacks mean that is no longer true. See
+// https://moderncrypto.org/mail-archive/curves/2016/000740.html.)
package bn256 // import "golang.org/x/crypto/bn256"
import (
