bn256: don't claim a 128-bit security level.

It's no longer true.

Fixes golang/go#19479

Change-Id: I85b0ce850ebde60b816924a25368208527a8e617
Reviewed-on: https://go-review.googlesource.com/79877
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

bn256/bn256.go

@@ -2,7 +2,7 @@
 // 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
@@ -14,6 +14,10 @@
 // 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 (