Switch to keybase go-crypto (for some elliptic curve key) + test (#1925)

* Switch to keybase go-crypto (for some elliptic curve key) + test

* Use assert.NoError 

and add a little more context to failing test description

* Use assert.(No)Error everywhere 🌈

and assert.Error in place of .Nil/.NotNil

vendor/github.com/keybase/go-crypto/brainpool/rcurve.go

@@ -0,0 +1,83 @@
+package brainpool
+
+import (
+	"crypto/elliptic"
+	"math/big"
+)
+
+var _ elliptic.Curve = (*rcurve)(nil)
+
+type rcurve struct {
+	twisted elliptic.Curve
+	params  *elliptic.CurveParams
+	z       *big.Int
+	zinv    *big.Int
+	z2      *big.Int
+	z3      *big.Int
+	zinv2   *big.Int
+	zinv3   *big.Int
+}
+
+var (
+	two   = big.NewInt(2)
+	three = big.NewInt(3)
+)
+
+func newrcurve(twisted elliptic.Curve, params *elliptic.CurveParams, z *big.Int) *rcurve {
+	zinv := new(big.Int).ModInverse(z, params.P)
+	return &rcurve{
+		twisted: twisted,
+		params:  params,
+		z:       z,
+		zinv:    zinv,
+		z2:      new(big.Int).Exp(z, two, params.P),
+		z3:      new(big.Int).Exp(z, three, params.P),
+		zinv2:   new(big.Int).Exp(zinv, two, params.P),
+		zinv3:   new(big.Int).Exp(zinv, three, params.P),
+	}
+}
+
+func (curve *rcurve) toTwisted(x, y *big.Int) (*big.Int, *big.Int) {
+	var tx, ty big.Int
+	tx.Mul(x, curve.z2)
+	tx.Mod(&tx, curve.params.P)
+	ty.Mul(y, curve.z3)
+	ty.Mod(&ty, curve.params.P)
+	return &tx, &ty
+}
+
+func (curve *rcurve) fromTwisted(tx, ty *big.Int) (*big.Int, *big.Int) {
+	var x, y big.Int
+	x.Mul(tx, curve.zinv2)
+	x.Mod(&x, curve.params.P)
+	y.Mul(ty, curve.zinv3)
+	y.Mod(&y, curve.params.P)
+	return &x, &y
+}
+
+func (curve *rcurve) Params() *elliptic.CurveParams {
+	return curve.params
+}
+
+func (curve *rcurve) IsOnCurve(x, y *big.Int) bool {
+	return curve.twisted.IsOnCurve(curve.toTwisted(x, y))
+}
+
+func (curve *rcurve) Add(x1, y1, x2, y2 *big.Int) (x, y *big.Int) {
+	tx1, ty1 := curve.toTwisted(x1, y1)
+	tx2, ty2 := curve.toTwisted(x2, y2)
+	return curve.fromTwisted(curve.twisted.Add(tx1, ty1, tx2, ty2))
+}
+
+func (curve *rcurve) Double(x1, y1 *big.Int) (x, y *big.Int) {
+	return curve.fromTwisted(curve.twisted.Double(curve.toTwisted(x1, y1)))
+}
+
+func (curve *rcurve) ScalarMult(x1, y1 *big.Int, scalar []byte) (x, y *big.Int) {
+	tx1, ty1 := curve.toTwisted(x1, y1)
+	return curve.fromTwisted(curve.twisted.ScalarMult(tx1, ty1, scalar))
+}
+
+func (curve *rcurve) ScalarBaseMult(scalar []byte) (x, y *big.Int) {
+	return curve.fromTwisted(curve.twisted.ScalarBaseMult(scalar))
+}