go.cypherpunks.ru namespace usage

[gogost.git] / gost3410 / edwards.go

diff --git a/gost3410/edwards.go b/gost3410/edwards.go
new file mode 100644 (file)
index 0000000..10b1c6a
--- /dev/null
+++ b/gost3410/edwards.go
@@ -0,0 +1,92 @@
+// GoGOST -- Pure Go GOST cryptographic functions library
+// Copyright (C) 2015-2019 Sergey Matveev <stargrave@stargrave.org>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, version 3 of the License.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package gost3410
+
+import (
+       "math/big"
+)
+
+func (c *Curve) IsEdwards() bool {
+       return c.E != nil
+}
+
+func (c *Curve) EdwardsST() (*big.Int, *big.Int) {
+       if c.edS != nil {
+               return c.edS, c.edT
+       }
+       c.edS = big.NewInt(0)
+       c.edS.Set(c.E)
+       c.edS.Sub(c.edS, c.D)
+       c.pos(c.edS)
+       c.t.SetUint64(4)
+       c.t.ModInverse(c.t, c.P)
+       c.edS.Mul(c.edS, c.t)
+       c.edS.Mod(c.edS, c.P)
+       c.edT = big.NewInt(0)
+       c.edT.Set(c.E)
+       c.edT.Add(c.edT, c.D)
+       c.t.SetUint64(6)
+       c.t.ModInverse(c.t, c.P)
+       c.edT.Mul(c.edT, c.t)
+       c.edT.Mod(c.edT, c.P)
+       return c.edS, c.edT
+}
+
+// Convert Weierstrass X,Y coordinates to twisted Edwards U,V
+func XY2UV(curve *Curve, x, y *big.Int) (*big.Int, *big.Int) {
+       if !curve.IsEdwards() {
+               panic("non twisted Edwards curve")
+       }
+       edS, edT := curve.EdwardsST()
+       curve.t.Sub(x, edT)
+       curve.pos(curve.t)
+       u := big.NewInt(0)
+       u.ModInverse(y, curve.P)
+       u.Mul(u, curve.t)
+       u.Mod(u, curve.P)
+       v := big.NewInt(0).Set(curve.t)
+       v.Sub(v, edS)
+       curve.pos(v)
+       curve.t.Add(curve.t, edS)
+       curve.t.ModInverse(curve.t, curve.P)
+       v.Mul(v, curve.t)
+       v.Mod(v, curve.P)
+       return u, v
+}
+
+// Convert twisted Edwards U,V coordinates to Weierstrass X,Y
+func UV2XY(curve *Curve, u, v *big.Int) (*big.Int, *big.Int) {
+       if !curve.IsEdwards() {
+               panic("non twisted Edwards curve")
+       }
+       edS, edT := curve.EdwardsST()
+       curve.tx.Add(bigInt1, v)
+       curve.tx.Mul(curve.tx, edS)
+       curve.tx.Mod(curve.tx, curve.P)
+       curve.ty.Sub(bigInt1, v)
+       curve.pos(curve.ty)
+       x := big.NewInt(0)
+       x.ModInverse(curve.ty, curve.P)
+       x.Mul(x, curve.tx)
+       x.Add(x, edT)
+       x.Mod(x, curve.P)
+       y := big.NewInt(0)
+       y.Mul(u, curve.ty)
+       y.ModInverse(y, curve.P)
+       y.Mul(y, curve.tx)
+       y.Mod(y, curve.P)
+       return x, y
+}