Separate GF^64 and GF^128 multiplier implementations

[gogost.git] / mgm / mul128.go

diff --git a/mgm/mul128.go b/mgm/mul128.go
new file mode 100644 (file)
index 0000000..07f105b
--- /dev/null
+++ b/mgm/mul128.go
@@ -0,0 +1,68 @@
+// GoGOST -- Pure Go GOST cryptographic functions library
+// Copyright (C) 2015-2021 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 mgm
+
+import (
+       "math/big"
+)
+
+const Mul128MaxBit = 128 - 1
+
+var R128 = big.NewInt(0).SetBytes([]byte{
+       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x87,
+})
+
+type mul128 struct {
+       x   *big.Int
+       y   *big.Int
+       z   *big.Int
+       buf [16]byte
+}
+
+func newMul128() *mul128 {
+       return &mul128{
+               x: big.NewInt(0),
+               y: big.NewInt(0),
+               z: big.NewInt(0),
+       }
+}
+
+func (mul *mul128) Mul(x, y []byte) []byte {
+       mul.x.SetBytes(x)
+       mul.y.SetBytes(y)
+       mul.z.SetInt64(0)
+       for mul.y.BitLen() != 0 {
+               if mul.y.Bit(0) == 1 {
+                       mul.z.Xor(mul.z, mul.x)
+               }
+               if mul.x.Bit(Mul128MaxBit) == 1 {
+                       mul.x.SetBit(mul.x, Mul128MaxBit, 0)
+                       mul.x.Lsh(mul.x, 1)
+                       mul.x.Xor(mul.x, R128)
+               } else {
+                       mul.x.Lsh(mul.x, 1)
+               }
+               mul.y.Rsh(mul.y, 1)
+       }
+       zBytes := mul.z.Bytes()
+       rem := len(x) - len(zBytes)
+       for i := 0; i < rem; i++ {
+               mul.buf[i] = 0
+       }
+       copy(mul.buf[rem:], zBytes)
+       return mul.buf[:]
+}