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// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build amd64 || arm64
package nistec_test
import (
"bytes"
"crypto/elliptic"
"crypto/internal/nistec"
"math/big"
"testing"
)
func TestP256OrdInverse(t *testing.T) {
N := elliptic.P256().Params().N
// inv(0) is expected to be 0.
zero := make([]byte, 32)
out, err := nistec.P256OrdInverse(zero)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, zero) {
t.Error("unexpected output for inv(0)")
}
// inv(N) is also 0 mod N.
input := make([]byte, 32)
N.FillBytes(input)
out, err = nistec.P256OrdInverse(input)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, zero) {
t.Error("unexpected output for inv(N)")
}
if !bytes.Equal(input, N.Bytes()) {
t.Error("input was modified")
}
// Check inv(1) and inv(N+1) against math/big
exp := new(big.Int).ModInverse(big.NewInt(1), N).FillBytes(make([]byte, 32))
big.NewInt(1).FillBytes(input)
out, err = nistec.P256OrdInverse(input)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, exp) {
t.Error("unexpected output for inv(1)")
}
new(big.Int).Add(N, big.NewInt(1)).FillBytes(input)
out, err = nistec.P256OrdInverse(input)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, exp) {
t.Error("unexpected output for inv(N+1)")
}
// Check inv(20) and inv(N+20) against math/big
exp = new(big.Int).ModInverse(big.NewInt(20), N).FillBytes(make([]byte, 32))
big.NewInt(20).FillBytes(input)
out, err = nistec.P256OrdInverse(input)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, exp) {
t.Error("unexpected output for inv(20)")
}
new(big.Int).Add(N, big.NewInt(20)).FillBytes(input)
out, err = nistec.P256OrdInverse(input)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, exp) {
t.Error("unexpected output for inv(N+20)")
}
// Check inv(2^256-1) against math/big
bigInput := new(big.Int).Lsh(big.NewInt(1), 256)
bigInput.Sub(bigInput, big.NewInt(1))
exp = new(big.Int).ModInverse(bigInput, N).FillBytes(make([]byte, 32))
bigInput.FillBytes(input)
out, err = nistec.P256OrdInverse(input)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(out, exp) {
t.Error("unexpected output for inv(2^256-1)")
}
}
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