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key.go
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key.go
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// Licensed to Elasticsearch B.V. under one or more contributor
// license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright
// ownership. Elasticsearch B.V. licenses this file to you under
// the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.
package x25519
import (
"crypto/rand"
"crypto/subtle"
"golang.org/x/crypto/curve25519"
)
// IsValidPublicKey returns true if corrent input could be a valid x25519 key.
func IsValidPublicKey(key []byte) bool {
// Check length
if len(key) != curve25519.PointSize {
return false
}
// Is blacklisted ?
if hasSmallOrder(key) {
return false
}
// Try to do DH
// Generate ephemeral key
ephemeral := make([]byte, curve25519.ScalarSize)
if _, err := rand.Read(ephemeral); err != nil {
return false
}
_, err := curve25519.X25519(ephemeral, key)
return err == nil
}
// -----------------------------------------------------------------------------
// use the same blacklist as lib sodium (see https://eprint.iacr.org/2017/806.pdf for reference):
// https://github.com/jedisct1/libsodium/blob/536ed00d2c5e0c65ac01e29141d69a30455f2038/src/libsodium/crypto_scalarmult/curve25519/ref10/x25519_ref10.c#L11-L17
var blacklist = [][32]byte{
// 0 (order 4)
{
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
},
// 1 (order 1)
{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
},
// 325606250916557431795983626356110631294008115727848805560023387167927233504
// (order 8)
{
0xe0, 0xeb, 0x7a, 0x7c, 0x3b, 0x41, 0xb8, 0xae, 0x16, 0x56, 0xe3,
0xfa, 0xf1, 0x9f, 0xc4, 0x6a, 0xda, 0x09, 0x8d, 0xeb, 0x9c, 0x32,
0xb1, 0xfd, 0x86, 0x62, 0x05, 0x16, 0x5f, 0x49, 0xb8, 0x00,
},
// 39382357235489614581723060781553021112529911719440698176882885853963445705823
// (order 8)
{
0x5f, 0x9c, 0x95, 0xbc, 0xa3, 0x50, 0x8c, 0x24, 0xb1, 0xd0, 0xb1,
0x55, 0x9c, 0x83, 0xef, 0x5b, 0x04, 0x44, 0x5c, 0xc4, 0x58, 0x1c,
0x8e, 0x86, 0xd8, 0x22, 0x4e, 0xdd, 0xd0, 0x9f, 0x11, 0x57,
},
// p-1 (order 2)
{
0xec, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f,
},
// p (=0, order 4)
{
0xed, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f,
},
// p+1 (=1, order 1)
{
0xee, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f,
},
}
func hasSmallOrder(pubKey []byte) bool {
isSmallOrderPoint := false
for _, bl := range blacklist {
if subtle.ConstantTimeCompare(pubKey, bl[:]) == 1 {
isSmallOrderPoint = true
break
}
}
return isSmallOrderPoint
}