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crypto.go
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crypto.go
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// Copyright (c) 2020 RethinkDNS and its authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
//
// This file incorporates work covered by the following copyright and
// permission notice:
//
// ISC License
//
// Copyright (c) 2018-2021
// Frank Denis <j at pureftpd dot org>
package dnscrypt
import (
"bytes"
crypto_rand "crypto/rand"
"github.com/celzero/firestack/intra/log"
"github.com/celzero/firestack/intra/xdns"
"github.com/jedisct1/xsecretbox"
"golang.org/x/crypto/nacl/box"
"golang.org/x/crypto/nacl/secretbox"
)
const (
// NonceSize is what the name suggests
NonceSize = 24
// TagSize is what the name suggests
TagSize = 16
// HalfNonceSize is half of NonceSize
HalfNonceSize = NonceSize / 2
// PublicKeySize is the size of a public key
PublicKeySize = 32
// QueryOverhead is the amount of request overhead due to client-magic, public key, nonce, and tag
QueryOverhead = xdns.ClientMagicLen + PublicKeySize + HalfNonceSize + TagSize
// ResponseOverhead is the amount of answer overhead due to server-magic, nonce, and tag
ResponseOverhead = len(xdns.ServerMagic) + NonceSize + TagSize
)
func pad(packet []byte, minSize int) []byte {
packet = append(packet, 0x80)
for len(packet) < minSize {
packet = append(packet, 0)
}
return packet
}
func unpad(packet []byte) ([]byte, error) {
for i := len(packet); ; {
if i == 0 { // short packet
return nil, errIncorrectPad
}
i--
if packet[i] == 0x80 {
return packet[:i], nil
} else if packet[i] != 0x00 { // delimiter not found
return nil, errIncorrectPad
}
}
}
func computeSharedKey(cryptoConstruction xdns.CryptoConstruction, secretKey *[32]byte, serverPk *[32]byte, providerName *string) (sharedKey [32]byte) {
if cryptoConstruction == xdns.XChacha20Poly1305 {
var err error
sharedKey, err = xsecretbox.SharedKey(*secretKey, *serverPk)
if err != nil {
log.W("dnscrypt: [%v] Weak public key", providerName)
}
} else {
box.Precompute(&sharedKey, serverPk, secretKey)
}
return
}
func encrypt(
serverInfo *serverinfo,
packet []byte,
useudp bool,
) (sharedKey *[32]byte, encrypted []byte, clientNonce []byte, err error) {
nonce := make([]byte, NonceSize)
clientNonce = make([]byte, HalfNonceSize)
if _, err = crypto_rand.Read(clientNonce); err != nil {
return
}
copy(nonce, clientNonce)
var publicKey *[PublicKeySize]byte
sharedKey = &serverInfo.SharedKey
publicKey = serverInfo.ClientPubKey
var paddedLength int
if useudp { // using udp
paddedLength = xdns.MaxDNSUDPSafePacketSize
} else if len(serverInfo.RelayTCPAddrs) > 0 { // tcp, with relay
paddedLength = xdns.MaxDNSPacketSize
} else { // tcp, without relay
minQuestionSize := QueryOverhead + len(packet)
// random pad if tcp without relay
var xpad [1]byte
if _, err = crypto_rand.Read(xpad[:]); err != nil {
return
}
minQuestionSize += int(xpad[0])
paddedLength = xdns.Min(xdns.MaxDNSUDPPacketSize, (xdns.Max(minQuestionSize, QueryOverhead)+1+63) & ^63)
}
if QueryOverhead+len(packet)+1 > paddedLength {
err = errQueryTooLarge
return
}
encrypted = append(serverInfo.MagicQuery[:], publicKey[:]...)
encrypted = append(encrypted, nonce[:HalfNonceSize]...)
padded := pad(packet, paddedLength-QueryOverhead)
if serverInfo.CryptoConstruction == xdns.XChacha20Poly1305 {
encrypted = xsecretbox.Seal(encrypted, nonce, padded, sharedKey[:])
} else {
var xsalsaNonce [24]byte
copy(xsalsaNonce[:], nonce)
encrypted = secretbox.Seal(encrypted, padded, &xsalsaNonce, sharedKey)
}
return
}
func decrypt(serverInfo *serverinfo, sharedKey *[32]byte, encrypted []byte, nonce []byte) ([]byte, error) {
serverMagicLen := len(xdns.ServerMagic)
responseHeaderLen := serverMagicLen + NonceSize
if len(encrypted) < responseHeaderLen+TagSize+int(xdns.MinDNSPacketSize) ||
len(encrypted) > responseHeaderLen+TagSize+int(xdns.MaxDNSPacketSize) ||
!bytes.Equal(encrypted[:serverMagicLen], xdns.ServerMagic[:]) {
return encrypted, errInvalidResponse
}
serverNonce := encrypted[serverMagicLen:responseHeaderLen]
if !bytes.Equal(nonce[:HalfNonceSize], serverNonce[:HalfNonceSize]) {
return encrypted, errNonceUnexpected
}
var packet []byte
var err error
if serverInfo.CryptoConstruction == xdns.XChacha20Poly1305 {
packet, err = xsecretbox.Open(nil, serverNonce, encrypted[responseHeaderLen:], sharedKey[:])
} else {
var xsalsaServerNonce [24]byte
copy(xsalsaServerNonce[:], serverNonce)
var ok bool
packet, ok = secretbox.Open(nil, encrypted[responseHeaderLen:], &xsalsaServerNonce, sharedKey)
if !ok {
err = errIncorrectTag
}
}
if err != nil {
return encrypted, err
}
if len(packet) <= 0 {
return encrypted, errInvalidResponse
}
packet, err = unpad(packet)
if err != nil || len(packet) < xdns.MinDNSPacketSize {
return encrypted, errIncorrectPad
}
return packet, nil
}