// Copyright (C) 2014 The Syncthing 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/.

package main

import (
	"bufio"
	"crypto/rand"
	"crypto/rsa"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"io"
	"math/big"
	mr "math/rand"
	"net"
	"os"
	"time"
)

const (
	tlsRSABits           = 3072
	tlsDefaultCommonName = "syncthing"
)

func newCertificate(certFile, keyFile, name string) (tls.Certificate, error) {
	l.Infof("Generating RSA key and certificate for %s...", name)

	priv, err := rsa.GenerateKey(rand.Reader, tlsRSABits)
	if err != nil {
		l.Fatalln("generate key:", err)
	}

	notBefore := time.Now()
	notAfter := time.Date(2049, 12, 31, 23, 59, 59, 0, time.UTC)

	template := x509.Certificate{
		SerialNumber: new(big.Int).SetInt64(mr.Int63()),
		Subject: pkix.Name{
			CommonName: name,
		},
		NotBefore: notBefore,
		NotAfter:  notAfter,

		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageClientAuth},
		BasicConstraintsValid: true,
	}

	derBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, &priv.PublicKey, priv)
	if err != nil {
		l.Fatalln("create cert:", err)
	}

	certOut, err := os.Create(certFile)
	if err != nil {
		l.Fatalln("save cert:", err)
	}
	err = pem.Encode(certOut, &pem.Block{Type: "CERTIFICATE", Bytes: derBytes})
	if err != nil {
		l.Fatalln("save cert:", err)
	}
	err = certOut.Close()
	if err != nil {
		l.Fatalln("save cert:", err)
	}

	keyOut, err := os.OpenFile(keyFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		l.Fatalln("save key:", err)
	}
	err = pem.Encode(keyOut, &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(priv)})
	if err != nil {
		l.Fatalln("save key:", err)
	}
	err = keyOut.Close()
	if err != nil {
		l.Fatalln("save key:", err)
	}

	return tls.LoadX509KeyPair(certFile, keyFile)
}

type DowngradingListener struct {
	net.Listener
	TLSConfig *tls.Config
}

type WrappedConnection struct {
	io.Reader
	net.Conn
}

func (l *DowngradingListener) Accept() (net.Conn, error) {
	conn, err := l.Listener.Accept()
	if err != nil {
		return nil, err
	}

	br := bufio.NewReader(conn)
	bs, err := br.Peek(1)
	if err != nil {
		// We hit a read error here, but the Accept() call succeeded so we must not return an error.
		// We return the connection as is and let whoever tries to use it deal with the error.
		return conn, nil
	}

	wrapper := &WrappedConnection{br, conn}

	// 0x16 is the first byte of a TLS handshake
	if bs[0] == 0x16 {
		return tls.Server(wrapper, l.TLSConfig), nil
	}

	return wrapper, nil
}

func (c *WrappedConnection) Read(b []byte) (n int, err error) {
	return c.Reader.Read(b)
}
