psiphon-tunnel-core/psiphon/tlsDialer.go

/*
 * Copyright (c) 2015, Psiphon Inc.
 * All rights reserved.
 *
 * 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, either version 3 of the License, or
 * (at your option) any later version.
 *
 * 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/>.
 *
 */

/*
Copyright (c) 2012 The Go Authors. All rights reserved.

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modification, are permitted provided that the following conditions are
met:

   * Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following disclaimer
in the documentation and/or other materials provided with the
distribution.
   * Neither the name of Google Inc. nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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*/

// Fork of https://github.com/getlantern/tlsdialer (http://gopkg.in/getlantern/tlsdialer.v1)
// which itself is a "Fork of crypto/tls.Dial and DialWithDialer"

// Adds two capabilities to tlsdialer:
//
// 1. HTTP proxy support, so the dialer may be used with http.Transport.
//
// 2. Support for self-signed Psiphon server certificates, which Go's certificate
//    verification rejects due to two short comings:
//    - lack of IP address SANs.
//      see: "...because it doesn't contain any IP SANs" case in crypto/x509/verify.go
//    - non-compliant constraint configuration (RFC 5280, 4.2.1.9).
//      see: CheckSignatureFrom() in crypto/x509/x509.go
//    Since the client has to be able to handle existing Psiphon server certificates,
//    we need to be able to perform some form of verification in these cases.

// tlsdialer:
// package tlsdialer contains a customized version of crypto/tls.Dial that
// allows control over whether or not to send the ServerName extension in the
// client handshake.

package psiphon

import (
	"bytes"
	"crypto/x509"
	"encoding/hex"
	"errors"
	"net"
	"time"

	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/tls"
)

// CustomTLSConfig contains parameters to determine the behavior
// of CustomTLSDial.
type CustomTLSConfig struct {

	// Dial is the network connection dialer. TLS is layered on
	// top of a new network connection created with dialer.
	Dial Dialer

	// Timeout is and optional timeout for combined network
	// connection dial and TLS handshake.
	Timeout time.Duration

	// DialAddr overrides the "addr" input to Dial when specified
	DialAddr string

	// SNIServerName specifies the value to set in the SNI
	// server_name field. When blank, SNI is omitted. Note that
	// underlying TLS code also automatically omits SNI when
	// the server_name is an IP address.
	SNIServerName string

	// SkipVerify completely disables server certificate verification.
	SkipVerify bool

	// VerifyLegacyCertificate is a special case self-signed server
	// certificate case. Ignores IP SANs and basic constraints. No
	// certificate chain. Just checks that the server presented the
	// specified certificate. SNI is disbled when this is set.
	VerifyLegacyCertificate *x509.Certificate

	// UseIndistinguishableTLS specifies whether to try to use an
	// alternative stack for TLS. From a circumvention perspective,
	// Go's TLS has a distinct fingerprint that may be used for blocking.
	UseIndistinguishableTLS bool

	// TrustedCACertificatesFilename specifies a file containing trusted
	// CA certs. Directory contents should be compatible with OpenSSL's
	// SSL_CTX_load_verify_locations
	// Only applies to UseIndistinguishableTLS connections.
	TrustedCACertificatesFilename string

	// ObfuscatedSessionTicketKey enables obfuscated session tickets
	// using the specified key.
	ObfuscatedSessionTicketKey string
}

func NewCustomTLSDialer(config *CustomTLSConfig) Dialer {
	return func(network, addr string) (net.Conn, error) {
		return CustomTLSDial(network, addr, config)
	}
}

// handshakeConn is a net.Conn that can perform a TLS handshake
type handshakeConn interface {
	net.Conn
	Handshake() error
}

// CustomTLSDialWithDialer is a customized replacement for tls.Dial.
// Based on tlsdialer.DialWithDialer which is based on crypto/tls.DialWithDialer.
//
// tlsdialer comment:
//   Note - if sendServerName is false, the VerifiedChains field on the
//   connection's ConnectionState will never get populated.
func CustomTLSDial(network, addr string, config *CustomTLSConfig) (net.Conn, error) {

	// We want the Timeout and Deadline values from dialer to cover the
	// whole process: TCP connection and TLS handshake. This means that we
	// also need to start our own timers now.
	var errChannel chan error
	if config.Timeout != 0 {
		errChannel = make(chan error, 2)
		time.AfterFunc(config.Timeout, func() {
			errChannel <- TimeoutError{}
		})
	}

	dialAddr := addr
	if config.DialAddr != "" {
		dialAddr = config.DialAddr
	}

	rawConn, err := config.Dial(network, dialAddr)
	if err != nil {
		return nil, common.ContextError(err)
	}

	hostname, _, err := net.SplitHostPort(dialAddr)
	if err != nil {
		rawConn.Close()
		return nil, common.ContextError(err)
	}

	tlsConfig := &tls.Config{}

	if config.SkipVerify {
		tlsConfig.InsecureSkipVerify = true
	}

	if config.SNIServerName != "" && config.VerifyLegacyCertificate == nil {
		// Set the ServerName and rely on the usual logic in
		// tls.Conn.Handshake() to do its verification.
		// Note: Go TLS will automatically omit this ServerName when it's an IP address
		if net.ParseIP(hostname) == nil {
			tlsConfig.ServerName = config.SNIServerName
		}
	} else {
		// No SNI.
		// Disable verification in tls.Conn.Handshake().  We'll verify manually
		// after handshaking
		tlsConfig.InsecureSkipVerify = true
	}

	if config.ObfuscatedSessionTicketKey != "" {

		// See obfuscated session ticket overview
		// in tls.NewObfuscatedClientSessionCache

		var obfuscatedSessionTicketKey [32]byte
		key, err := hex.DecodeString(config.ObfuscatedSessionTicketKey)
		if err == nil && len(key) != 32 {
			err = errors.New("invalid obfuscated session key length")
		}
		if err != nil {
			return nil, common.ContextError(err)
		}
		copy(obfuscatedSessionTicketKey[:], key)

		tlsConfig.ClientSessionCache = tls.NewObfuscatedClientSessionCache(
			obfuscatedSessionTicketKey)
	}

	var conn handshakeConn

	// When supported, use OpenSSL TLS as a more indistinguishable TLS.
	if config.UseIndistinguishableTLS &&
		config.ObfuscatedSessionTicketKey == "" &&
		(config.SkipVerify ||
			// TODO: config.VerifyLegacyCertificate != nil ||
			config.TrustedCACertificatesFilename != "") {

		conn, err = newOpenSSLConn(rawConn, hostname, config)
		if err != nil {
			rawConn.Close()
			return nil, common.ContextError(err)
		}
	} else {
		conn = tls.Client(rawConn, tlsConfig)
	}

	if config.Timeout == 0 {
		err = conn.Handshake()
	} else {
		go func() {
			errChannel <- conn.Handshake()
		}()
		err = <-errChannel
	}

	// openSSLConns complete verification automatically. For Go TLS,
	// we need to complete the process from crypto/tls.Dial.

	// NOTE: for (config.SendServerName && !config.tlsConfig.InsecureSkipVerify),
	// the tls.Conn.Handshake() does the complete verification, including host name.
	tlsConn, isTlsConn := conn.(*tls.Conn)
	if err == nil && isTlsConn &&
		!config.SkipVerify && tlsConfig.InsecureSkipVerify {

		if config.VerifyLegacyCertificate != nil {
			err = verifyLegacyCertificate(tlsConn, config.VerifyLegacyCertificate)
		} else {
			// Manually verify certificates
			err = verifyServerCerts(tlsConn, hostname, tlsConfig)
		}
	}

	if err != nil {
		rawConn.Close()
		return nil, common.ContextError(err)
	}

	return conn, nil
}

func verifyLegacyCertificate(conn *tls.Conn, expectedCertificate *x509.Certificate) error {
	certs := conn.ConnectionState().PeerCertificates
	if len(certs) < 1 {
		return common.ContextError(errors.New("no certificate to verify"))
	}
	if !bytes.Equal(certs[0].Raw, expectedCertificate.Raw) {
		return common.ContextError(errors.New("unexpected certificate"))
	}
	return nil
}

func verifyServerCerts(conn *tls.Conn, hostname string, config *tls.Config) error {
	certs := conn.ConnectionState().PeerCertificates

	opts := x509.VerifyOptions{
		Roots:         config.RootCAs,
		CurrentTime:   time.Now(),
		DNSName:       hostname,
		Intermediates: x509.NewCertPool(),
	}

	for i, cert := range certs {
		if i == 0 {
			continue
		}
		opts.Intermediates.AddCert(cert)
	}

	_, err := certs[0].Verify(opts)
	if err != nil {
		return common.ContextError(err)
	}
	return nil
}