ticket.go

// Copyright 2012 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.

package smtls

import (
	"bytes"
	"crypto/aes"
	"crypto/cipher"
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/binary"
	"errors"
	"io"
	"time"

	"golang.org/x/crypto/cryptobyte"
)

// sessionState contains the information that is serialized into a session
// ticket in order to later resume a connection.
type sessionState struct {
	vers         uint16
	cipherSuite  uint16
	createdAt    uint64
	masterSecret []byte // opaque master_secret<1..2^16-1>;
	// struct { opaque certificate<1..2^24-1> } Certificate;
	certificates [][]byte // Certificate certificate_list<0..2^24-1>;

	// usedOldKey is true if the ticket from which this session came from
	// was encrypted with an older key and thus should be refreshed.
	usedOldKey bool
}

func (m *sessionState) marshal() ([]byte, error) {
	var b cryptobyte.Builder
	b.AddUint16(m.vers)
	b.AddUint16(m.cipherSuite)
	addUint64(&b, m.createdAt)
	b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes(m.masterSecret)
	})
	b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
		for _, cert := range m.certificates {
			b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
				b.AddBytes(cert)
			})
		}
	})
	return b.Bytes()
}

func (m *sessionState) unmarshal(data []byte) bool {
	*m = sessionState{usedOldKey: m.usedOldKey}
	s := cryptobyte.String(data)
	if ok := s.ReadUint16(&m.vers) &&
		s.ReadUint16(&m.cipherSuite) &&
		readUint64(&s, &m.createdAt) &&
		readUint16LengthPrefixed(&s, &m.masterSecret) &&
		len(m.masterSecret) != 0; !ok {
		return false
	}
	var certList cryptobyte.String
	if !s.ReadUint24LengthPrefixed(&certList) {
		return false
	}
	for !certList.Empty() {
		var cert []byte
		if !readUint24LengthPrefixed(&certList, &cert) {
			return false
		}
		m.certificates = append(m.certificates, cert)
	}
	return s.Empty()
}

// sessionStateTLS13 is the content of a TLS 1.3 session ticket. Its first
// version (revision = 0) doesn't carry any of the information needed for 0-RTT
// validation and the nonce is always empty.
// version (revision = 1) carries the max_early_data_size sent in the ticket.
// version (revision = 2) carries the ALPN sent in the ticket.
type sessionStateTLS13 struct {
	// uint8 version  = 0x0304;
	// uint8 revision = 2;
	cipherSuite      uint16
	createdAt        uint64
	resumptionSecret []byte      // opaque resumption_master_secret<1..2^8-1>;
	certificate      Certificate // CertificateEntry certificate_list<0..2^24-1>;
	maxEarlyData     uint32
	alpn             string

	appData []byte
}

func (m *sessionStateTLS13) marshal() ([]byte, error) {
	var b cryptobyte.Builder
	b.AddUint16(VersionTLS13)
	b.AddUint8(2) // revision
	b.AddUint16(m.cipherSuite)
	addUint64(&b, m.createdAt)
	b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes(m.resumptionSecret)
	})
	marshalCertificate(&b, m.certificate)
	b.AddUint32(m.maxEarlyData)
	b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes([]byte(m.alpn))
	})
	b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes(m.appData)
	})
	return b.Bytes()
}

func (m *sessionStateTLS13) unmarshal(data []byte) bool {
	*m = sessionStateTLS13{}
	s := cryptobyte.String(data)
	var version uint16
	var revision uint8
	var alpn []byte
	ret := s.ReadUint16(&version) &&
		version == VersionTLS13 &&
		s.ReadUint8(&revision) &&
		revision == 2 &&
		s.ReadUint16(&m.cipherSuite) &&
		readUint64(&s, &m.createdAt) &&
		readUint8LengthPrefixed(&s, &m.resumptionSecret) &&
		len(m.resumptionSecret) != 0 &&
		unmarshalCertificate(&s, &m.certificate) &&
		s.ReadUint32(&m.maxEarlyData) &&
		readUint8LengthPrefixed(&s, &alpn) &&
		readUint16LengthPrefixed(&s, &m.appData) &&
		s.Empty()
	m.alpn = string(alpn)
	return ret
}

func (c *Conn) encryptTicket(state []byte) ([]byte, error) {
	if len(c.ticketKeys) == 0 {
		return nil, errors.New("tls: internal error: session ticket keys unavailable")
	}

	encrypted := make([]byte, ticketKeyNameLen+aes.BlockSize+len(state)+sha256.Size)
	keyName := encrypted[:ticketKeyNameLen]
	iv := encrypted[ticketKeyNameLen : ticketKeyNameLen+aes.BlockSize]
	macBytes := encrypted[len(encrypted)-sha256.Size:]

	if _, err := io.ReadFull(c.config.rand(), iv); err != nil {
		return nil, err
	}
	key := c.ticketKeys[0]
	copy(keyName, key.keyName[:])
	block, err := aes.NewCipher(key.aesKey[:])
	if err != nil {
		return nil, errors.New("tls: failed to create cipher while encrypting ticket: " + err.Error())
	}
	cipher.NewCTR(block, iv).XORKeyStream(encrypted[ticketKeyNameLen+aes.BlockSize:], state)

	mac := hmac.New(sha256.New, key.hmacKey[:])
	mac.Write(encrypted[:len(encrypted)-sha256.Size])
	mac.Sum(macBytes[:0])

	return encrypted, nil
}

func (c *Conn) decryptTicket(encrypted []byte) (plaintext []byte, usedOldKey bool) {
	if len(encrypted) < ticketKeyNameLen+aes.BlockSize+sha256.Size {
		return nil, false
	}

	keyName := encrypted[:ticketKeyNameLen]
	iv := encrypted[ticketKeyNameLen : ticketKeyNameLen+aes.BlockSize]
	macBytes := encrypted[len(encrypted)-sha256.Size:]
	ciphertext := encrypted[ticketKeyNameLen+aes.BlockSize : len(encrypted)-sha256.Size]

	keyIndex := -1
	for i, candidateKey := range c.ticketKeys {
		if bytes.Equal(keyName, candidateKey.keyName[:]) {
			keyIndex = i
			break
		}
	}
	if keyIndex == -1 {
		return nil, false
	}
	key := &c.ticketKeys[keyIndex]

	mac := hmac.New(sha256.New, key.hmacKey[:])
	mac.Write(encrypted[:len(encrypted)-sha256.Size])
	expected := mac.Sum(nil)

	if subtle.ConstantTimeCompare(macBytes, expected) != 1 {
		return nil, false
	}

	block, err := aes.NewCipher(key.aesKey[:])
	if err != nil {
		return nil, false
	}
	plaintext = make([]byte, len(ciphertext))
	cipher.NewCTR(block, iv).XORKeyStream(plaintext, ciphertext)

	return plaintext, keyIndex > 0
}

func (c *Conn) getSessionTicketMsg(appData []byte) (*newSessionTicketMsgTLS13, error) {
	m := new(newSessionTicketMsgTLS13)

	var certsFromClient [][]byte
	for _, cert := range c.peerCertificates {
		certsFromClient = append(certsFromClient, cert.Raw)
	}
	state := sessionStateTLS13{
		cipherSuite:      c.cipherSuite,
		createdAt:        uint64(c.config.time().Unix()),
		resumptionSecret: c.resumptionSecret,
		certificate: Certificate{
			Certificate:                 certsFromClient,
			OCSPStaple:                  c.ocspResponse,
			SignedCertificateTimestamps: c.scts,
		},
		appData: appData,
		alpn:    c.clientProtocol,
	}
	if c.extraConfig != nil {
		state.maxEarlyData = c.extraConfig.MaxEarlyData
	}
	stateBytes, err := state.marshal()
	if err != nil {
		return nil, err
	}
	m.label, err = c.encryptTicket(stateBytes)
	if err != nil {
		return nil, err
	}
	m.lifetime = uint32(maxSessionTicketLifetime / time.Second)

	// ticket_age_add is a random 32-bit value. See RFC 8446, section 4.6.1
	// The value is not stored anywhere; we never need to check the ticket age
	// because 0-RTT is not supported.
	ageAdd := make([]byte, 4)
	_, err = c.config.rand().Read(ageAdd)
	if err != nil {
		return nil, err
	}
	m.ageAdd = binary.LittleEndian.Uint32(ageAdd)

	// ticket_nonce, which must be unique per connection, is always left at
	// zero because we only ever send one ticket per connection.

	if c.extraConfig != nil {
		m.maxEarlyData = c.extraConfig.MaxEarlyData
	}
	return m, nil
}

// GetSessionTicket generates a new session ticket.
// It should only be called after the handshake completes.
// It can only be used for servers, and only if the alternative record layer is set.
// The ticket may be nil if config.SessionTicketsDisabled is set,
// or if the client isn't able to receive session tickets.
func (c *Conn) GetSessionTicket(appData []byte) ([]byte, error) {
	if c.isClient || !c.handshakeComplete() || c.extraConfig == nil || c.extraConfig.AlternativeRecordLayer == nil {
		return nil, errors.New("GetSessionTicket is only valid for servers after completion of the handshake, and if an alternative record layer is set.")
	}
	if c.config.SessionTicketsDisabled {
		return nil, nil
	}

	m, err := c.getSessionTicketMsg(appData)
	if err != nil {
		return nil, err
	}
	return m.marshal()
}