utls/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 tls

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"crypto/x509"
	"errors"
	"io"

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

// A SessionState is a resumable session.
type SessionState struct {
	// Encoded as a SessionState (in the language of RFC 8446, Section 3).
	//
	//   enum { server(1), client(2) } SessionStateType;
	//
	//   opaque Certificate<1..2^24-1>;
	//
	//   Certificate CertificateChain<0..2^24-1>;
	//
	//   struct {
	//       uint16 version;
	//       SessionStateType type;
	//       uint16 cipher_suite;
	//       uint64 created_at;
	//       opaque secret<1..2^8-1>;
	//       CertificateEntry certificate_list<0..2^24-1>;
	//       select (SessionState.type) {
	//           case server: /* empty */;
	//           case client: struct {
	//               CertificateChain verified_chains<0..2^24-1>; /* excluding leaf */
	//               select (SessionState.version) {
	//                   case VersionTLS10..VersionTLS12: /* empty */;
	//                   case VersionTLS13: struct {
	//                       uint64 use_by;
	//                       uint32 age_add;
	//                   };
	//               };
	//           };
	//       };
	//   } SessionState;
	//

	version     uint16
	isClient    bool
	cipherSuite uint16
	// createdAt is the generation time of the secret on the sever (which for
	// TLS 1.0–1.2 might be earlier than the current session) and the time at
	// which the ticket was received on the client.
	createdAt         uint64 // seconds since UNIX epoch
	secret            []byte // master secret for TLS 1.2, or the PSK for TLS 1.3
	peerCertificates  []*x509.Certificate
	activeCertHandles []*activeCert
	ocspResponse      []byte
	scts              [][]byte

	// Client-side fields.
	verifiedChains [][]*x509.Certificate

	// Client-side TLS 1.3-only fields.
	useBy  uint64 // seconds since UNIX epoch
	ageAdd uint32
}

// Bytes encodes the session, including any private fields, so that it can be
// parsed by [ParseSessionState]. The encoding contains secret values critical
// to the security of future and possibly past sessions.
//
// The specific encoding should be considered opaque and may change incompatibly
// between Go versions.
func (s *SessionState) Bytes() ([]byte, error) {
	var b cryptobyte.Builder
	b.AddUint16(s.version)
	if s.isClient {
		b.AddUint8(2) // client
	} else {
		b.AddUint8(1) // server
	}
	b.AddUint16(s.cipherSuite)
	addUint64(&b, s.createdAt)
	b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes(s.secret)
	})
	marshalCertificate(&b, s.certificate())
	if s.isClient {
		b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
			for _, chain := range s.verifiedChains {
				b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
					// We elide the first certificate because it's always the leaf.
					if len(chain) == 0 {
						b.SetError(errors.New("tls: internal error: empty verified chain"))
						return
					}
					for _, cert := range chain[1:] {
						b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
							b.AddBytes(cert.Raw)
						})
					}
				})
			}
		})
		if s.version >= VersionTLS13 {
			addUint64(&b, s.useBy)
			b.AddUint32(s.ageAdd)
		}
	}
	return b.Bytes()
}

func (s *SessionState) certificate() Certificate {
	return Certificate{
		Certificate:                 certificatesToBytesSlice(s.peerCertificates),
		OCSPStaple:                  s.ocspResponse,
		SignedCertificateTimestamps: s.scts,
	}
}

func certificatesToBytesSlice(certs []*x509.Certificate) [][]byte {
	s := make([][]byte, 0, len(certs))
	for _, c := range certs {
		s = append(s, c.Raw)
	}
	return s
}

// ParseSessionState parses a [SessionState] encoded by [SessionState.Bytes].
func ParseSessionState(data []byte) (*SessionState, error) {
	ss := &SessionState{}
	s := cryptobyte.String(data)
	var typ uint8
	var cert Certificate
	if !s.ReadUint16(&ss.version) ||
		!s.ReadUint8(&typ) ||
		(typ != 1 && typ != 2) ||
		!s.ReadUint16(&ss.cipherSuite) ||
		!readUint64(&s, &ss.createdAt) ||
		!readUint8LengthPrefixed(&s, &ss.secret) ||
		len(ss.secret) == 0 ||
		!unmarshalCertificate(&s, &cert) {
		return nil, errors.New("tls: invalid session encoding")
	}
	for _, cert := range cert.Certificate {
		c, err := globalCertCache.newCert(cert)
		if err != nil {
			return nil, err
		}
		ss.activeCertHandles = append(ss.activeCertHandles, c)
		ss.peerCertificates = append(ss.peerCertificates, c.cert)
	}
	ss.ocspResponse = cert.OCSPStaple
	ss.scts = cert.SignedCertificateTimestamps
	if isClient := typ == 2; !isClient {
		if !s.Empty() {
			return nil, errors.New("tls: invalid session encoding")
		}
		return ss, nil
	}
	ss.isClient = true
	if len(ss.peerCertificates) == 0 {
		return nil, errors.New("tls: no server certificates in client session")
	}
	var chainList cryptobyte.String
	if !s.ReadUint24LengthPrefixed(&chainList) {
		return nil, errors.New("tls: invalid session encoding")
	}
	for !chainList.Empty() {
		var certList cryptobyte.String
		if !chainList.ReadUint24LengthPrefixed(&certList) {
			return nil, errors.New("tls: invalid session encoding")
		}
		var chain []*x509.Certificate
		chain = append(chain, ss.peerCertificates[0])
		for !certList.Empty() {
			var cert []byte
			if !readUint24LengthPrefixed(&certList, &cert) {
				return nil, errors.New("tls: invalid session encoding")
			}
			c, err := globalCertCache.newCert(cert)
			if err != nil {
				return nil, err
			}
			ss.activeCertHandles = append(ss.activeCertHandles, c)
			chain = append(chain, c.cert)
		}
		ss.verifiedChains = append(ss.verifiedChains, chain)
	}
	if ss.version < VersionTLS13 {
		if !s.Empty() {
			return nil, errors.New("tls: invalid session encoding")
		}
		return ss, nil
	}
	if !s.ReadUint64(&ss.useBy) || !s.ReadUint32(&ss.ageAdd) || !s.Empty() {
		return nil, errors.New("tls: invalid session encoding")
	}
	return ss, nil
}

// sessionState returns a partially filled-out [SessionState] with information
// from the current connection.
func (c *Conn) sessionState() (*SessionState, error) {
	var verifiedChains [][]*x509.Certificate
	if c.isClient {
		verifiedChains = c.verifiedChains
		if len(c.peerCertificates) == 0 {
			return nil, errors.New("tls: internal error: empty peer certificates")
		}
	}
	return &SessionState{
		version:           c.vers,
		cipherSuite:       c.cipherSuite,
		createdAt:         uint64(c.config.time().Unix()),
		peerCertificates:  c.peerCertificates,
		activeCertHandles: c.activeCertHandles,
		ocspResponse:      c.ocspResponse,
		scts:              c.scts,
		isClient:          c.isClient,
		verifiedChains:    verifiedChains,
	}, nil
}

// EncryptTicket encrypts a ticket with the Config's configured (or default)
// session ticket keys. It can be used as a [Config.WrapSession] implementation.
func (c *Config) EncryptTicket(cs ConnectionState, ss *SessionState) ([]byte, error) {
	ticketKeys := c.ticketKeys(nil)
	stateBytes, err := ss.Bytes()
	if err != nil {
		return nil, err
	}
	return c.encryptTicket(stateBytes, ticketKeys)
}

var _ = &Config{WrapSession: (&Config{}).EncryptTicket}

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

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

	if _, err := io.ReadFull(c.rand(), iv); err != nil {
		return nil, err
	}
	key := ticketKeys[0]
	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(ciphertext, state)

	mac := hmac.New(sha256.New, key.hmacKey[:])
	mac.Write(authenticated)
	mac.Sum(macBytes[:0])

	return encrypted, nil
}

// DecryptTicket decrypts a ticket encrypted by [Config.EncryptTicket]. It can
// be used as a [Config.UnwrapSession] implementation.
//
// If the ticket can't be decrypted or parsed, DecryptTicket returns (nil, nil).
func (c *Config) DecryptTicket(identity []byte, cs ConnectionState) (*SessionState, error) {
	ticketKeys := c.ticketKeys(nil)
	stateBytes := c.decryptTicket(identity, ticketKeys)
	if stateBytes == nil {
		return nil, nil
	}
	s, err := ParseSessionState(stateBytes)
	if err != nil {
		return nil, nil // drop unparsable tickets on the floor
	}
	return s, nil
}

var _ = &Config{UnwrapSession: (&Config{}).DecryptTicket}

func (c *Config) decryptTicket(encrypted []byte, ticketKeys []ticketKey) []byte {
	if len(encrypted) < aes.BlockSize+sha256.Size {
		return nil
	}

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

	for _, key := range ticketKeys {
		mac := hmac.New(sha256.New, key.hmacKey[:])
		mac.Write(authenticated)
		expected := mac.Sum(nil)

		if subtle.ConstantTimeCompare(macBytes, expected) != 1 {
			continue
		}

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

		return plaintext
	}

	return nil
}

// ClientSessionState contains the state needed by a client to
// resume a previous TLS session.
type ClientSessionState struct {
	ticket  []byte
	session *SessionState
}

// ResumptionState returns the session ticket sent by the server (also known as
// the session's identity) and the state necessary to resume this session.
//
// It can be called by [ClientSessionCache.Put] to serialize (with
// [SessionState.Bytes]) and store the session.
func (cs *ClientSessionState) ResumptionState() (ticket []byte, state *SessionState, err error) {
	return cs.ticket, cs.session, nil
}

// NewResumptionState returns a state value that can be returned by
// [ClientSessionCache.Get] to resume a previous session.
//
// state needs to be returned by [ParseSessionState], and the ticket and session
// state must have been returned by [ClientSessionState.ResumptionState].
func NewResumptionState(ticket []byte, state *SessionState) (*ClientSessionState, error) {
	return &ClientSessionState{
		ticket: ticket, session: state,
	}, nil
}