th5/common.go

// Copyright 2009 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 (
	"container/list"
	"crypto"
	"crypto/rand"
	"crypto/sha512"
	"crypto/x509"
	"errors"
	"fmt"
	"io"
	"math/big"
	"strings"
	"sync"
	"time"
)

const (
	VersionSSL30 = 0x0300
	VersionTLS10 = 0x0301
	VersionTLS11 = 0x0302
	VersionTLS12 = 0x0303
)

const (
	maxPlaintext    = 16384        // maximum plaintext payload length
	maxCiphertext   = 16384 + 2048 // maximum ciphertext payload length
	recordHeaderLen = 5            // record header length
	maxHandshake    = 65536        // maximum handshake we support (protocol max is 16 MB)

	minVersion = VersionTLS10
	maxVersion = VersionTLS12
)

// TLS record types.
type recordType uint8

const (
	recordTypeChangeCipherSpec recordType = 20
	recordTypeAlert            recordType = 21
	recordTypeHandshake        recordType = 22
	recordTypeApplicationData  recordType = 23
)

// TLS handshake message types.
const (
	typeHelloRequest       uint8 = 0
	typeClientHello        uint8 = 1
	typeServerHello        uint8 = 2
	typeNewSessionTicket   uint8 = 4
	typeCertificate        uint8 = 11
	typeServerKeyExchange  uint8 = 12
	typeCertificateRequest uint8 = 13
	typeServerHelloDone    uint8 = 14
	typeCertificateVerify  uint8 = 15
	typeClientKeyExchange  uint8 = 16
	typeFinished           uint8 = 20
	typeCertificateStatus  uint8 = 22
	typeNextProtocol       uint8 = 67 // Not IANA assigned
)

// TLS compression types.
const (
	compressionNone uint8 = 0
)

// TLS extension numbers
const (
	extensionServerName          uint16 = 0
	extensionStatusRequest       uint16 = 5
	extensionSupportedCurves     uint16 = 10
	extensionSupportedPoints     uint16 = 11
	extensionSignatureAlgorithms uint16 = 13
	extensionALPN                uint16 = 16
	extensionSCT                 uint16 = 18 // https://tools.ietf.org/html/rfc6962#section-6
	extensionSessionTicket       uint16 = 35
	extensionNextProtoNeg        uint16 = 13172 // not IANA assigned
	extensionRenegotiationInfo   uint16 = 0xff01
)

// TLS signaling cipher suite values
const (
	scsvRenegotiation uint16 = 0x00ff
)

// CurveID is the type of a TLS identifier for an elliptic curve. See
// http://www.iana.org/assignments/tls-parameters/tls-parameters.xml#tls-parameters-8
type CurveID uint16

const (
	CurveP256 CurveID = 23
	CurveP384 CurveID = 24
	CurveP521 CurveID = 25
)

// TLS Elliptic Curve Point Formats
// http://www.iana.org/assignments/tls-parameters/tls-parameters.xml#tls-parameters-9
const (
	pointFormatUncompressed uint8 = 0
)

// TLS CertificateStatusType (RFC 3546)
const (
	statusTypeOCSP uint8 = 1
)

// Certificate types (for certificateRequestMsg)
const (
	certTypeRSASign    = 1 // A certificate containing an RSA key
	certTypeDSSSign    = 2 // A certificate containing a DSA key
	certTypeRSAFixedDH = 3 // A certificate containing a static DH key
	certTypeDSSFixedDH = 4 // A certificate containing a static DH key

	// See RFC 4492 sections 3 and 5.5.
	certTypeECDSASign      = 64 // A certificate containing an ECDSA-capable public key, signed with ECDSA.
	certTypeRSAFixedECDH   = 65 // A certificate containing an ECDH-capable public key, signed with RSA.
	certTypeECDSAFixedECDH = 66 // A certificate containing an ECDH-capable public key, signed with ECDSA.

	// Rest of these are reserved by the TLS spec
)

// Hash functions for TLS 1.2 (See RFC 5246, section A.4.1)
const (
	hashSHA1   uint8 = 2
	hashSHA256 uint8 = 4
	hashSHA384 uint8 = 5
)

// Signature algorithms for TLS 1.2 (See RFC 5246, section A.4.1)
const (
	signatureRSA   uint8 = 1
	signatureECDSA uint8 = 3
)

// signatureAndHash mirrors the TLS 1.2, SignatureAndHashAlgorithm struct. See
// RFC 5246, section A.4.1.
type signatureAndHash struct {
	hash, signature uint8
}

// supportedSignatureAlgorithms contains the signature and hash algorithms that
// the code advertises as supported in a TLS 1.2 ClientHello and in a TLS 1.2
// CertificateRequest.
var supportedSignatureAlgorithms = []signatureAndHash{
	{hashSHA256, signatureRSA},
	{hashSHA256, signatureECDSA},
	{hashSHA384, signatureRSA},
	{hashSHA384, signatureECDSA},
	{hashSHA1, signatureRSA},
	{hashSHA1, signatureECDSA},
}

// ConnectionState records basic TLS details about the connection.
type ConnectionState struct {
	Version                     uint16                // TLS version used by the connection (e.g. VersionTLS12)
	HandshakeComplete           bool                  // TLS handshake is complete
	DidResume                   bool                  // connection resumes a previous TLS connection
	CipherSuite                 uint16                // cipher suite in use (TLS_RSA_WITH_RC4_128_SHA, ...)
	NegotiatedProtocol          string                // negotiated next protocol (from Config.NextProtos)
	NegotiatedProtocolIsMutual  bool                  // negotiated protocol was advertised by server
	ServerName                  string                // server name requested by client, if any (server side only)
	PeerCertificates            []*x509.Certificate   // certificate chain presented by remote peer
	VerifiedChains              [][]*x509.Certificate // verified chains built from PeerCertificates
	SignedCertificateTimestamps [][]byte              // SCTs from the server, if any
	OCSPResponse                []byte                // stapled OCSP response from server, if any

	// TLSUnique contains the "tls-unique" channel binding value (see RFC
	// 5929, section 3). For resumed sessions this value will be nil
	// because resumption does not include enough context (see
	// https://secure-resumption.com/#channelbindings). This will change in
	// future versions of Go once the TLS master-secret fix has been
	// standardized and implemented.
	TLSUnique []byte
}

// ClientAuthType declares the policy the server will follow for
// TLS Client Authentication.
type ClientAuthType int

const (
	NoClientCert ClientAuthType = iota
	RequestClientCert
	RequireAnyClientCert
	VerifyClientCertIfGiven
	RequireAndVerifyClientCert
)

// ClientSessionState contains the state needed by clients to resume TLS
// sessions.
type ClientSessionState struct {
	sessionTicket      []uint8               // Encrypted ticket used for session resumption with server
	vers               uint16                // SSL/TLS version negotiated for the session
	cipherSuite        uint16                // Ciphersuite negotiated for the session
	masterSecret       []byte                // MasterSecret generated by client on a full handshake
	serverCertificates []*x509.Certificate   // Certificate chain presented by the server
	verifiedChains     [][]*x509.Certificate // Certificate chains we built for verification
}

// ClientSessionCache is a cache of ClientSessionState objects that can be used
// by a client to resume a TLS session with a given server. ClientSessionCache
// implementations should expect to be called concurrently from different
// goroutines.
type ClientSessionCache interface {
	// Get searches for a ClientSessionState associated with the given key.
	// On return, ok is true if one was found.
	Get(sessionKey string) (session *ClientSessionState, ok bool)

	// Put adds the ClientSessionState to the cache with the given key.
	Put(sessionKey string, cs *ClientSessionState)
}

// ClientHelloInfo contains information from a ClientHello message in order to
// guide certificate selection in the GetCertificate callback.
type ClientHelloInfo struct {
	// CipherSuites lists the CipherSuites supported by the client (e.g.
	// TLS_RSA_WITH_RC4_128_SHA).
	CipherSuites []uint16

	// ServerName indicates the name of the server requested by the client
	// in order to support virtual hosting. ServerName is only set if the
	// client is using SNI (see
	// http://tools.ietf.org/html/rfc4366#section-3.1).
	ServerName string

	// SupportedCurves lists the elliptic curves supported by the client.
	// SupportedCurves is set only if the Supported Elliptic Curves
	// Extension is being used (see
	// http://tools.ietf.org/html/rfc4492#section-5.1.1).
	SupportedCurves []CurveID

	// SupportedPoints lists the point formats supported by the client.
	// SupportedPoints is set only if the Supported Point Formats Extension
	// is being used (see
	// http://tools.ietf.org/html/rfc4492#section-5.1.2).
	SupportedPoints []uint8
}

// RenegotiationSupport enumerates the different levels of support for TLS
// renegotiation. TLS renegotiation is the act of performing subsequent
// handshakes on a connection after the first. This significantly complicates
// the state machine and has been the source of numerous, subtle security
// issues. Initiating a renegotiation is not supported, but support for
// accepting renegotiation requests may be enabled.
//
// Even when enabled, the server may not change its identity between handshakes
// (i.e. the leaf certificate must be the same). Additionally, concurrent
// handshake and application data flow is not permitted so renegotiation can
// only be used with protocols that synchronise with the renegotiation, such as
// HTTPS.
type RenegotiationSupport int

const (
	// RenegotiateNever disables renegotiation.
	RenegotiateNever RenegotiationSupport = iota

	// RenegotiateOnceAsClient allows a remote server to request
	// renegotiation once per connection.
	RenegotiateOnceAsClient

	// RenegotiateFreelyAsClient allows a remote server to repeatedly
	// request renegotiation.
	RenegotiateFreelyAsClient
)

// A Config structure is used to configure a TLS client or server.
// After one has been passed to a TLS function it must not be
// modified. A Config may be reused; the tls package will also not
// modify it.
type Config struct {
	// Rand provides the source of entropy for nonces and RSA blinding.
	// If Rand is nil, TLS uses the cryptographic random reader in package
	// crypto/rand.
	// The Reader must be safe for use by multiple goroutines.
	Rand io.Reader

	// Time returns the current time as the number of seconds since the epoch.
	// If Time is nil, TLS uses time.Now.
	Time func() time.Time

	// Certificates contains one or more certificate chains
	// to present to the other side of the connection.
	// Server configurations must include at least one certificate
	// or else set GetCertificate.
	Certificates []Certificate

	// NameToCertificate maps from a certificate name to an element of
	// Certificates. Note that a certificate name can be of the form
	// '*.example.com' and so doesn't have to be a domain name as such.
	// See Config.BuildNameToCertificate
	// The nil value causes the first element of Certificates to be used
	// for all connections.
	NameToCertificate map[string]*Certificate

	// GetCertificate returns a Certificate based on the given
	// ClientHelloInfo. It will only be called if the client supplies SNI
	// information or if Certificates is empty.
	//
	// If GetCertificate is nil or returns nil, then the certificate is
	// retrieved from NameToCertificate. If NameToCertificate is nil, the
	// first element of Certificates will be used.
	GetCertificate func(clientHello *ClientHelloInfo) (*Certificate, error)

	// RootCAs defines the set of root certificate authorities
	// that clients use when verifying server certificates.
	// If RootCAs is nil, TLS uses the host's root CA set.
	RootCAs *x509.CertPool

	// NextProtos is a list of supported, application level protocols.
	NextProtos []string

	// ServerName is used to verify the hostname on the returned
	// certificates unless InsecureSkipVerify is given. It is also included
	// in the client's handshake to support virtual hosting unless it is
	// an IP address.
	ServerName string

	// ClientAuth determines the server's policy for
	// TLS Client Authentication. The default is NoClientCert.
	ClientAuth ClientAuthType

	// ClientCAs defines the set of root certificate authorities
	// that servers use if required to verify a client certificate
	// by the policy in ClientAuth.
	ClientCAs *x509.CertPool

	// InsecureSkipVerify controls whether a client verifies the
	// server's certificate chain and host name.
	// If InsecureSkipVerify is true, TLS accepts any certificate
	// presented by the server and any host name in that certificate.
	// In this mode, TLS is susceptible to man-in-the-middle attacks.
	// This should be used only for testing.
	InsecureSkipVerify bool

	// CipherSuites is a list of supported cipher suites. If CipherSuites
	// is nil, TLS uses a list of suites supported by the implementation.
	CipherSuites []uint16

	// PreferServerCipherSuites controls whether the server selects the
	// client's most preferred ciphersuite, or the server's most preferred
	// ciphersuite. If true then the server's preference, as expressed in
	// the order of elements in CipherSuites, is used.
	PreferServerCipherSuites bool

	// SessionTicketsDisabled may be set to true to disable session ticket
	// (resumption) support.
	SessionTicketsDisabled bool

	// SessionTicketKey is used by TLS servers to provide session
	// resumption. See RFC 5077. If zero, it will be filled with
	// random data before the first server handshake.
	//
	// If multiple servers are terminating connections for the same host
	// they should all have the same SessionTicketKey. If the
	// SessionTicketKey leaks, previously recorded and future TLS
	// connections using that key are compromised.
	SessionTicketKey [32]byte

	// SessionCache is a cache of ClientSessionState entries for TLS session
	// resumption.
	ClientSessionCache ClientSessionCache

	// MinVersion contains the minimum SSL/TLS version that is acceptable.
	// If zero, then TLS 1.0 is taken as the minimum.
	MinVersion uint16

	// MaxVersion contains the maximum SSL/TLS version that is acceptable.
	// If zero, then the maximum version supported by this package is used,
	// which is currently TLS 1.2.
	MaxVersion uint16

	// CurvePreferences contains the elliptic curves that will be used in
	// an ECDHE handshake, in preference order. If empty, the default will
	// be used.
	CurvePreferences []CurveID

	// DynamicRecordSizingDisabled disables adaptive sizing of TLS records.
	// When true, the largest possible TLS record size is always used. When
	// false, the size of TLS records may be adjusted in an attempt to
	// improve latency.
	DynamicRecordSizingDisabled bool

	// Renegotiation controls what types of renegotiation are supported.
	// The default, none, is correct for the vast majority of applications.
	Renegotiation RenegotiationSupport

	// KeyLogWriter optionally specifies a destination for TLS master secrets
	// in NSS key log format that can be used to allow external programs
	// such as Wireshark to decrypt TLS connections.
	// See https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format.
	// Use of KeyLogWriter compromises security and should only be
	// used for debugging.
	KeyLogWriter io.Writer

	serverInitOnce sync.Once // guards calling (*Config).serverInit

	// mutex protects sessionTicketKeys
	mutex sync.RWMutex
	// sessionTicketKeys contains zero or more ticket keys. If the length
	// is zero, SessionTicketsDisabled must be true. The first key is used
	// for new tickets and any subsequent keys can be used to decrypt old
	// tickets.
	sessionTicketKeys []ticketKey
}

// ticketKeyNameLen is the number of bytes of identifier that is prepended to
// an encrypted session ticket in order to identify the key used to encrypt it.
const ticketKeyNameLen = 16

// ticketKey is the internal representation of a session ticket key.
type ticketKey struct {
	// keyName is an opaque byte string that serves to identify the session
	// ticket key. It's exposed as plaintext in every session ticket.
	keyName [ticketKeyNameLen]byte
	aesKey  [16]byte
	hmacKey [16]byte
}

// ticketKeyFromBytes converts from the external representation of a session
// ticket key to a ticketKey. Externally, session ticket keys are 32 random
// bytes and this function expands that into sufficient name and key material.
func ticketKeyFromBytes(b [32]byte) (key ticketKey) {
	hashed := sha512.Sum512(b[:])
	copy(key.keyName[:], hashed[:ticketKeyNameLen])
	copy(key.aesKey[:], hashed[ticketKeyNameLen:ticketKeyNameLen+16])
	copy(key.hmacKey[:], hashed[ticketKeyNameLen+16:ticketKeyNameLen+32])
	return key
}

// Clone returns a shallow clone of c.
// Only the exported fields are copied.
func (c *Config) Clone() *Config {
	return &Config{
		Rand:                        c.Rand,
		Time:                        c.Time,
		Certificates:                c.Certificates,
		NameToCertificate:           c.NameToCertificate,
		GetCertificate:              c.GetCertificate,
		RootCAs:                     c.RootCAs,
		NextProtos:                  c.NextProtos,
		ServerName:                  c.ServerName,
		ClientAuth:                  c.ClientAuth,
		ClientCAs:                   c.ClientCAs,
		InsecureSkipVerify:          c.InsecureSkipVerify,
		CipherSuites:                c.CipherSuites,
		PreferServerCipherSuites:    c.PreferServerCipherSuites,
		SessionTicketsDisabled:      c.SessionTicketsDisabled,
		SessionTicketKey:            c.SessionTicketKey,
		ClientSessionCache:          c.ClientSessionCache,
		MinVersion:                  c.MinVersion,
		MaxVersion:                  c.MaxVersion,
		CurvePreferences:            c.CurvePreferences,
		DynamicRecordSizingDisabled: c.DynamicRecordSizingDisabled,
		Renegotiation:               c.Renegotiation,
		KeyLogWriter:                c.KeyLogWriter,
	}
}

func (c *Config) serverInit() {
	if c.SessionTicketsDisabled || len(c.ticketKeys()) != 0 {
		return
	}

	alreadySet := false
	for _, b := range c.SessionTicketKey {
		if b != 0 {
			alreadySet = true
			break
		}
	}

	if !alreadySet {
		if _, err := io.ReadFull(c.rand(), c.SessionTicketKey[:]); err != nil {
			c.SessionTicketsDisabled = true
			return
		}
	}

	c.sessionTicketKeys = []ticketKey{ticketKeyFromBytes(c.SessionTicketKey)}
}

func (c *Config) ticketKeys() []ticketKey {
	c.mutex.RLock()
	// c.sessionTicketKeys is constant once created. SetSessionTicketKeys
	// will only update it by replacing it with a new value.
	ret := c.sessionTicketKeys
	c.mutex.RUnlock()
	return ret
}

// SetSessionTicketKeys updates the session ticket keys for a server. The first
// key will be used when creating new tickets, while all keys can be used for
// decrypting tickets. It is safe to call this function while the server is
// running in order to rotate the session ticket keys. The function will panic
// if keys is empty.
func (c *Config) SetSessionTicketKeys(keys [][32]byte) {
	if len(keys) == 0 {
		panic("tls: keys must have at least one key")
	}

	newKeys := make([]ticketKey, len(keys))
	for i, bytes := range keys {
		newKeys[i] = ticketKeyFromBytes(bytes)
	}

	c.mutex.Lock()
	c.sessionTicketKeys = newKeys
	c.mutex.Unlock()
}

func (c *Config) rand() io.Reader {
	r := c.Rand
	if r == nil {
		return rand.Reader
	}
	return r
}

func (c *Config) time() time.Time {
	t := c.Time
	if t == nil {
		t = time.Now
	}
	return t()
}

func (c *Config) cipherSuites() []uint16 {
	s := c.CipherSuites
	if s == nil {
		s = defaultCipherSuites()
	}
	return s
}

func (c *Config) minVersion() uint16 {
	if c == nil || c.MinVersion == 0 {
		return minVersion
	}
	return c.MinVersion
}

func (c *Config) maxVersion() uint16 {
	if c == nil || c.MaxVersion == 0 {
		return maxVersion
	}
	return c.MaxVersion
}

var defaultCurvePreferences = []CurveID{CurveP256, CurveP384, CurveP521}

func (c *Config) curvePreferences() []CurveID {
	if c == nil || len(c.CurvePreferences) == 0 {
		return defaultCurvePreferences
	}
	return c.CurvePreferences
}

// mutualVersion returns the protocol version to use given the advertised
// version of the peer.
func (c *Config) mutualVersion(vers uint16) (uint16, bool) {
	minVersion := c.minVersion()
	maxVersion := c.maxVersion()

	if vers < minVersion {
		return 0, false
	}
	if vers > maxVersion {
		vers = maxVersion
	}
	return vers, true
}

// getCertificate returns the best certificate for the given ClientHelloInfo,
// defaulting to the first element of c.Certificates.
func (c *Config) getCertificate(clientHello *ClientHelloInfo) (*Certificate, error) {
	if c.GetCertificate != nil &&
		(len(c.Certificates) == 0 || len(clientHello.ServerName) > 0) {
		cert, err := c.GetCertificate(clientHello)
		if cert != nil || err != nil {
			return cert, err
		}
	}

	if len(c.Certificates) == 0 {
		return nil, errors.New("tls: no certificates configured")
	}

	if len(c.Certificates) == 1 || c.NameToCertificate == nil {
		// There's only one choice, so no point doing any work.
		return &c.Certificates[0], nil
	}

	name := strings.ToLower(clientHello.ServerName)
	for len(name) > 0 && name[len(name)-1] == '.' {
		name = name[:len(name)-1]
	}

	if cert, ok := c.NameToCertificate[name]; ok {
		return cert, nil
	}

	// try replacing labels in the name with wildcards until we get a
	// match.
	labels := strings.Split(name, ".")
	for i := range labels {
		labels[i] = "*"
		candidate := strings.Join(labels, ".")
		if cert, ok := c.NameToCertificate[candidate]; ok {
			return cert, nil
		}
	}

	// If nothing matches, return the first certificate.
	return &c.Certificates[0], nil
}

// BuildNameToCertificate parses c.Certificates and builds c.NameToCertificate
// from the CommonName and SubjectAlternateName fields of each of the leaf
// certificates.
func (c *Config) BuildNameToCertificate() {
	c.NameToCertificate = make(map[string]*Certificate)
	for i := range c.Certificates {
		cert := &c.Certificates[i]
		x509Cert, err := x509.ParseCertificate(cert.Certificate[0])
		if err != nil {
			continue
		}
		if len(x509Cert.Subject.CommonName) > 0 {
			c.NameToCertificate[x509Cert.Subject.CommonName] = cert
		}
		for _, san := range x509Cert.DNSNames {
			c.NameToCertificate[san] = cert
		}
	}
}

// writeKeyLog logs client random and master secret if logging enabled
// by setting KeyLogWriter.
func (c *Config) writeKeyLog(clientRandom, masterSecret []byte) error {
	if c.KeyLogWriter == nil {
		return nil
	}
	writerMutex.Lock()
	_, err := fmt.Fprintf(c.KeyLogWriter, "CLIENT_RANDOM %x %x\n", clientRandom, masterSecret)
	writerMutex.Unlock()
	return err
}

// writerMutex protects all KeyLogWriters globally. It is rarely enabled,
// and is only for debugging, so a global mutex saves space.
var writerMutex sync.Mutex

// A Certificate is a chain of one or more certificates, leaf first.
type Certificate struct {
	Certificate [][]byte
	// PrivateKey contains the private key corresponding to the public key
	// in Leaf. For a server, this must implement crypto.Signer and/or
	// crypto.Decrypter, with an RSA or ECDSA PublicKey. For a client
	// (performing client authentication), this must be a crypto.Signer
	// with an RSA or ECDSA PublicKey.
	PrivateKey crypto.PrivateKey
	// OCSPStaple contains an optional OCSP response which will be served
	// to clients that request it.
	OCSPStaple []byte
	// SignedCertificateTimestamps contains an optional list of Signed
	// Certificate Timestamps which will be served to clients that request it.
	SignedCertificateTimestamps [][]byte
	// Leaf is the parsed form of the leaf certificate, which may be
	// initialized using x509.ParseCertificate to reduce per-handshake
	// processing for TLS clients doing client authentication. If nil, the
	// leaf certificate will be parsed as needed.
	Leaf *x509.Certificate
}

type handshakeMessage interface {
	marshal() []byte
	unmarshal([]byte) bool
}

// lruSessionCache is a ClientSessionCache implementation that uses an LRU
// caching strategy.
type lruSessionCache struct {
	sync.Mutex

	m        map[string]*list.Element
	q        *list.List
	capacity int
}

type lruSessionCacheEntry struct {
	sessionKey string
	state      *ClientSessionState
}

// NewLRUClientSessionCache returns a ClientSessionCache with the given
// capacity that uses an LRU strategy. If capacity is < 1, a default capacity
// is used instead.
func NewLRUClientSessionCache(capacity int) ClientSessionCache {
	const defaultSessionCacheCapacity = 64

	if capacity < 1 {
		capacity = defaultSessionCacheCapacity
	}
	return &lruSessionCache{
		m:        make(map[string]*list.Element),
		q:        list.New(),
		capacity: capacity,
	}
}

// Put adds the provided (sessionKey, cs) pair to the cache.
func (c *lruSessionCache) Put(sessionKey string, cs *ClientSessionState) {
	c.Lock()
	defer c.Unlock()

	if elem, ok := c.m[sessionKey]; ok {
		entry := elem.Value.(*lruSessionCacheEntry)
		entry.state = cs
		c.q.MoveToFront(elem)
		return
	}

	if c.q.Len() < c.capacity {
		entry := &lruSessionCacheEntry{sessionKey, cs}
		c.m[sessionKey] = c.q.PushFront(entry)
		return
	}

	elem := c.q.Back()
	entry := elem.Value.(*lruSessionCacheEntry)
	delete(c.m, entry.sessionKey)
	entry.sessionKey = sessionKey
	entry.state = cs
	c.q.MoveToFront(elem)
	c.m[sessionKey] = elem
}

// Get returns the ClientSessionState value associated with a given key. It
// returns (nil, false) if no value is found.
func (c *lruSessionCache) Get(sessionKey string) (*ClientSessionState, bool) {
	c.Lock()
	defer c.Unlock()

	if elem, ok := c.m[sessionKey]; ok {
		c.q.MoveToFront(elem)
		return elem.Value.(*lruSessionCacheEntry).state, true
	}
	return nil, false
}

// TODO(jsing): Make these available to both crypto/x509 and crypto/tls.
type dsaSignature struct {
	R, S *big.Int
}

type ecdsaSignature dsaSignature

var emptyConfig Config

func defaultConfig() *Config {
	return &emptyConfig
}

var (
	once                   sync.Once
	varDefaultCipherSuites []uint16
)

func defaultCipherSuites() []uint16 {
	once.Do(initDefaultCipherSuites)
	return varDefaultCipherSuites
}

func initDefaultCipherSuites() {
	varDefaultCipherSuites = make([]uint16, 0, len(cipherSuites))
	for _, suite := range cipherSuites {
		if suite.flags&suiteDefaultOff != 0 {
			continue
		}
		varDefaultCipherSuites = append(varDefaultCipherSuites, suite.id)
	}
}

func unexpectedMessageError(wanted, got interface{}) error {
	return fmt.Errorf("tls: received unexpected handshake message of type %T when waiting for %T", got, wanted)
}

func isSupportedSignatureAndHash(sigHash signatureAndHash, sigHashes []signatureAndHash) bool {
	for _, s := range sigHashes {
		if s == sigHash {
			return true
		}
	}
	return false
}