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 qtls

import (
	"bytes"
	"container/list"
	"crypto"
	"crypto/ecdsa"
	"crypto/ed25519"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha512"
	"crypto/tls"
	"crypto/x509"
	"errors"
	"fmt"
	"io"
	"math/big"
	"net"
	"strings"
	"sync"
	"time"

	"golang.org/x/sys/cpu"
)

const (
	VersionTLS10 = 0x0301
	VersionTLS11 = 0x0302
	VersionTLS12 = 0x0303
	VersionTLS13 = 0x0304

	// Deprecated: SSLv3 is cryptographically broken, and is no longer
	// supported by this package. See golang.org/issue/32716.
	VersionSSL30 = 0x0300
)

const (
	maxPlaintext       = 16384        // maximum plaintext payload length
	maxCiphertext      = 16384 + 2048 // maximum ciphertext payload length
	maxCiphertextTLS13 = 16384 + 256  // maximum ciphertext length in TLS 1.3
	recordHeaderLen    = 5            // record header length
	maxHandshake       = 65536        // maximum handshake we support (protocol max is 16 MB)
	maxUselessRecords  = 16           // maximum number of consecutive non-advancing records
)

// 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
	typeEndOfEarlyData      uint8 = 5
	typeEncryptedExtensions uint8 = 8
	typeCertificate         uint8 = 11
	typeServerKeyExchange   uint8 = 12
	typeCertificateRequest  uint8 = 13
	typeServerHelloDone     uint8 = 14
	typeCertificateVerify   uint8 = 15
	typeClientKeyExchange   uint8 = 16
	typeFinished            uint8 = 20
	typeCertificateStatus   uint8 = 22
	typeKeyUpdate           uint8 = 24
	typeNextProtocol        uint8 = 67  // Not IANA assigned
	typeMessageHash         uint8 = 254 // synthetic message
)

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

type Extension struct {
	Type uint16
	Data []byte
}

// TLS extension numbers
const (
	extensionServerName              uint16 = 0
	extensionStatusRequest           uint16 = 5
	extensionSupportedCurves         uint16 = 10 // supported_groups in TLS 1.3, see RFC 8446, Section 4.2.7
	extensionSupportedPoints         uint16 = 11
	extensionSignatureAlgorithms     uint16 = 13
	extensionALPN                    uint16 = 16
	extensionSCT                     uint16 = 18
	extensionSessionTicket           uint16 = 35
	extensionPreSharedKey            uint16 = 41
	extensionEarlyData               uint16 = 42
	extensionSupportedVersions       uint16 = 43
	extensionCookie                  uint16 = 44
	extensionPSKModes                uint16 = 45
	extensionCertificateAuthorities  uint16 = 47
	extensionSignatureAlgorithmsCert uint16 = 50
	extensionKeyShare                uint16 = 51
	extensionRenegotiationInfo       uint16 = 0xff01
)

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

type EncryptionLevel uint8

const (
	EncryptionHandshake EncryptionLevel = iota
	Encryption0RTT
	EncryptionApplication
)

// CurveID is a tls.CurveID
type CurveID = tls.CurveID

const (
	CurveP256 CurveID = 23
	CurveP384 CurveID = 24
	CurveP521 CurveID = 25
	X25519    CurveID = 29
)

// TLS 1.3 Key Share. See RFC 8446, Section 4.2.8.
type keyShare struct {
	group CurveID
	data  []byte
}

// TLS 1.3 PSK Key Exchange Modes. See RFC 8446, Section 4.2.9.
const (
	pskModePlain uint8 = 0
	pskModeDHE   uint8 = 1
)

// TLS 1.3 PSK Identity. Can be a Session Ticket, or a reference to a saved
// session. See RFC 8446, Section 4.2.11.
type pskIdentity struct {
	label               []byte
	obfuscatedTicketAge uint32
}

// TLS Elliptic Curve Point Formats
// https://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
	certTypeECDSASign = 64 // ECDSA or EdDSA keys, see RFC 8422, Section 3.
)

// Signature algorithms (for internal signaling use). Starting at 225 to avoid overlap with
// TLS 1.2 codepoints (RFC 5246, Appendix A.4.1), with which these have nothing to do.
const (
	signaturePKCS1v15 uint8 = iota + 225
	signatureRSAPSS
	signatureECDSA
	signatureEd25519
)

// directSigning is a standard Hash value that signals that no pre-hashing
// should be performed, and that the input should be signed directly. It is the
// hash function associated with the Ed25519 signature scheme.
var directSigning crypto.Hash = 0

// 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. The two fields are merged to match with TLS 1.3.
// Note that in TLS 1.2, the ECDSA algorithms are not constrained to P-256, etc.
var supportedSignatureAlgorithms = []SignatureScheme{
	PSSWithSHA256,
	ECDSAWithP256AndSHA256,
	Ed25519,
	PSSWithSHA384,
	PSSWithSHA512,
	PKCS1WithSHA256,
	PKCS1WithSHA384,
	PKCS1WithSHA512,
	ECDSAWithP384AndSHA384,
	ECDSAWithP521AndSHA512,
	PKCS1WithSHA1,
	ECDSAWithSHA1,
}

// helloRetryRequestRandom is set as the Random value of a ServerHello
// to signal that the message is actually a HelloRetryRequest.
var helloRetryRequestRandom = []byte{ // See RFC 8446, Section 4.1.3.
	0xCF, 0x21, 0xAD, 0x74, 0xE5, 0x9A, 0x61, 0x11,
	0xBE, 0x1D, 0x8C, 0x02, 0x1E, 0x65, 0xB8, 0x91,
	0xC2, 0xA2, 0x11, 0x16, 0x7A, 0xBB, 0x8C, 0x5E,
	0x07, 0x9E, 0x09, 0xE2, 0xC8, 0xA8, 0x33, 0x9C,
}

const (
	// downgradeCanaryTLS12 or downgradeCanaryTLS11 is embedded in the server
	// random as a downgrade protection if the server would be capable of
	// negotiating a higher version. See RFC 8446, Section 4.1.3.
	downgradeCanaryTLS12 = "DOWNGRD\x01"
	downgradeCanaryTLS11 = "DOWNGRD\x00"
)

// 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_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, ...)
	NegotiatedProtocol          string                // negotiated next protocol (not guaranteed to be from Config.NextProtos)
	NegotiatedProtocolIsMutual  bool                  // negotiated protocol was advertised by server (client side only)
	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 peer, if any
	OCSPResponse                []byte                // stapled OCSP response from peer, if any

	Used0RTT bool // true if 0-RTT was both offered and accepted

	// ekm is a closure exposed via ExportKeyingMaterial.
	ekm func(label string, context []byte, length int) ([]byte, error)

	// 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://mitls.org/pages/attacks/3SHAKE#channelbindings). This will
	// change in future versions of Go once the TLS master-secret fix has
	// been standardized and implemented. It is not defined in TLS 1.3.
	TLSUnique []byte
}

// ExportKeyingMaterial returns length bytes of exported key material in a new
// slice as defined in RFC 5705. If context is nil, it is not used as part of
// the seed. If the connection was set to allow renegotiation via
// Config.Renegotiation, this function will return an error.
func (cs *ConnectionState) ExportKeyingMaterial(label string, context []byte, length int) ([]byte, error) {
	return cs.ekm(label, context, length)
}

// ClientAuthType is tls.ClientAuthType
type ClientAuthType = tls.ClientAuthType

const (
	NoClientCert ClientAuthType = iota
	RequestClientCert
	RequireAnyClientCert
	VerifyClientCertIfGiven
	RequireAndVerifyClientCert
)

// requiresClientCert reports whether the ClientAuthType requires a client
// certificate to be provided.
func requiresClientCert(c ClientAuthType) bool {
	switch c {
	case RequireAnyClientCert, RequireAndVerifyClientCert:
		return true
	default:
		return false
	}
}

// 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                // TLS version negotiated for the session
	cipherSuite        uint16                // Ciphersuite negotiated for the session
	masterSecret       []byte                // Full handshake MasterSecret, or TLS 1.3 resumption_master_secret
	serverCertificates []*x509.Certificate   // Certificate chain presented by the server
	verifiedChains     [][]*x509.Certificate // Certificate chains we built for verification
	receivedAt         time.Time             // When the session ticket was received from the server

	// TLS 1.3 fields.
	nonce  []byte    // Ticket nonce sent by the server, to derive PSK
	useBy  time.Time // Expiration of the ticket lifetime as set by the server
	ageAdd uint32    // Random obfuscation factor for sending the ticket age
}

// 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. Up to TLS 1.2, only ticket-based resumption is supported, not
// SessionID-based resumption. In TLS 1.3 they were merged into PSK modes, which
// are supported via this interface.
//go:generate sh -c "mockgen -package qtls -self_package github.com/marten-seemann/qtls -destination mock_client_session_cache_test.go github.com/marten-seemann/qtls ClientSessionCache"
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. It might
	// get called multiple times in a connection if a TLS 1.3 server provides
	// more than one session ticket. If called with a nil *ClientSessionState,
	// it should remove the cache entry.
	Put(sessionKey string, cs *ClientSessionState)
}

// SignatureScheme is a tls.SignatureScheme
type SignatureScheme = tls.SignatureScheme

const (
	// RSASSA-PKCS1-v1_5 algorithms.
	PKCS1WithSHA256 SignatureScheme = 0x0401
	PKCS1WithSHA384 SignatureScheme = 0x0501
	PKCS1WithSHA512 SignatureScheme = 0x0601

	// RSASSA-PSS algorithms with public key OID rsaEncryption.
	PSSWithSHA256 SignatureScheme = 0x0804
	PSSWithSHA384 SignatureScheme = 0x0805
	PSSWithSHA512 SignatureScheme = 0x0806

	// ECDSA algorithms. Only constrained to a specific curve in TLS 1.3.
	ECDSAWithP256AndSHA256 SignatureScheme = 0x0403
	ECDSAWithP384AndSHA384 SignatureScheme = 0x0503
	ECDSAWithP521AndSHA512 SignatureScheme = 0x0603

	// EdDSA algorithms.
	Ed25519 SignatureScheme = 0x0807

	// Legacy signature and hash algorithms for TLS 1.2.
	PKCS1WithSHA1 SignatureScheme = 0x0201
	ECDSAWithSHA1 SignatureScheme = 0x0203
)

// ClientHelloInfo contains information from a ClientHello message in order to
// guide application logic in the GetCertificate and GetConfigForClient callbacks.
type ClientHelloInfo struct {
	// CipherSuites lists the CipherSuites supported by the client (e.g.
	// TLS_AES_128_GCM_SHA256, TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256).
	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 RFC 4366, 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 RFC 4492, 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 RFC 4492, Section 5.1.2).
	SupportedPoints []uint8

	// SignatureSchemes lists the signature and hash schemes that the client
	// is willing to verify. SignatureSchemes is set only if the Signature
	// Algorithms Extension is being used (see RFC 5246, Section 7.4.1.4.1).
	SignatureSchemes []SignatureScheme

	// SupportedProtos lists the application protocols supported by the client.
	// SupportedProtos is set only if the Application-Layer Protocol
	// Negotiation Extension is being used (see RFC 7301, Section 3.1).
	//
	// Servers can select a protocol by setting Config.NextProtos in a
	// GetConfigForClient return value.
	SupportedProtos []string

	// SupportedVersions lists the TLS versions supported by the client.
	// For TLS versions less than 1.3, this is extrapolated from the max
	// version advertised by the client, so values other than the greatest
	// might be rejected if used.
	SupportedVersions []uint16

	// Conn is the underlying net.Conn for the connection. Do not read
	// from, or write to, this connection; that will cause the TLS
	// connection to fail.
	Conn net.Conn

	// config is embedded by the GetCertificate or GetConfigForClient caller,
	// for use with SupportsCertificate.
	config *Config
}

// CertificateRequestInfo contains information from a server's
// CertificateRequest message, which is used to demand a certificate and proof
// of control from a client.
type CertificateRequestInfo struct {
	// AcceptableCAs contains zero or more, DER-encoded, X.501
	// Distinguished Names. These are the names of root or intermediate CAs
	// that the server wishes the returned certificate to be signed by. An
	// empty slice indicates that the server has no preference.
	AcceptableCAs [][]byte

	// SignatureSchemes lists the signature schemes that the server is
	// willing to verify.
	SignatureSchemes []SignatureScheme

	// Version is the TLS version that was negotiated for this connection.
	Version uint16
}

// 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.
//
// Renegotiation is not defined in TLS 1.3.
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. The first certificate compatible with the
	// peer's requirements is selected automatically.
	//
	// Server configurations must set one of Certificates, GetCertificate or
	// GetConfigForClient. Clients doing client-authentication may set either
	// Certificates or GetClientCertificate.
	//
	// Note: if there are multiple Certificates, and they don't have the
	// optional field Leaf set, certificate selection will incur a significant
	// per-handshake performance cost.
	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.
	//
	// Deprecated: NameToCertificate only allows associating a single
	// certificate with a given name. Leave this field nil to let the library
	// select the first compatible chain from Certificates.
	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
	// best element of Certificates will be used.
	GetCertificate func(*ClientHelloInfo) (*Certificate, error)

	// GetClientCertificate, if not nil, is called when a server requests a
	// certificate from a client. If set, the contents of Certificates will
	// be ignored.
	//
	// If GetClientCertificate returns an error, the handshake will be
	// aborted and that error will be returned. Otherwise
	// GetClientCertificate must return a non-nil Certificate. If
	// Certificate.Certificate is empty then no certificate will be sent to
	// the server. If this is unacceptable to the server then it may abort
	// the handshake.
	//
	// GetClientCertificate may be called multiple times for the same
	// connection if renegotiation occurs or if TLS 1.3 is in use.
	GetClientCertificate func(*CertificateRequestInfo) (*Certificate, error)

	// GetConfigForClient, if not nil, is called after a ClientHello is
	// received from a client. It may return a non-nil Config in order to
	// change the Config that will be used to handle this connection. If
	// the returned Config is nil, the original Config will be used. The
	// Config returned by this callback may not be subsequently modified.
	//
	// If GetConfigForClient is nil, the Config passed to Server() will be
	// used for all connections.
	//
	// Uniquely for the fields in the returned Config, session ticket keys
	// will be duplicated from the original Config if not set.
	// Specifically, if SetSessionTicketKeys was called on the original
	// config but not on the returned config then the ticket keys from the
	// original config will be copied into the new config before use.
	// Otherwise, if SessionTicketKey was set in the original config but
	// not in the returned config then it will be copied into the returned
	// config before use. If neither of those cases applies then the key
	// material from the returned config will be used for session tickets.
	GetConfigForClient func(*ClientHelloInfo) (*Config, error)

	// VerifyPeerCertificate, if not nil, is called after normal
	// certificate verification by either a TLS client or server. It
	// receives the raw ASN.1 certificates provided by the peer and also
	// any verified chains that normal processing found. If it returns a
	// non-nil error, the handshake is aborted and that error results.
	//
	// If normal verification fails then the handshake will abort before
	// considering this callback. If normal verification is disabled by
	// setting InsecureSkipVerify, or (for a server) when ClientAuth is
	// RequestClientCert or RequireAnyClientCert, then this callback will
	// be considered but the verifiedChains argument will always be nil.
	VerifyPeerCertificate func(rawCerts [][]byte, verifiedChains [][]*x509.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, in
	// order of preference.
	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 for TLS versions up to
	// TLS 1.2. If CipherSuites is nil, a default list of secure cipher suites
	// is used, with a preference order based on hardware performance. The
	// default cipher suites might change over Go versions. Note that TLS 1.3
	// ciphersuites are not configurable.
	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 and
	// PSK (resumption) support. Note that on clients, session ticket support is
	// also disabled if ClientSessionCache is nil.
	SessionTicketsDisabled bool

	// SessionTicketKey is used by TLS servers to provide session resumption.
	// See RFC 5077 and the PSK mode of RFC 8446. 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 might be compromised.
	SessionTicketKey [32]byte

	// ClientSessionCache is a cache of ClientSessionState entries for TLS
	// session resumption. It is only used by clients.
	ClientSessionCache ClientSessionCache

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

	// MaxVersion contains the maximum TLS version that is acceptable.
	// If zero, the maximum version supported by this package is used,
	// which is currently TLS 1.3.
	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. The client will use the first preference as the type for
	// its key share in TLS 1.3. This may change in the future.
	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

	// GetExtensions, if not nil, is called before a message that allows
	// sending of extensions is sent.
	// Currently only implemented for the ClientHello message (for the client)
	// and for the EncryptedExtensions message (for the server).
	// Only valid for TLS 1.3.
	GetExtensions func(handshakeMessageType uint8) []Extension

	// ReceivedExtensions, if not nil, is called when a message that allows the
	// inclusion of extensions is received.
	// It is called with an empty slice of extensions, if the message didn't
	// contain any extensions.
	// Currently only implemented for the ClientHello message (sent by the
	// client) and for the EncryptedExtensions message (sent by the server).
	// Only valid for TLS 1.3.
	ReceivedExtensions func(handshakeMessageType uint8, exts []Extension)

	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

	// AlternativeRecordLayer is used by QUIC
	AlternativeRecordLayer RecordLayer

	// Enforce the selection of a supported application protocol.
	// Only works for TLS 1.3.
	// If enabled, client and server have to agree on an application protocol.
	// Otherwise, connection establishment fails.
	EnforceNextProtoSelection bool

	// If MaxEarlyData is greater than 0, the client will be allowed to send early
	// data when resuming a session.
	// Requires the AlternativeRecordLayer to be set.
	//
	// It has no meaning on the client.
	MaxEarlyData uint32

	// The Accept0RTT callback is called when the client offers 0-RTT.
	// The server then has to decide if it wants to accept or reject 0-RTT.
	// It is only used for servers.
	Accept0RTT func(appData []byte) bool

	// 0RTTRejected is called when the server rejectes 0-RTT.
	// It is only used for clients.
	Rejected0RTT func()

	// If set, the client will export the 0-RTT key when resuming a session that
	// allows sending of early data.
	// Requires the AlternativeRecordLayer to be set.
	//
	// It has no meaning to the server.
	Enable0RTT bool

	// Is called when the client saves a session ticket to the session ticket.
	// This gives the application the opportunity to save some data along with the ticket,
	// which can be restored when the session ticket is used.
	GetAppDataForSessionState func() []byte

	// Is called when the client uses a session ticket.
	// Restores the application data that was saved earlier on GetAppDataForSessionTicket.
	SetAppDataFromSessionState func([]byte)
}

// A RecordLayer handles encrypting and decrypting of TLS messages.
type RecordLayer interface {
	SetReadKey(encLevel EncryptionLevel, suite *CipherSuiteTLS13, trafficSecret []byte)
	SetWriteKey(encLevel EncryptionLevel, suite *CipherSuiteTLS13, trafficSecret []byte)
	ReadHandshakeMessage() ([]byte, error)
	WriteRecord([]byte) (int, error)
	SendAlert(uint8)
}

// 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
}

// maxSessionTicketLifetime is the maximum allowed lifetime of a TLS 1.3 session
// ticket, and the lifetime we set for tickets we send.
const maxSessionTicketLifetime = 7 * 24 * time.Hour

// Clone returns a shallow clone of c. It is safe to clone a Config that is
// being used concurrently by a TLS client or server.
func (c *Config) Clone() *Config {
	// Running serverInit ensures that it's safe to read
	// SessionTicketsDisabled.
	c.serverInitOnce.Do(func() { c.serverInit(nil) })

	var sessionTicketKeys []ticketKey
	c.mutex.RLock()
	sessionTicketKeys = c.sessionTicketKeys
	c.mutex.RUnlock()

	return &Config{
		Rand:                        c.Rand,
		Time:                        c.Time,
		Certificates:                c.Certificates,
		NameToCertificate:           c.NameToCertificate,
		GetCertificate:              c.GetCertificate,
		GetClientCertificate:        c.GetClientCertificate,
		GetConfigForClient:          c.GetConfigForClient,
		VerifyPeerCertificate:       c.VerifyPeerCertificate,
		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,
		GetExtensions:               c.GetExtensions,
		ReceivedExtensions:          c.ReceivedExtensions,
		sessionTicketKeys:           sessionTicketKeys,
		EnforceNextProtoSelection:   c.EnforceNextProtoSelection,
		MaxEarlyData:                c.MaxEarlyData,
		Enable0RTT:                  c.Enable0RTT,
		Accept0RTT:                  c.Accept0RTT,
		Rejected0RTT:                c.Rejected0RTT,
		GetAppDataForSessionState:   c.GetAppDataForSessionState,
		SetAppDataFromSessionState:  c.SetAppDataFromSessionState,
	}
}

// serverInit is run under c.serverInitOnce to do initialization of c. If c was
// returned by a GetConfigForClient callback then the argument should be the
// Config that was passed to Server, otherwise it should be nil.
func (c *Config) serverInit(originalConfig *Config) {
	if c.SessionTicketsDisabled || len(c.ticketKeys()) != 0 {
		return
	}

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

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

	if originalConfig != nil {
		originalConfig.mutex.RLock()
		c.sessionTicketKeys = originalConfig.sessionTicketKeys
		originalConfig.mutex.RUnlock()
	} else {
		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
}

var supportedVersions = []uint16{
	VersionTLS13,
	VersionTLS12,
	VersionTLS11,
	VersionTLS10,
}

func (c *Config) supportedVersions() []uint16 {
	versions := make([]uint16, 0, len(supportedVersions))
	for _, v := range supportedVersions {
		if c != nil && c.MinVersion != 0 && v < c.MinVersion {
			continue
		}
		if c != nil && c.MaxVersion != 0 && v > c.MaxVersion {
			continue
		}
		versions = append(versions, v)
	}
	return versions
}

func (c *Config) maxSupportedVersion() uint16 {
	supportedVersions := c.supportedVersions()
	if len(supportedVersions) == 0 {
		return 0
	}
	return supportedVersions[0]
}

// supportedVersionsFromMax returns a list of supported versions derived from a
// legacy maximum version value. Note that only versions supported by this
// library are returned. Any newer peer will use supportedVersions anyway.
func supportedVersionsFromMax(maxVersion uint16) []uint16 {
	versions := make([]uint16, 0, len(supportedVersions))
	for _, v := range supportedVersions {
		if v > maxVersion {
			continue
		}
		versions = append(versions, v)
	}
	return versions
}

var defaultCurvePreferences = []CurveID{X25519, CurveP256, CurveP384, CurveP521}

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

func (c *Config) supportsCurve(curve CurveID) bool {
	for _, cc := range c.curvePreferences() {
		if cc == curve {
			return true
		}
	}
	return false
}

// mutualVersion returns the protocol version to use given the advertised
// versions of the peer. Priority is given to the peer preference order.
func (c *Config) mutualVersion(peerVersions []uint16) (uint16, bool) {
	supportedVersions := c.supportedVersions()
	for _, peerVersion := range peerVersions {
		for _, v := range supportedVersions {
			if v == peerVersion {
				return v, true
			}
		}
	}
	return 0, false
}

var errNoCertificates = errors.New("tls: no certificates configured")

// 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, errNoCertificates
	}

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

	if c.NameToCertificate != nil {
		name := strings.ToLower(clientHello.ServerName)
		if cert, ok := c.NameToCertificate[name]; ok {
			return cert, nil
		}
		if len(name) > 0 {
			labels := strings.Split(name, ".")
			labels[0] = "*"
			wildcardName := strings.Join(labels, ".")
			if cert, ok := c.NameToCertificate[wildcardName]; ok {
				return cert, nil
			}
		}
	}

	for _, cert := range c.Certificates {
		if err := clientHello.SupportsCertificate(&cert); err == nil {
			return &cert, nil
		}
	}

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

// SupportsCertificate returns nil if the provided certificate is supported by
// the client that sent the ClientHello. Otherwise, it returns an error
// describing the reason for the incompatibility.
//
// If this ClientHelloInfo was passed to a GetConfigForClient or GetCertificate
// callback, this method will take into account the associated Config. Note that
// if GetConfigForClient returns a different Config, the change can't be
// accounted for by this method.
//
// This function will call x509.ParseCertificate unless c.Leaf is set, which can
// incur a significant performance cost.
func (chi *ClientHelloInfo) SupportsCertificate(c *Certificate) error {
	// Note we don't currently support certificate_authorities nor
	// signature_algorithms_cert, and don't check the algorithms of the
	// signatures on the chain (which anyway are a SHOULD, see RFC 8446,
	// Section 4.4.2.2).

	config := chi.config
	if config == nil {
		config = &Config{}
	}
	vers, ok := config.mutualVersion(chi.SupportedVersions)
	if !ok {
		return errors.New("no mutually supported protocol versions")
	}

	// If the client specified the name they are trying to connect to, the
	// certificate needs to be valid for it.
	if chi.ServerName != "" {
		x509Cert, err := c.leaf()
		if err != nil {
			return fmt.Errorf("failed to parse certificate: %w", err)
		}
		if err := x509Cert.VerifyHostname(chi.ServerName); err != nil {
			return fmt.Errorf("certificate is not valid for requested server name: %w", err)
		}
	}

	// supportsRSAFallback returns nil if the certificate and connection support
	// the static RSA key exchange, and unsupported otherwise. The logic for
	// supporting static RSA is completely disjoint from the logic for
	// supporting signed key exchanges, so we just check it as a fallback.
	supportsRSAFallback := func(unsupported error) error {
		// TLS 1.3 dropped support for the static RSA key exchange.
		if vers == VersionTLS13 {
			return unsupported
		}
		// The static RSA key exchange works by decrypting a challenge with the
		// RSA private key, not by signing, so check the PrivateKey implements
		// crypto.Decrypter, like *rsa.PrivateKey does.
		if priv, ok := c.PrivateKey.(crypto.Decrypter); ok {
			if _, ok := priv.Public().(*rsa.PublicKey); !ok {
				return unsupported
			}
		} else {
			return unsupported
		}
		// Finally, there needs to be a mutual cipher suite that uses the static
		// RSA key exchange instead of ECDHE.
		rsaCipherSuite := selectCipherSuite(chi.CipherSuites, config.cipherSuites(), func(c *cipherSuite) bool {
			if c.flags&suiteECDHE != 0 {
				return false
			}
			if vers < VersionTLS12 && c.flags&suiteTLS12 != 0 {
				return false
			}
			return true
		})
		if rsaCipherSuite == nil {
			return unsupported
		}
		return nil
	}

	// If the client sent the signature_algorithms extension, ensure it supports
	// schemes we can use with this certificate and TLS version.
	if len(chi.SignatureSchemes) > 0 {
		if _, err := selectSignatureScheme(vers, c, chi.SignatureSchemes); err != nil {
			return supportsRSAFallback(err)
		}
	}

	// In TLS 1.3 we are done because supported_groups is only relevant to the
	// ECDHE computation, point format negotiation is removed, cipher suites are
	// only relevant to the AEAD choice, and static RSA does not exist.
	if vers == VersionTLS13 {
		return nil
	}

	// The only signed key exchange we support is ECDHE.
	if !supportsECDHE(config, chi.SupportedCurves, chi.SupportedPoints) {
		return supportsRSAFallback(errors.New("client doesn't support ECDHE, can only use legacy RSA key exchange"))
	}

	var ecdsaCipherSuite bool
	if priv, ok := c.PrivateKey.(crypto.Signer); ok {
		switch pub := priv.Public().(type) {
		case *ecdsa.PublicKey:
			var curve CurveID
			switch pub.Curve {
			case elliptic.P256():
				curve = CurveP256
			case elliptic.P384():
				curve = CurveP384
			case elliptic.P521():
				curve = CurveP521
			default:
				return supportsRSAFallback(unsupportedCertificateError(c))
			}
			var curveOk bool
			for _, c := range chi.SupportedCurves {
				if c == curve && config.supportsCurve(c) {
					curveOk = true
					break
				}
			}
			if !curveOk {
				return errors.New("client doesn't support certificate curve")
			}
			ecdsaCipherSuite = true
		case ed25519.PublicKey:
			if vers < VersionTLS12 || len(chi.SignatureSchemes) == 0 {
				return errors.New("connection doesn't support Ed25519")
			}
			ecdsaCipherSuite = true
		case *rsa.PublicKey:
		default:
			return supportsRSAFallback(unsupportedCertificateError(c))
		}
	} else {
		return supportsRSAFallback(unsupportedCertificateError(c))
	}

	// Make sure that there is a mutually supported cipher suite that works with
	// this certificate. Cipher suite selection will then apply the logic in
	// reverse to pick it. See also serverHandshakeState.cipherSuiteOk.
	cipherSuite := selectCipherSuite(chi.CipherSuites, config.cipherSuites(), func(c *cipherSuite) bool {
		if c.flags&suiteECDHE == 0 {
			return false
		}
		if c.flags&suiteECSign != 0 {
			if !ecdsaCipherSuite {
				return false
			}
		} else {
			if ecdsaCipherSuite {
				return false
			}
		}
		if vers < VersionTLS12 && c.flags&suiteTLS12 != 0 {
			return false
		}
		return true
	})
	if cipherSuite == nil {
		return supportsRSAFallback(errors.New("client doesn't support any cipher suites compatible with the certificate"))
	}

	return nil
}

// SupportsCertificate returns nil if the provided certificate is supported by
// the server that sent the CertificateRequest. Otherwise, it returns an error
// describing the reason for the incompatibility.
func (cri *CertificateRequestInfo) SupportsCertificate(c *Certificate) error {
	if _, err := selectSignatureScheme(cri.Version, c, cri.SignatureSchemes); err != nil {
		return err
	}

	if len(cri.AcceptableCAs) == 0 {
		return nil
	}

	for j, cert := range c.Certificate {
		x509Cert := c.Leaf
		// Parse the certificate if this isn't the leaf node, or if
		// chain.Leaf was nil.
		if j != 0 || x509Cert == nil {
			var err error
			if x509Cert, err = x509.ParseCertificate(cert); err != nil {
				return fmt.Errorf("failed to parse certificate #%d in the chain: %w", j, err)
			}
		}

		for _, ca := range cri.AcceptableCAs {
			if bytes.Equal(x509Cert.RawIssuer, ca) {
				return nil
			}
		}
	}
	return errors.New("chain is not signed by an acceptable CA")
}

// BuildNameToCertificate parses c.Certificates and builds c.NameToCertificate
// from the CommonName and SubjectAlternateName fields of each of the leaf
// certificates.
//
// Deprecated: NameToCertificate only allows associating a single certificate
// with a given name. Leave that field nil to let the library select the first
// compatible chain from Certificates.
func (c *Config) BuildNameToCertificate() {
	c.NameToCertificate = make(map[string]*Certificate)
	for i := range c.Certificates {
		cert := &c.Certificates[i]
		x509Cert, err := cert.leaf()
		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
		}
	}
}

const (
	keyLogLabelTLS12           = "CLIENT_RANDOM"
	keyLogLabelEarlyTraffic    = "CLIENT_EARLY_TRAFFIC_SECRET"
	keyLogLabelClientHandshake = "CLIENT_HANDSHAKE_TRAFFIC_SECRET"
	keyLogLabelServerHandshake = "SERVER_HANDSHAKE_TRAFFIC_SECRET"
	keyLogLabelClientTraffic   = "CLIENT_TRAFFIC_SECRET_0"
	keyLogLabelServerTraffic   = "SERVER_TRAFFIC_SECRET_0"
)

func (c *Config) writeKeyLog(label string, clientRandom, secret []byte) error {
	if c.KeyLogWriter == nil {
		return nil
	}

	logLine := []byte(fmt.Sprintf("%s %x %x\n", label, clientRandom, secret))

	writerMutex.Lock()
	_, err := c.KeyLogWriter.Write(logLine)
	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. This must implement crypto.Signer with an RSA, ECDSA or Ed25519 PublicKey.
	// For a server up to TLS 1.2, it can also implement crypto.Decrypter with
	// an RSA PublicKey.
	PrivateKey crypto.PrivateKey
	// SupportedSignatureAlgorithms is an optional list restricting what
	// signature algorithms the PrivateKey can be used for.
	SupportedSignatureAlgorithms []SignatureScheme
	// 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. If nil,
	// the leaf certificate will be parsed as needed.
	Leaf *x509.Certificate
}

// leaf returns the parsed leaf certificate, either from c.Leaf or by parsing
// the corresponding c.Certificate[0].
func (c *Certificate) leaf() (*x509.Certificate, error) {
	if c.Leaf != nil {
		return c.Leaf, nil
	}
	return x509.ParseCertificate(c.Certificate[0])
}

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. If cs is nil, the entry
// corresponding to sessionKey is removed from the cache instead.
func (c *lruSessionCache) Put(sessionKey string, cs *ClientSessionState) {
	c.Lock()
	defer c.Unlock()

	if elem, ok := c.m[sessionKey]; ok {
		if cs == nil {
			c.q.Remove(elem)
			delete(c.m, sessionKey)
		} else {
			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
	varDefaultCipherSuitesTLS13 []uint16
)

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

func defaultCipherSuitesTLS13() []uint16 {
	once.Do(initDefaultCipherSuites)
	return varDefaultCipherSuitesTLS13
}

func initDefaultCipherSuites() {
	var topCipherSuites []uint16

	// Check the cpu flags for each platform that has optimized GCM implementations.
	// Worst case, these variables will just all be false.
	var (
		hasGCMAsmAMD64 = cpu.X86.HasAES && cpu.X86.HasPCLMULQDQ
		hasGCMAsmARM64 = cpu.ARM64.HasAES && cpu.ARM64.HasPMULL
		// Keep in sync with crypto/aes/cipher_s390x.go.
		// TODO: check for s390
		// hasGCMAsmS390X = cpu.S390X.HasAES && cpu.S390X.HasAESCBC && cpu.S390X.HasAESCTR && (cpu.S390X.HasGHASH || cpu.S390X.HasAESGCM)
		hasGCMAsmS390X = false

		hasGCMAsm = hasGCMAsmAMD64 || hasGCMAsmARM64 || hasGCMAsmS390X
	)

	if hasGCMAsm {
		// If AES-GCM hardware is provided then prioritise AES-GCM
		// cipher suites.
		topCipherSuites = []uint16{
			TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
			TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
			TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
			TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
			TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
			TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
		}
		varDefaultCipherSuitesTLS13 = []uint16{
			TLS_AES_128_GCM_SHA256,
			TLS_CHACHA20_POLY1305_SHA256,
			TLS_AES_256_GCM_SHA384,
		}
	} else {
		// Without AES-GCM hardware, we put the ChaCha20-Poly1305
		// cipher suites first.
		topCipherSuites = []uint16{
			TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
			TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
			TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
			TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
			TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
			TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
		}
		varDefaultCipherSuitesTLS13 = []uint16{
			TLS_CHACHA20_POLY1305_SHA256,
			TLS_AES_128_GCM_SHA256,
			TLS_AES_256_GCM_SHA384,
		}
	}

	varDefaultCipherSuites = make([]uint16, 0, len(cipherSuites))
	varDefaultCipherSuites = append(varDefaultCipherSuites, topCipherSuites...)

NextCipherSuite:
	for _, suite := range cipherSuites {
		if suite.flags&suiteDefaultOff != 0 {
			continue
		}
		for _, existing := range varDefaultCipherSuites {
			if existing == suite.id {
				continue NextCipherSuite
			}
		}
		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 isSupportedSignatureAlgorithm(sigAlg SignatureScheme, supportedSignatureAlgorithms []SignatureScheme) bool {
	for _, s := range supportedSignatureAlgorithms {
		if s == sigAlg {
			return true
		}
	}
	return false
}