libgo: update to go1.9

[official-gcc.git] / libgo / go / crypto / tls / handshake_server.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 (
        "crypto"
        "crypto/ecdsa"
        "crypto/rsa"
        "crypto/subtle"
        "crypto/x509"
        "encoding/asn1"
        "errors"
        "fmt"
        "io"
)

// serverHandshakeState contains details of a server handshake in progress.
// It's discarded once the handshake has completed.
type serverHandshakeState struct {
        c                     *Conn
        clientHello           *clientHelloMsg
        hello                 *serverHelloMsg
        suite                 *cipherSuite
        ellipticOk            bool
        ecdsaOk               bool
        rsaDecryptOk          bool
        rsaSignOk             bool
        sessionState          *sessionState
        finishedHash          finishedHash
        masterSecret          []byte
        certsFromClient       [][]byte
        cert                  *Certificate
        cachedClientHelloInfo *ClientHelloInfo
}

// serverHandshake performs a TLS handshake as a server.
// c.out.Mutex <= L; c.handshakeMutex <= L.
func (c *Conn) serverHandshake() error {
        // If this is the first server handshake, we generate a random key to
        // encrypt the tickets with.
        c.config.serverInitOnce.Do(func() { c.config.serverInit(nil) })

        hs := serverHandshakeState{
                c: c,
        }
        isResume, err := hs.readClientHello()
        if err != nil {
                return err
        }

        // For an overview of TLS handshaking, see https://tools.ietf.org/html/rfc5246#section-7.3
        c.buffering = true
        if isResume {
                // The client has included a session ticket and so we do an abbreviated handshake.
                if err := hs.doResumeHandshake(); err != nil {
                        return err
                }
                if err := hs.establishKeys(); err != nil {
                        return err
                }
                // ticketSupported is set in a resumption handshake if the
                // ticket from the client was encrypted with an old session
                // ticket key and thus a refreshed ticket should be sent.
                if hs.hello.ticketSupported {
                        if err := hs.sendSessionTicket(); err != nil {
                                return err
                        }
                }
                if err := hs.sendFinished(c.serverFinished[:]); err != nil {
                        return err
                }
                if _, err := c.flush(); err != nil {
                        return err
                }
                c.clientFinishedIsFirst = false
                if err := hs.readFinished(nil); err != nil {
                        return err
                }
                c.didResume = true
        } else {
                // The client didn't include a session ticket, or it wasn't
                // valid so we do a full handshake.
                if err := hs.doFullHandshake(); err != nil {
                        return err
                }
                if err := hs.establishKeys(); err != nil {
                        return err
                }
                if err := hs.readFinished(c.clientFinished[:]); err != nil {
                        return err
                }
                c.clientFinishedIsFirst = true
                c.buffering = true
                if err := hs.sendSessionTicket(); err != nil {
                        return err
                }
                if err := hs.sendFinished(nil); err != nil {
                        return err
                }
                if _, err := c.flush(); err != nil {
                        return err
                }
        }
        c.handshakeComplete = true

        return nil
}

// readClientHello reads a ClientHello message from the client and decides
// whether we will perform session resumption.
func (hs *serverHandshakeState) readClientHello() (isResume bool, err error) {
        c := hs.c

        msg, err := c.readHandshake()
        if err != nil {
                return false, err
        }
        var ok bool
        hs.clientHello, ok = msg.(*clientHelloMsg)
        if !ok {
                c.sendAlert(alertUnexpectedMessage)
                return false, unexpectedMessageError(hs.clientHello, msg)
        }

        if c.config.GetConfigForClient != nil {
                if newConfig, err := c.config.GetConfigForClient(hs.clientHelloInfo()); err != nil {
                        c.sendAlert(alertInternalError)
                        return false, err
                } else if newConfig != nil {
                        newConfig.serverInitOnce.Do(func() { newConfig.serverInit(c.config) })
                        c.config = newConfig
                }
        }

        c.vers, ok = c.config.mutualVersion(hs.clientHello.vers)
        if !ok {
                c.sendAlert(alertProtocolVersion)
                return false, fmt.Errorf("tls: client offered an unsupported, maximum protocol version of %x", hs.clientHello.vers)
        }
        c.haveVers = true

        hs.hello = new(serverHelloMsg)

        supportedCurve := false
        preferredCurves := c.config.curvePreferences()
Curves:
        for _, curve := range hs.clientHello.supportedCurves {
                for _, supported := range preferredCurves {
                        if supported == curve {
                                supportedCurve = true
                                break Curves
                        }
                }
        }

        supportedPointFormat := false
        for _, pointFormat := range hs.clientHello.supportedPoints {
                if pointFormat == pointFormatUncompressed {
                        supportedPointFormat = true
                        break
                }
        }
        hs.ellipticOk = supportedCurve && supportedPointFormat

        foundCompression := false
        // We only support null compression, so check that the client offered it.
        for _, compression := range hs.clientHello.compressionMethods {
                if compression == compressionNone {
                        foundCompression = true
                        break
                }
        }

        if !foundCompression {
                c.sendAlert(alertHandshakeFailure)
                return false, errors.New("tls: client does not support uncompressed connections")
        }

        hs.hello.vers = c.vers
        hs.hello.random = make([]byte, 32)
        _, err = io.ReadFull(c.config.rand(), hs.hello.random)
        if err != nil {
                c.sendAlert(alertInternalError)
                return false, err
        }

        if len(hs.clientHello.secureRenegotiation) != 0 {
                c.sendAlert(alertHandshakeFailure)
                return false, errors.New("tls: initial handshake had non-empty renegotiation extension")
        }

        hs.hello.secureRenegotiationSupported = hs.clientHello.secureRenegotiationSupported
        hs.hello.compressionMethod = compressionNone
        if len(hs.clientHello.serverName) > 0 {
                c.serverName = hs.clientHello.serverName
        }

        if len(hs.clientHello.alpnProtocols) > 0 {
                if selectedProto, fallback := mutualProtocol(hs.clientHello.alpnProtocols, c.config.NextProtos); !fallback {
                        hs.hello.alpnProtocol = selectedProto
                        c.clientProtocol = selectedProto
                }
        } else {
                // Although sending an empty NPN extension is reasonable, Firefox has
                // had a bug around this. Best to send nothing at all if
                // c.config.NextProtos is empty. See
                // https://golang.org/issue/5445.
                if hs.clientHello.nextProtoNeg && len(c.config.NextProtos) > 0 {
                        hs.hello.nextProtoNeg = true
                        hs.hello.nextProtos = c.config.NextProtos
                }
        }

        hs.cert, err = c.config.getCertificate(hs.clientHelloInfo())
        if err != nil {
                c.sendAlert(alertInternalError)
                return false, err
        }
        if hs.clientHello.scts {
                hs.hello.scts = hs.cert.SignedCertificateTimestamps
        }

        if priv, ok := hs.cert.PrivateKey.(crypto.Signer); ok {
                switch priv.Public().(type) {
                case *ecdsa.PublicKey:
                        hs.ecdsaOk = true
                case *rsa.PublicKey:
                        hs.rsaSignOk = true
                default:
                        c.sendAlert(alertInternalError)
                        return false, fmt.Errorf("tls: unsupported signing key type (%T)", priv.Public())
                }
        }
        if priv, ok := hs.cert.PrivateKey.(crypto.Decrypter); ok {
                switch priv.Public().(type) {
                case *rsa.PublicKey:
                        hs.rsaDecryptOk = true
                default:
                        c.sendAlert(alertInternalError)
                        return false, fmt.Errorf("tls: unsupported decryption key type (%T)", priv.Public())
                }
        }

        if hs.checkForResumption() {
                return true, nil
        }

        var preferenceList, supportedList []uint16
        if c.config.PreferServerCipherSuites {
                preferenceList = c.config.cipherSuites()
                supportedList = hs.clientHello.cipherSuites
        } else {
                preferenceList = hs.clientHello.cipherSuites
                supportedList = c.config.cipherSuites()
        }

        for _, id := range preferenceList {
                if hs.setCipherSuite(id, supportedList, c.vers) {
                        break
                }
        }

        if hs.suite == nil {
                c.sendAlert(alertHandshakeFailure)
                return false, errors.New("tls: no cipher suite supported by both client and server")
        }

        // See https://tools.ietf.org/html/rfc7507.
        for _, id := range hs.clientHello.cipherSuites {
                if id == TLS_FALLBACK_SCSV {
                        // The client is doing a fallback connection.
                        if hs.clientHello.vers < c.config.maxVersion() {
                                c.sendAlert(alertInappropriateFallback)
                                return false, errors.New("tls: client using inappropriate protocol fallback")
                        }
                        break
                }
        }

        return false, nil
}

// checkForResumption reports whether we should perform resumption on this connection.
func (hs *serverHandshakeState) checkForResumption() bool {
        c := hs.c

        if c.config.SessionTicketsDisabled {
                return false
        }

        var ok bool
        var sessionTicket = append([]uint8{}, hs.clientHello.sessionTicket...)
        if hs.sessionState, ok = c.decryptTicket(sessionTicket); !ok {
                return false
        }

        // Never resume a session for a different TLS version.
        if c.vers != hs.sessionState.vers {
                return false
        }

        cipherSuiteOk := false
        // Check that the client is still offering the ciphersuite in the session.
        for _, id := range hs.clientHello.cipherSuites {
                if id == hs.sessionState.cipherSuite {
                        cipherSuiteOk = true
                        break
                }
        }
        if !cipherSuiteOk {
                return false
        }

        // Check that we also support the ciphersuite from the session.
        if !hs.setCipherSuite(hs.sessionState.cipherSuite, c.config.cipherSuites(), hs.sessionState.vers) {
                return false
        }

        sessionHasClientCerts := len(hs.sessionState.certificates) != 0
        needClientCerts := c.config.ClientAuth == RequireAnyClientCert || c.config.ClientAuth == RequireAndVerifyClientCert
        if needClientCerts && !sessionHasClientCerts {
                return false
        }
        if sessionHasClientCerts && c.config.ClientAuth == NoClientCert {
                return false
        }

        return true
}

func (hs *serverHandshakeState) doResumeHandshake() error {
        c := hs.c

        hs.hello.cipherSuite = hs.suite.id
        // We echo the client's session ID in the ServerHello to let it know
        // that we're doing a resumption.
        hs.hello.sessionId = hs.clientHello.sessionId
        hs.hello.ticketSupported = hs.sessionState.usedOldKey
        hs.finishedHash = newFinishedHash(c.vers, hs.suite)
        hs.finishedHash.discardHandshakeBuffer()
        hs.finishedHash.Write(hs.clientHello.marshal())
        hs.finishedHash.Write(hs.hello.marshal())
        if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
                return err
        }

        if len(hs.sessionState.certificates) > 0 {
                if _, err := hs.processCertsFromClient(hs.sessionState.certificates); err != nil {
                        return err
                }
        }

        hs.masterSecret = hs.sessionState.masterSecret

        return nil
}

func (hs *serverHandshakeState) doFullHandshake() error {
        c := hs.c

        if hs.clientHello.ocspStapling && len(hs.cert.OCSPStaple) > 0 {
                hs.hello.ocspStapling = true
        }

        hs.hello.ticketSupported = hs.clientHello.ticketSupported && !c.config.SessionTicketsDisabled
        hs.hello.cipherSuite = hs.suite.id

        hs.finishedHash = newFinishedHash(hs.c.vers, hs.suite)
        if c.config.ClientAuth == NoClientCert {
                // No need to keep a full record of the handshake if client
                // certificates won't be used.
                hs.finishedHash.discardHandshakeBuffer()
        }
        hs.finishedHash.Write(hs.clientHello.marshal())
        hs.finishedHash.Write(hs.hello.marshal())
        if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
                return err
        }

        certMsg := new(certificateMsg)
        certMsg.certificates = hs.cert.Certificate
        hs.finishedHash.Write(certMsg.marshal())
        if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
                return err
        }

        if hs.hello.ocspStapling {
                certStatus := new(certificateStatusMsg)
                certStatus.statusType = statusTypeOCSP
                certStatus.response = hs.cert.OCSPStaple
                hs.finishedHash.Write(certStatus.marshal())
                if _, err := c.writeRecord(recordTypeHandshake, certStatus.marshal()); err != nil {
                        return err
                }
        }

        keyAgreement := hs.suite.ka(c.vers)
        skx, err := keyAgreement.generateServerKeyExchange(c.config, hs.cert, hs.clientHello, hs.hello)
        if err != nil {
                c.sendAlert(alertHandshakeFailure)
                return err
        }
        if skx != nil {
                hs.finishedHash.Write(skx.marshal())
                if _, err := c.writeRecord(recordTypeHandshake, skx.marshal()); err != nil {
                        return err
                }
        }

        if c.config.ClientAuth >= RequestClientCert {
                // Request a client certificate
                certReq := new(certificateRequestMsg)
                certReq.certificateTypes = []byte{
                        byte(certTypeRSASign),
                        byte(certTypeECDSASign),
                }
                if c.vers >= VersionTLS12 {
                        certReq.hasSignatureAndHash = true
                        certReq.signatureAndHashes = supportedSignatureAlgorithms
                }

                // An empty list of certificateAuthorities signals to
                // the client that it may send any certificate in response
                // to our request. When we know the CAs we trust, then
                // we can send them down, so that the client can choose
                // an appropriate certificate to give to us.
                if c.config.ClientCAs != nil {
                        certReq.certificateAuthorities = c.config.ClientCAs.Subjects()
                }
                hs.finishedHash.Write(certReq.marshal())
                if _, err := c.writeRecord(recordTypeHandshake, certReq.marshal()); err != nil {
                        return err
                }
        }

        helloDone := new(serverHelloDoneMsg)
        hs.finishedHash.Write(helloDone.marshal())
        if _, err := c.writeRecord(recordTypeHandshake, helloDone.marshal()); err != nil {
                return err
        }

        if _, err := c.flush(); err != nil {
                return err
        }

        var pub crypto.PublicKey // public key for client auth, if any

        msg, err := c.readHandshake()
        if err != nil {
                return err
        }

        var ok bool
        // If we requested a client certificate, then the client must send a
        // certificate message, even if it's empty.
        if c.config.ClientAuth >= RequestClientCert {
                if certMsg, ok = msg.(*certificateMsg); !ok {
                        c.sendAlert(alertUnexpectedMessage)
                        return unexpectedMessageError(certMsg, msg)
                }
                hs.finishedHash.Write(certMsg.marshal())

                if len(certMsg.certificates) == 0 {
                        // The client didn't actually send a certificate
                        switch c.config.ClientAuth {
                        case RequireAnyClientCert, RequireAndVerifyClientCert:
                                c.sendAlert(alertBadCertificate)
                                return errors.New("tls: client didn't provide a certificate")
                        }
                }

                pub, err = hs.processCertsFromClient(certMsg.certificates)
                if err != nil {
                        return err
                }

                msg, err = c.readHandshake()
                if err != nil {
                        return err
                }
        }

        // Get client key exchange
        ckx, ok := msg.(*clientKeyExchangeMsg)
        if !ok {
                c.sendAlert(alertUnexpectedMessage)
                return unexpectedMessageError(ckx, msg)
        }
        hs.finishedHash.Write(ckx.marshal())

        preMasterSecret, err := keyAgreement.processClientKeyExchange(c.config, hs.cert, ckx, c.vers)
        if err != nil {
                c.sendAlert(alertHandshakeFailure)
                return err
        }
        hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.clientHello.random, hs.hello.random)
        if err := c.config.writeKeyLog(hs.clientHello.random, hs.masterSecret); err != nil {
                c.sendAlert(alertInternalError)
                return err
        }

        // If we received a client cert in response to our certificate request message,
        // the client will send us a certificateVerifyMsg immediately after the
        // clientKeyExchangeMsg. This message is a digest of all preceding
        // handshake-layer messages that is signed using the private key corresponding
        // to the client's certificate. This allows us to verify that the client is in
        // possession of the private key of the certificate.
        if len(c.peerCertificates) > 0 {
                msg, err = c.readHandshake()
                if err != nil {
                        return err
                }
                certVerify, ok := msg.(*certificateVerifyMsg)
                if !ok {
                        c.sendAlert(alertUnexpectedMessage)
                        return unexpectedMessageError(certVerify, msg)
                }

                // Determine the signature type.
                var signatureAndHash signatureAndHash
                if certVerify.hasSignatureAndHash {
                        signatureAndHash = certVerify.signatureAndHash
                        if !isSupportedSignatureAndHash(signatureAndHash, supportedSignatureAlgorithms) {
                                return errors.New("tls: unsupported hash function for client certificate")
                        }
                } else {
                        // Before TLS 1.2 the signature algorithm was implicit
                        // from the key type, and only one hash per signature
                        // algorithm was possible. Leave the hash as zero.
                        switch pub.(type) {
                        case *ecdsa.PublicKey:
                                signatureAndHash.signature = signatureECDSA
                        case *rsa.PublicKey:
                                signatureAndHash.signature = signatureRSA
                        }
                }

                switch key := pub.(type) {
                case *ecdsa.PublicKey:
                        if signatureAndHash.signature != signatureECDSA {
                                err = errors.New("tls: bad signature type for client's ECDSA certificate")
                                break
                        }
                        ecdsaSig := new(ecdsaSignature)
                        if _, err = asn1.Unmarshal(certVerify.signature, ecdsaSig); err != nil {
                                break
                        }
                        if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
                                err = errors.New("tls: ECDSA signature contained zero or negative values")
                                break
                        }
                        var digest []byte
                        if digest, _, err = hs.finishedHash.hashForClientCertificate(signatureAndHash, hs.masterSecret); err != nil {
                                break
                        }
                        if !ecdsa.Verify(key, digest, ecdsaSig.R, ecdsaSig.S) {
                                err = errors.New("tls: ECDSA verification failure")
                        }
                case *rsa.PublicKey:
                        if signatureAndHash.signature != signatureRSA {
                                err = errors.New("tls: bad signature type for client's RSA certificate")
                                break
                        }
                        var digest []byte
                        var hashFunc crypto.Hash
                        if digest, hashFunc, err = hs.finishedHash.hashForClientCertificate(signatureAndHash, hs.masterSecret); err != nil {
                                break
                        }
                        err = rsa.VerifyPKCS1v15(key, hashFunc, digest, certVerify.signature)
                }
                if err != nil {
                        c.sendAlert(alertBadCertificate)
                        return errors.New("tls: could not validate signature of connection nonces: " + err.Error())
                }

                hs.finishedHash.Write(certVerify.marshal())
        }

        hs.finishedHash.discardHandshakeBuffer()

        return nil
}

func (hs *serverHandshakeState) establishKeys() error {
        c := hs.c

        clientMAC, serverMAC, clientKey, serverKey, clientIV, serverIV :=
                keysFromMasterSecret(c.vers, hs.suite, hs.masterSecret, hs.clientHello.random, hs.hello.random, hs.suite.macLen, hs.suite.keyLen, hs.suite.ivLen)

        var clientCipher, serverCipher interface{}
        var clientHash, serverHash macFunction

        if hs.suite.aead == nil {
                clientCipher = hs.suite.cipher(clientKey, clientIV, true /* for reading */)
                clientHash = hs.suite.mac(c.vers, clientMAC)
                serverCipher = hs.suite.cipher(serverKey, serverIV, false /* not for reading */)
                serverHash = hs.suite.mac(c.vers, serverMAC)
        } else {
                clientCipher = hs.suite.aead(clientKey, clientIV)
                serverCipher = hs.suite.aead(serverKey, serverIV)
        }

        c.in.prepareCipherSpec(c.vers, clientCipher, clientHash)
        c.out.prepareCipherSpec(c.vers, serverCipher, serverHash)

        return nil
}

func (hs *serverHandshakeState) readFinished(out []byte) error {
        c := hs.c

        c.readRecord(recordTypeChangeCipherSpec)
        if c.in.err != nil {
                return c.in.err
        }

        if hs.hello.nextProtoNeg {
                msg, err := c.readHandshake()
                if err != nil {
                        return err
                }
                nextProto, ok := msg.(*nextProtoMsg)
                if !ok {
                        c.sendAlert(alertUnexpectedMessage)
                        return unexpectedMessageError(nextProto, msg)
                }
                hs.finishedHash.Write(nextProto.marshal())
                c.clientProtocol = nextProto.proto
        }

        msg, err := c.readHandshake()
        if err != nil {
                return err
        }
        clientFinished, ok := msg.(*finishedMsg)
        if !ok {
                c.sendAlert(alertUnexpectedMessage)
                return unexpectedMessageError(clientFinished, msg)
        }

        verify := hs.finishedHash.clientSum(hs.masterSecret)
        if len(verify) != len(clientFinished.verifyData) ||
                subtle.ConstantTimeCompare(verify, clientFinished.verifyData) != 1 {
                c.sendAlert(alertHandshakeFailure)
                return errors.New("tls: client's Finished message is incorrect")
        }

        hs.finishedHash.Write(clientFinished.marshal())
        copy(out, verify)
        return nil
}

func (hs *serverHandshakeState) sendSessionTicket() error {
        if !hs.hello.ticketSupported {
                return nil
        }

        c := hs.c
        m := new(newSessionTicketMsg)

        var err error
        state := sessionState{
                vers:         c.vers,
                cipherSuite:  hs.suite.id,
                masterSecret: hs.masterSecret,
                certificates: hs.certsFromClient,
        }
        m.ticket, err = c.encryptTicket(&state)
        if err != nil {
                return err
        }

        hs.finishedHash.Write(m.marshal())
        if _, err := c.writeRecord(recordTypeHandshake, m.marshal()); err != nil {
                return err
        }

        return nil
}

func (hs *serverHandshakeState) sendFinished(out []byte) error {
        c := hs.c

        if _, err := c.writeRecord(recordTypeChangeCipherSpec, []byte{1}); err != nil {
                return err
        }

        finished := new(finishedMsg)
        finished.verifyData = hs.finishedHash.serverSum(hs.masterSecret)
        hs.finishedHash.Write(finished.marshal())
        if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
                return err
        }

        c.cipherSuite = hs.suite.id
        copy(out, finished.verifyData)

        return nil
}

// processCertsFromClient takes a chain of client certificates either from a
// Certificates message or from a sessionState and verifies them. It returns
// the public key of the leaf certificate.
func (hs *serverHandshakeState) processCertsFromClient(certificates [][]byte) (crypto.PublicKey, error) {
        c := hs.c

        hs.certsFromClient = certificates
        certs := make([]*x509.Certificate, len(certificates))
        var err error
        for i, asn1Data := range certificates {
                if certs[i], err = x509.ParseCertificate(asn1Data); err != nil {
                        c.sendAlert(alertBadCertificate)
                        return nil, errors.New("tls: failed to parse client certificate: " + err.Error())
                }
        }

        if c.config.ClientAuth >= VerifyClientCertIfGiven && len(certs) > 0 {
                opts := x509.VerifyOptions{
                        Roots:         c.config.ClientCAs,
                        CurrentTime:   c.config.time(),
                        Intermediates: x509.NewCertPool(),
                        KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
                }

                for _, cert := range certs[1:] {
                        opts.Intermediates.AddCert(cert)
                }

                chains, err := certs[0].Verify(opts)
                if err != nil {
                        c.sendAlert(alertBadCertificate)
                        return nil, errors.New("tls: failed to verify client's certificate: " + err.Error())
                }

                c.verifiedChains = chains
        }

        if c.config.VerifyPeerCertificate != nil {
                if err := c.config.VerifyPeerCertificate(certificates, c.verifiedChains); err != nil {
                        c.sendAlert(alertBadCertificate)
                        return nil, err
                }
        }

        if len(certs) == 0 {
                return nil, nil
        }

        var pub crypto.PublicKey
        switch key := certs[0].PublicKey.(type) {
        case *ecdsa.PublicKey, *rsa.PublicKey:
                pub = key
        default:
                c.sendAlert(alertUnsupportedCertificate)
                return nil, fmt.Errorf("tls: client's certificate contains an unsupported public key of type %T", certs[0].PublicKey)
        }
        c.peerCertificates = certs
        return pub, nil
}

// setCipherSuite sets a cipherSuite with the given id as the serverHandshakeState
// suite if that cipher suite is acceptable to use.
// It returns a bool indicating if the suite was set.
func (hs *serverHandshakeState) setCipherSuite(id uint16, supportedCipherSuites []uint16, version uint16) bool {
        for _, supported := range supportedCipherSuites {
                if id == supported {
                        var candidate *cipherSuite

                        for _, s := range cipherSuites {
                                if s.id == id {
                                        candidate = s
                                        break
                                }
                        }
                        if candidate == nil {
                                continue
                        }
                        // Don't select a ciphersuite which we can't
                        // support for this client.
                        if candidate.flags&suiteECDHE != 0 {
                                if !hs.ellipticOk {
                                        continue
                                }
                                if candidate.flags&suiteECDSA != 0 {
                                        if !hs.ecdsaOk {
                                                continue
                                        }
                                } else if !hs.rsaSignOk {
                                        continue
                                }
                        } else if !hs.rsaDecryptOk {
                                continue
                        }
                        if version < VersionTLS12 && candidate.flags&suiteTLS12 != 0 {
                                continue
                        }
                        hs.suite = candidate
                        return true
                }
        }
        return false
}

// suppVersArray is the backing array of ClientHelloInfo.SupportedVersions
var suppVersArray = [...]uint16{VersionTLS12, VersionTLS11, VersionTLS10, VersionSSL30}

func (hs *serverHandshakeState) clientHelloInfo() *ClientHelloInfo {
        if hs.cachedClientHelloInfo != nil {
                return hs.cachedClientHelloInfo
        }

        var supportedVersions []uint16
        if hs.clientHello.vers > VersionTLS12 {
                supportedVersions = suppVersArray[:]
        } else if hs.clientHello.vers >= VersionSSL30 {
                supportedVersions = suppVersArray[VersionTLS12-hs.clientHello.vers:]
        }

        signatureSchemes := make([]SignatureScheme, 0, len(hs.clientHello.signatureAndHashes))
        for _, sah := range hs.clientHello.signatureAndHashes {
                signatureSchemes = append(signatureSchemes, SignatureScheme(sah.hash)<<8+SignatureScheme(sah.signature))
        }

        hs.cachedClientHelloInfo = &ClientHelloInfo{
                CipherSuites:      hs.clientHello.cipherSuites,
                ServerName:        hs.clientHello.serverName,
                SupportedCurves:   hs.clientHello.supportedCurves,
                SupportedPoints:   hs.clientHello.supportedPoints,
                SignatureSchemes:  signatureSchemes,
                SupportedProtos:   hs.clientHello.alpnProtocols,
                SupportedVersions: supportedVersions,
                Conn:              hs.c.conn,
        }

        return hs.cachedClientHelloInfo
}