Rebase.

[official-gcc.git] / libgo / go / crypto / tls / conn.go

// Copyright 2010 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.

// TLS low level connection and record layer

package tls

import (
        "bytes"
        "crypto/cipher"
        "crypto/subtle"
        "crypto/x509"
        "errors"
        "fmt"
        "io"
        "net"
        "sync"
        "time"
)

// A Conn represents a secured connection.
// It implements the net.Conn interface.
type Conn struct {
        // constant
        conn     net.Conn
        isClient bool

        // constant after handshake; protected by handshakeMutex
        handshakeMutex    sync.Mutex // handshakeMutex < in.Mutex, out.Mutex, errMutex
        handshakeErr      error      // error resulting from handshake
        vers              uint16     // TLS version
        haveVers          bool       // version has been negotiated
        config            *Config    // configuration passed to constructor
        handshakeComplete bool
        didResume         bool // whether this connection was a session resumption
        cipherSuite       uint16
        ocspResponse      []byte // stapled OCSP response
        peerCertificates  []*x509.Certificate
        // verifiedChains contains the certificate chains that we built, as
        // opposed to the ones presented by the server.
        verifiedChains [][]*x509.Certificate
        // serverName contains the server name indicated by the client, if any.
        serverName string

        clientProtocol         string
        clientProtocolFallback bool

        // input/output
        in, out  halfConn     // in.Mutex < out.Mutex
        rawInput *block       // raw input, right off the wire
        input    *block       // application data waiting to be read
        hand     bytes.Buffer // handshake data waiting to be read

        tmp [16]byte
}

// Access to net.Conn methods.
// Cannot just embed net.Conn because that would
// export the struct field too.

// LocalAddr returns the local network address.
func (c *Conn) LocalAddr() net.Addr {
        return c.conn.LocalAddr()
}

// RemoteAddr returns the remote network address.
func (c *Conn) RemoteAddr() net.Addr {
        return c.conn.RemoteAddr()
}

// SetDeadline sets the read and write deadlines associated with the connection.
// A zero value for t means Read and Write will not time out.
// After a Write has timed out, the TLS state is corrupt and all future writes will return the same error.
func (c *Conn) SetDeadline(t time.Time) error {
        return c.conn.SetDeadline(t)
}

// SetReadDeadline sets the read deadline on the underlying connection.
// A zero value for t means Read will not time out.
func (c *Conn) SetReadDeadline(t time.Time) error {
        return c.conn.SetReadDeadline(t)
}

// SetWriteDeadline sets the write deadline on the underlying connection.
// A zero value for t means Write will not time out.
// After a Write has timed out, the TLS state is corrupt and all future writes will return the same error.
func (c *Conn) SetWriteDeadline(t time.Time) error {
        return c.conn.SetWriteDeadline(t)
}

// A halfConn represents one direction of the record layer
// connection, either sending or receiving.
type halfConn struct {
        sync.Mutex

        err     error       // first permanent error
        version uint16      // protocol version
        cipher  interface{} // cipher algorithm
        mac     macFunction
        seq     [8]byte // 64-bit sequence number
        bfree   *block  // list of free blocks

        nextCipher interface{} // next encryption state
        nextMac    macFunction // next MAC algorithm

        // used to save allocating a new buffer for each MAC.
        inDigestBuf, outDigestBuf []byte
}

func (hc *halfConn) setErrorLocked(err error) error {
        hc.err = err
        return err
}

func (hc *halfConn) error() error {
        hc.Lock()
        err := hc.err
        hc.Unlock()
        return err
}

// prepareCipherSpec sets the encryption and MAC states
// that a subsequent changeCipherSpec will use.
func (hc *halfConn) prepareCipherSpec(version uint16, cipher interface{}, mac macFunction) {
        hc.version = version
        hc.nextCipher = cipher
        hc.nextMac = mac
}

// changeCipherSpec changes the encryption and MAC states
// to the ones previously passed to prepareCipherSpec.
func (hc *halfConn) changeCipherSpec() error {
        if hc.nextCipher == nil {
                return alertInternalError
        }
        hc.cipher = hc.nextCipher
        hc.mac = hc.nextMac
        hc.nextCipher = nil
        hc.nextMac = nil
        for i := range hc.seq {
                hc.seq[i] = 0
        }
        return nil
}

// incSeq increments the sequence number.
func (hc *halfConn) incSeq() {
        for i := 7; i >= 0; i-- {
                hc.seq[i]++
                if hc.seq[i] != 0 {
                        return
                }
        }

        // Not allowed to let sequence number wrap.
        // Instead, must renegotiate before it does.
        // Not likely enough to bother.
        panic("TLS: sequence number wraparound")
}

// resetSeq resets the sequence number to zero.
func (hc *halfConn) resetSeq() {
        for i := range hc.seq {
                hc.seq[i] = 0
        }
}

// removePadding returns an unpadded slice, in constant time, which is a prefix
// of the input. It also returns a byte which is equal to 255 if the padding
// was valid and 0 otherwise. See RFC 2246, section 6.2.3.2
func removePadding(payload []byte) ([]byte, byte) {
        if len(payload) < 1 {
                return payload, 0
        }

        paddingLen := payload[len(payload)-1]
        t := uint(len(payload)-1) - uint(paddingLen)
        // if len(payload) >= (paddingLen - 1) then the MSB of t is zero
        good := byte(int32(^t) >> 31)

        toCheck := 255 // the maximum possible padding length
        // The length of the padded data is public, so we can use an if here
        if toCheck+1 > len(payload) {
                toCheck = len(payload) - 1
        }

        for i := 0; i < toCheck; i++ {
                t := uint(paddingLen) - uint(i)
                // if i <= paddingLen then the MSB of t is zero
                mask := byte(int32(^t) >> 31)
                b := payload[len(payload)-1-i]
                good &^= mask&paddingLen ^ mask&b
        }

        // We AND together the bits of good and replicate the result across
        // all the bits.
        good &= good << 4
        good &= good << 2
        good &= good << 1
        good = uint8(int8(good) >> 7)

        toRemove := good&paddingLen + 1
        return payload[:len(payload)-int(toRemove)], good
}

// removePaddingSSL30 is a replacement for removePadding in the case that the
// protocol version is SSLv3. In this version, the contents of the padding
// are random and cannot be checked.
func removePaddingSSL30(payload []byte) ([]byte, byte) {
        if len(payload) < 1 {
                return payload, 0
        }

        paddingLen := int(payload[len(payload)-1]) + 1
        if paddingLen > len(payload) {
                return payload, 0
        }

        return payload[:len(payload)-paddingLen], 255
}

func roundUp(a, b int) int {
        return a + (b-a%b)%b
}

// cbcMode is an interface for block ciphers using cipher block chaining.
type cbcMode interface {
        cipher.BlockMode
        SetIV([]byte)
}

// decrypt checks and strips the mac and decrypts the data in b. Returns a
// success boolean, the number of bytes to skip from the start of the record in
// order to get the application payload, and an optional alert value.
func (hc *halfConn) decrypt(b *block) (ok bool, prefixLen int, alertValue alert) {
        // pull out payload
        payload := b.data[recordHeaderLen:]

        macSize := 0
        if hc.mac != nil {
                macSize = hc.mac.Size()
        }

        paddingGood := byte(255)
        explicitIVLen := 0

        // decrypt
        if hc.cipher != nil {
                switch c := hc.cipher.(type) {
                case cipher.Stream:
                        c.XORKeyStream(payload, payload)
                case cipher.AEAD:
                        explicitIVLen = 8
                        if len(payload) < explicitIVLen {
                                return false, 0, alertBadRecordMAC
                        }
                        nonce := payload[:8]
                        payload = payload[8:]

                        var additionalData [13]byte
                        copy(additionalData[:], hc.seq[:])
                        copy(additionalData[8:], b.data[:3])
                        n := len(payload) - c.Overhead()
                        additionalData[11] = byte(n >> 8)
                        additionalData[12] = byte(n)
                        var err error
                        payload, err = c.Open(payload[:0], nonce, payload, additionalData[:])
                        if err != nil {
                                return false, 0, alertBadRecordMAC
                        }
                        b.resize(recordHeaderLen + explicitIVLen + len(payload))
                case cbcMode:
                        blockSize := c.BlockSize()
                        if hc.version >= VersionTLS11 {
                                explicitIVLen = blockSize
                        }

                        if len(payload)%blockSize != 0 || len(payload) < roundUp(explicitIVLen+macSize+1, blockSize) {
                                return false, 0, alertBadRecordMAC
                        }

                        if explicitIVLen > 0 {
                                c.SetIV(payload[:explicitIVLen])
                                payload = payload[explicitIVLen:]
                        }
                        c.CryptBlocks(payload, payload)
                        if hc.version == VersionSSL30 {
                                payload, paddingGood = removePaddingSSL30(payload)
                        } else {
                                payload, paddingGood = removePadding(payload)
                        }
                        b.resize(recordHeaderLen + explicitIVLen + len(payload))

                        // note that we still have a timing side-channel in the
                        // MAC check, below. An attacker can align the record
                        // so that a correct padding will cause one less hash
                        // block to be calculated. Then they can iteratively
                        // decrypt a record by breaking each byte. See
                        // "Password Interception in a SSL/TLS Channel", Brice
                        // Canvel et al.
                        //
                        // However, our behavior matches OpenSSL, so we leak
                        // only as much as they do.
                default:
                        panic("unknown cipher type")
                }
        }

        // check, strip mac
        if hc.mac != nil {
                if len(payload) < macSize {
                        return false, 0, alertBadRecordMAC
                }

                // strip mac off payload, b.data
                n := len(payload) - macSize
                b.data[3] = byte(n >> 8)
                b.data[4] = byte(n)
                b.resize(recordHeaderLen + explicitIVLen + n)
                remoteMAC := payload[n:]
                localMAC := hc.mac.MAC(hc.inDigestBuf, hc.seq[0:], b.data[:recordHeaderLen], payload[:n])

                if subtle.ConstantTimeCompare(localMAC, remoteMAC) != 1 || paddingGood != 255 {
                        return false, 0, alertBadRecordMAC
                }
                hc.inDigestBuf = localMAC
        }
        hc.incSeq()

        return true, recordHeaderLen + explicitIVLen, 0
}

// padToBlockSize calculates the needed padding block, if any, for a payload.
// On exit, prefix aliases payload and extends to the end of the last full
// block of payload. finalBlock is a fresh slice which contains the contents of
// any suffix of payload as well as the needed padding to make finalBlock a
// full block.
func padToBlockSize(payload []byte, blockSize int) (prefix, finalBlock []byte) {
        overrun := len(payload) % blockSize
        paddingLen := blockSize - overrun
        prefix = payload[:len(payload)-overrun]
        finalBlock = make([]byte, blockSize)
        copy(finalBlock, payload[len(payload)-overrun:])
        for i := overrun; i < blockSize; i++ {
                finalBlock[i] = byte(paddingLen - 1)
        }
        return
}

// encrypt encrypts and macs the data in b.
func (hc *halfConn) encrypt(b *block, explicitIVLen int) (bool, alert) {
        // mac
        if hc.mac != nil {
                mac := hc.mac.MAC(hc.outDigestBuf, hc.seq[0:], b.data[:recordHeaderLen], b.data[recordHeaderLen+explicitIVLen:])

                n := len(b.data)
                b.resize(n + len(mac))
                copy(b.data[n:], mac)
                hc.outDigestBuf = mac
        }

        payload := b.data[recordHeaderLen:]

        // encrypt
        if hc.cipher != nil {
                switch c := hc.cipher.(type) {
                case cipher.Stream:
                        c.XORKeyStream(payload, payload)
                case cipher.AEAD:
                        payloadLen := len(b.data) - recordHeaderLen - explicitIVLen
                        b.resize(len(b.data) + c.Overhead())
                        nonce := b.data[recordHeaderLen : recordHeaderLen+explicitIVLen]
                        payload := b.data[recordHeaderLen+explicitIVLen:]
                        payload = payload[:payloadLen]

                        var additionalData [13]byte
                        copy(additionalData[:], hc.seq[:])
                        copy(additionalData[8:], b.data[:3])
                        additionalData[11] = byte(payloadLen >> 8)
                        additionalData[12] = byte(payloadLen)

                        c.Seal(payload[:0], nonce, payload, additionalData[:])
                case cbcMode:
                        blockSize := c.BlockSize()
                        if explicitIVLen > 0 {
                                c.SetIV(payload[:explicitIVLen])
                                payload = payload[explicitIVLen:]
                        }
                        prefix, finalBlock := padToBlockSize(payload, blockSize)
                        b.resize(recordHeaderLen + explicitIVLen + len(prefix) + len(finalBlock))
                        c.CryptBlocks(b.data[recordHeaderLen+explicitIVLen:], prefix)
                        c.CryptBlocks(b.data[recordHeaderLen+explicitIVLen+len(prefix):], finalBlock)
                default:
                        panic("unknown cipher type")
                }
        }

        // update length to include MAC and any block padding needed.
        n := len(b.data) - recordHeaderLen
        b.data[3] = byte(n >> 8)
        b.data[4] = byte(n)
        hc.incSeq()

        return true, 0
}

// A block is a simple data buffer.
type block struct {
        data []byte
        off  int // index for Read
        link *block
}

// resize resizes block to be n bytes, growing if necessary.
func (b *block) resize(n int) {
        if n > cap(b.data) {
                b.reserve(n)
        }
        b.data = b.data[0:n]
}

// reserve makes sure that block contains a capacity of at least n bytes.
func (b *block) reserve(n int) {
        if cap(b.data) >= n {
                return
        }
        m := cap(b.data)
        if m == 0 {
                m = 1024
        }
        for m < n {
                m *= 2
        }
        data := make([]byte, len(b.data), m)
        copy(data, b.data)
        b.data = data
}

// readFromUntil reads from r into b until b contains at least n bytes
// or else returns an error.
func (b *block) readFromUntil(r io.Reader, n int) error {
        // quick case
        if len(b.data) >= n {
                return nil
        }

        // read until have enough.
        b.reserve(n)
        for {
                m, err := r.Read(b.data[len(b.data):cap(b.data)])
                b.data = b.data[0 : len(b.data)+m]
                if len(b.data) >= n {
                        // TODO(bradfitz,agl): slightly suspicious
                        // that we're throwing away r.Read's err here.
                        break
                }
                if err != nil {
                        return err
                }
        }
        return nil
}

func (b *block) Read(p []byte) (n int, err error) {
        n = copy(p, b.data[b.off:])
        b.off += n
        return
}

// newBlock allocates a new block, from hc's free list if possible.
func (hc *halfConn) newBlock() *block {
        b := hc.bfree
        if b == nil {
                return new(block)
        }
        hc.bfree = b.link
        b.link = nil
        b.resize(0)
        return b
}

// freeBlock returns a block to hc's free list.
// The protocol is such that each side only has a block or two on
// its free list at a time, so there's no need to worry about
// trimming the list, etc.
func (hc *halfConn) freeBlock(b *block) {
        b.link = hc.bfree
        hc.bfree = b
}

// splitBlock splits a block after the first n bytes,
// returning a block with those n bytes and a
// block with the remainder.  the latter may be nil.
func (hc *halfConn) splitBlock(b *block, n int) (*block, *block) {
        if len(b.data) <= n {
                return b, nil
        }
        bb := hc.newBlock()
        bb.resize(len(b.data) - n)
        copy(bb.data, b.data[n:])
        b.data = b.data[0:n]
        return b, bb
}

// readRecord reads the next TLS record from the connection
// and updates the record layer state.
// c.in.Mutex <= L; c.input == nil.
func (c *Conn) readRecord(want recordType) error {
        // Caller must be in sync with connection:
        // handshake data if handshake not yet completed,
        // else application data.  (We don't support renegotiation.)
        switch want {
        default:
                c.sendAlert(alertInternalError)
                return c.in.setErrorLocked(errors.New("tls: unknown record type requested"))
        case recordTypeHandshake, recordTypeChangeCipherSpec:
                if c.handshakeComplete {
                        c.sendAlert(alertInternalError)
                        return c.in.setErrorLocked(errors.New("tls: handshake or ChangeCipherSpec requested after handshake complete"))
                }
        case recordTypeApplicationData:
                if !c.handshakeComplete {
                        c.sendAlert(alertInternalError)
                        return c.in.setErrorLocked(errors.New("tls: application data record requested before handshake complete"))
                }
        }

Again:
        if c.rawInput == nil {
                c.rawInput = c.in.newBlock()
        }
        b := c.rawInput

        // Read header, payload.
        if err := b.readFromUntil(c.conn, recordHeaderLen); err != nil {
                // RFC suggests that EOF without an alertCloseNotify is
                // an error, but popular web sites seem to do this,
                // so we can't make it an error.
                // if err == io.EOF {
                //      err = io.ErrUnexpectedEOF
                // }
                if e, ok := err.(net.Error); !ok || !e.Temporary() {
                        c.in.setErrorLocked(err)
                }
                return err
        }
        typ := recordType(b.data[0])

        // No valid TLS record has a type of 0x80, however SSLv2 handshakes
        // start with a uint16 length where the MSB is set and the first record
        // is always < 256 bytes long. Therefore typ == 0x80 strongly suggests
        // an SSLv2 client.
        if want == recordTypeHandshake && typ == 0x80 {
                c.sendAlert(alertProtocolVersion)
                return c.in.setErrorLocked(errors.New("tls: unsupported SSLv2 handshake received"))
        }

        vers := uint16(b.data[1])<<8 | uint16(b.data[2])
        n := int(b.data[3])<<8 | int(b.data[4])
        if c.haveVers && vers != c.vers {
                c.sendAlert(alertProtocolVersion)
                return c.in.setErrorLocked(fmt.Errorf("tls: received record with version %x when expecting version %x", vers, c.vers))
        }
        if n > maxCiphertext {
                c.sendAlert(alertRecordOverflow)
                return c.in.setErrorLocked(fmt.Errorf("tls: oversized record received with length %d", n))
        }
        if !c.haveVers {
                // First message, be extra suspicious:
                // this might not be a TLS client.
                // Bail out before reading a full 'body', if possible.
                // The current max version is 3.1.
                // If the version is >= 16.0, it's probably not real.
                // Similarly, a clientHello message encodes in
                // well under a kilobyte.  If the length is >= 12 kB,
                // it's probably not real.
                if (typ != recordTypeAlert && typ != want) || vers >= 0x1000 || n >= 0x3000 {
                        c.sendAlert(alertUnexpectedMessage)
                        return c.in.setErrorLocked(fmt.Errorf("tls: first record does not look like a TLS handshake"))
                }
        }
        if err := b.readFromUntil(c.conn, recordHeaderLen+n); err != nil {
                if err == io.EOF {
                        err = io.ErrUnexpectedEOF
                }
                if e, ok := err.(net.Error); !ok || !e.Temporary() {
                        c.in.setErrorLocked(err)
                }
                return err
        }

        // Process message.
        b, c.rawInput = c.in.splitBlock(b, recordHeaderLen+n)
        ok, off, err := c.in.decrypt(b)
        if !ok {
                c.in.setErrorLocked(c.sendAlert(err))
        }
        b.off = off
        data := b.data[b.off:]
        if len(data) > maxPlaintext {
                err := c.sendAlert(alertRecordOverflow)
                c.in.freeBlock(b)
                return c.in.setErrorLocked(err)
        }

        switch typ {
        default:
                c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))

        case recordTypeAlert:
                if len(data) != 2 {
                        c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
                        break
                }
                if alert(data[1]) == alertCloseNotify {
                        c.in.setErrorLocked(io.EOF)
                        break
                }
                switch data[0] {
                case alertLevelWarning:
                        // drop on the floor
                        c.in.freeBlock(b)
                        goto Again
                case alertLevelError:
                        c.in.setErrorLocked(&net.OpError{Op: "remote error", Err: alert(data[1])})
                default:
                        c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
                }

        case recordTypeChangeCipherSpec:
                if typ != want || len(data) != 1 || data[0] != 1 {
                        c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
                        break
                }
                err := c.in.changeCipherSpec()
                if err != nil {
                        c.in.setErrorLocked(c.sendAlert(err.(alert)))
                }

        case recordTypeApplicationData:
                if typ != want {
                        c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
                        break
                }
                c.input = b
                b = nil

        case recordTypeHandshake:
                // TODO(rsc): Should at least pick off connection close.
                if typ != want {
                        return c.in.setErrorLocked(c.sendAlert(alertNoRenegotiation))
                }
                c.hand.Write(data)
        }

        if b != nil {
                c.in.freeBlock(b)
        }
        return c.in.err
}

// sendAlert sends a TLS alert message.
// c.out.Mutex <= L.
func (c *Conn) sendAlertLocked(err alert) error {
        switch err {
        case alertNoRenegotiation, alertCloseNotify:
                c.tmp[0] = alertLevelWarning
        default:
                c.tmp[0] = alertLevelError
        }
        c.tmp[1] = byte(err)
        c.writeRecord(recordTypeAlert, c.tmp[0:2])
        // closeNotify is a special case in that it isn't an error:
        if err != alertCloseNotify {
                return c.out.setErrorLocked(&net.OpError{Op: "local error", Err: err})
        }
        return nil
}

// sendAlert sends a TLS alert message.
// L < c.out.Mutex.
func (c *Conn) sendAlert(err alert) error {
        c.out.Lock()
        defer c.out.Unlock()
        return c.sendAlertLocked(err)
}

// writeRecord writes a TLS record with the given type and payload
// to the connection and updates the record layer state.
// c.out.Mutex <= L.
func (c *Conn) writeRecord(typ recordType, data []byte) (n int, err error) {
        b := c.out.newBlock()
        for len(data) > 0 {
                m := len(data)
                if m > maxPlaintext {
                        m = maxPlaintext
                }
                explicitIVLen := 0
                explicitIVIsSeq := false

                var cbc cbcMode
                if c.out.version >= VersionTLS11 {
                        var ok bool
                        if cbc, ok = c.out.cipher.(cbcMode); ok {
                                explicitIVLen = cbc.BlockSize()
                        }
                }
                if explicitIVLen == 0 {
                        if _, ok := c.out.cipher.(cipher.AEAD); ok {
                                explicitIVLen = 8
                                // The AES-GCM construction in TLS has an
                                // explicit nonce so that the nonce can be
                                // random. However, the nonce is only 8 bytes
                                // which is too small for a secure, random
                                // nonce. Therefore we use the sequence number
                                // as the nonce.
                                explicitIVIsSeq = true
                        }
                }
                b.resize(recordHeaderLen + explicitIVLen + m)
                b.data[0] = byte(typ)
                vers := c.vers
                if vers == 0 {
                        // Some TLS servers fail if the record version is
                        // greater than TLS 1.0 for the initial ClientHello.
                        vers = VersionTLS10
                }
                b.data[1] = byte(vers >> 8)
                b.data[2] = byte(vers)
                b.data[3] = byte(m >> 8)
                b.data[4] = byte(m)
                if explicitIVLen > 0 {
                        explicitIV := b.data[recordHeaderLen : recordHeaderLen+explicitIVLen]
                        if explicitIVIsSeq {
                                copy(explicitIV, c.out.seq[:])
                        } else {
                                if _, err = io.ReadFull(c.config.rand(), explicitIV); err != nil {
                                        break
                                }
                        }
                }
                copy(b.data[recordHeaderLen+explicitIVLen:], data)
                c.out.encrypt(b, explicitIVLen)
                _, err = c.conn.Write(b.data)
                if err != nil {
                        break
                }
                n += m
                data = data[m:]
        }
        c.out.freeBlock(b)

        if typ == recordTypeChangeCipherSpec {
                err = c.out.changeCipherSpec()
                if err != nil {
                        // Cannot call sendAlert directly,
                        // because we already hold c.out.Mutex.
                        c.tmp[0] = alertLevelError
                        c.tmp[1] = byte(err.(alert))
                        c.writeRecord(recordTypeAlert, c.tmp[0:2])
                        return n, c.out.setErrorLocked(&net.OpError{Op: "local error", Err: err})
                }
        }
        return
}

// readHandshake reads the next handshake message from
// the record layer.
// c.in.Mutex < L; c.out.Mutex < L.
func (c *Conn) readHandshake() (interface{}, error) {
        for c.hand.Len() < 4 {
                if err := c.in.err; err != nil {
                        return nil, err
                }
                if err := c.readRecord(recordTypeHandshake); err != nil {
                        return nil, err
                }
        }

        data := c.hand.Bytes()
        n := int(data[1])<<16 | int(data[2])<<8 | int(data[3])
        if n > maxHandshake {
                return nil, c.in.setErrorLocked(c.sendAlert(alertInternalError))
        }
        for c.hand.Len() < 4+n {
                if err := c.in.err; err != nil {
                        return nil, err
                }
                if err := c.readRecord(recordTypeHandshake); err != nil {
                        return nil, err
                }
        }
        data = c.hand.Next(4 + n)
        var m handshakeMessage
        switch data[0] {
        case typeClientHello:
                m = new(clientHelloMsg)
        case typeServerHello:
                m = new(serverHelloMsg)
        case typeNewSessionTicket:
                m = new(newSessionTicketMsg)
        case typeCertificate:
                m = new(certificateMsg)
        case typeCertificateRequest:
                m = &certificateRequestMsg{
                        hasSignatureAndHash: c.vers >= VersionTLS12,
                }
        case typeCertificateStatus:
                m = new(certificateStatusMsg)
        case typeServerKeyExchange:
                m = new(serverKeyExchangeMsg)
        case typeServerHelloDone:
                m = new(serverHelloDoneMsg)
        case typeClientKeyExchange:
                m = new(clientKeyExchangeMsg)
        case typeCertificateVerify:
                m = &certificateVerifyMsg{
                        hasSignatureAndHash: c.vers >= VersionTLS12,
                }
        case typeNextProtocol:
                m = new(nextProtoMsg)
        case typeFinished:
                m = new(finishedMsg)
        default:
                return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
        }

        // The handshake message unmarshallers
        // expect to be able to keep references to data,
        // so pass in a fresh copy that won't be overwritten.
        data = append([]byte(nil), data...)

        if !m.unmarshal(data) {
                return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
        }
        return m, nil
}

// Write writes data to the connection.
func (c *Conn) Write(b []byte) (int, error) {
        if err := c.Handshake(); err != nil {
                return 0, err
        }

        c.out.Lock()
        defer c.out.Unlock()

        if err := c.out.err; err != nil {
                return 0, err
        }

        if !c.handshakeComplete {
                return 0, alertInternalError
        }

        // SSL 3.0 and TLS 1.0 are susceptible to a chosen-plaintext
        // attack when using block mode ciphers due to predictable IVs.
        // This can be prevented by splitting each Application Data
        // record into two records, effectively randomizing the IV.
        //
        // http://www.openssl.org/~bodo/tls-cbc.txt
        // https://bugzilla.mozilla.org/show_bug.cgi?id=665814
        // http://www.imperialviolet.org/2012/01/15/beastfollowup.html

        var m int
        if len(b) > 1 && c.vers <= VersionTLS10 {
                if _, ok := c.out.cipher.(cipher.BlockMode); ok {
                        n, err := c.writeRecord(recordTypeApplicationData, b[:1])
                        if err != nil {
                                return n, c.out.setErrorLocked(err)
                        }
                        m, b = 1, b[1:]
                }
        }

        n, err := c.writeRecord(recordTypeApplicationData, b)
        return n + m, c.out.setErrorLocked(err)
}

// Read can be made to time out and return a net.Error with Timeout() == true
// after a fixed time limit; see SetDeadline and SetReadDeadline.
func (c *Conn) Read(b []byte) (n int, err error) {
        if err = c.Handshake(); err != nil {
                return
        }
        if len(b) == 0 {
                // Put this after Handshake, in case people were calling
                // Read(nil) for the side effect of the Handshake.
                return
        }

        c.in.Lock()
        defer c.in.Unlock()

        // Some OpenSSL servers send empty records in order to randomize the
        // CBC IV. So this loop ignores a limited number of empty records.
        const maxConsecutiveEmptyRecords = 100
        for emptyRecordCount := 0; emptyRecordCount <= maxConsecutiveEmptyRecords; emptyRecordCount++ {
                for c.input == nil && c.in.err == nil {
                        if err := c.readRecord(recordTypeApplicationData); err != nil {
                                // Soft error, like EAGAIN
                                return 0, err
                        }
                }
                if err := c.in.err; err != nil {
                        return 0, err
                }

                n, err = c.input.Read(b)
                if c.input.off >= len(c.input.data) {
                        c.in.freeBlock(c.input)
                        c.input = nil
                }

                // If a close-notify alert is waiting, read it so that
                // we can return (n, EOF) instead of (n, nil), to signal
                // to the HTTP response reading goroutine that the
                // connection is now closed. This eliminates a race
                // where the HTTP response reading goroutine would
                // otherwise not observe the EOF until its next read,
                // by which time a client goroutine might have already
                // tried to reuse the HTTP connection for a new
                // request.
                // See https://codereview.appspot.com/76400046
                // and http://golang.org/issue/3514
                if ri := c.rawInput; ri != nil &&
                        n != 0 && err == nil &&
                        c.input == nil && len(ri.data) > 0 && recordType(ri.data[0]) == recordTypeAlert {
                        if recErr := c.readRecord(recordTypeApplicationData); recErr != nil {
                                err = recErr // will be io.EOF on closeNotify
                        }
                }

                if n != 0 || err != nil {
                        return n, err
                }
        }

        return 0, io.ErrNoProgress
}

// Close closes the connection.
func (c *Conn) Close() error {
        var alertErr error

        c.handshakeMutex.Lock()
        defer c.handshakeMutex.Unlock()
        if c.handshakeComplete {
                alertErr = c.sendAlert(alertCloseNotify)
        }

        if err := c.conn.Close(); err != nil {
                return err
        }
        return alertErr
}

// Handshake runs the client or server handshake
// protocol if it has not yet been run.
// Most uses of this package need not call Handshake
// explicitly: the first Read or Write will call it automatically.
func (c *Conn) Handshake() error {
        c.handshakeMutex.Lock()
        defer c.handshakeMutex.Unlock()
        if err := c.handshakeErr; err != nil {
                return err
        }
        if c.handshakeComplete {
                return nil
        }

        if c.isClient {
                c.handshakeErr = c.clientHandshake()
        } else {
                c.handshakeErr = c.serverHandshake()
        }
        return c.handshakeErr
}

// ConnectionState returns basic TLS details about the connection.
func (c *Conn) ConnectionState() ConnectionState {
        c.handshakeMutex.Lock()
        defer c.handshakeMutex.Unlock()

        var state ConnectionState
        state.HandshakeComplete = c.handshakeComplete
        if c.handshakeComplete {
                state.Version = c.vers
                state.NegotiatedProtocol = c.clientProtocol
                state.DidResume = c.didResume
                state.NegotiatedProtocolIsMutual = !c.clientProtocolFallback
                state.CipherSuite = c.cipherSuite
                state.PeerCertificates = c.peerCertificates
                state.VerifiedChains = c.verifiedChains
                state.ServerName = c.serverName
        }

        return state
}

// OCSPResponse returns the stapled OCSP response from the TLS server, if
// any. (Only valid for client connections.)
func (c *Conn) OCSPResponse() []byte {
        c.handshakeMutex.Lock()
        defer c.handshakeMutex.Unlock()

        return c.ocspResponse
}

// VerifyHostname checks that the peer certificate chain is valid for
// connecting to host.  If so, it returns nil; if not, it returns an error
// describing the problem.
func (c *Conn) VerifyHostname(host string) error {
        c.handshakeMutex.Lock()
        defer c.handshakeMutex.Unlock()
        if !c.isClient {
                return errors.New("tls: VerifyHostname called on TLS server connection")
        }
        if !c.handshakeComplete {
                return errors.New("tls: handshake has not yet been performed")
        }
        return c.peerCertificates[0].VerifyHostname(host)
}