ssl/test/runner/dtls.go - boringssl.git - Git at Google

 // Copyright 2014 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.

 // DTLS implementation.
 //
 // NOTE: This is a not even a remotely production-quality DTLS
 // implementation. It is the bare minimum necessary to be able to
 // achieve coverage on BoringSSL's implementation. Of note is that
 // this implementation assumes the underlying net.PacketConn is not
 // only reliable but also ordered. BoringSSL will be expected to deal
 // with simulated loss, but there is no point in forcing the test
 // driver to.

 package runner

 import (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"math/rand"
 	"net"
 )

 func (c *Conn) dtlsDoReadRecord(want recordType) (recordType, *block, error) {
 	recordHeaderLen := dtlsRecordHeaderLen

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

 	// Read a new packet only if the current one is empty.
 	var newPacket bool
 	if len(b.data) == 0 {
 		// Pick some absurdly large buffer size.
 		b.resize(maxCiphertext + recordHeaderLen)
 		n, err := c.conn.Read(c.rawInput.data)
 		if err != nil {
 			return 0, nil, err
 		}
 		if c.config.Bugs.MaxPacketLength != 0 && n > c.config.Bugs.MaxPacketLength {
 			return 0, nil, fmt.Errorf("dtls: exceeded maximum packet length")
 		}
 		c.rawInput.resize(n)
 		newPacket = true
 	}

 	// Read out one record.
 	//
 	// A real DTLS implementation should be tolerant of errors,
 	// but this is test code. We should not be tolerant of our
 	// peer sending garbage.
 	if len(b.data) < recordHeaderLen {
 		return 0, nil, errors.New("dtls: failed to read record header")
 	}
 	typ := recordType(b.data[0])
 	vers := uint16(b.data[1])<<8 | uint16(b.data[2])
 	// Alerts sent near version negotiation do not have a well-defined
 	// record-layer version prior to TLS 1.3. (In TLS 1.3, the record-layer
 	// version is irrelevant.)
 	if typ != recordTypeAlert {
 		if c.haveVers {
 			if vers != c.wireVersion {
 				c.sendAlert(alertProtocolVersion)
 				return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, c.wireVersion))
 			}
 		} else {
 			// Pre-version-negotiation alerts may be sent with any version.
 			if expect := c.config.Bugs.ExpectInitialRecordVersion; expect != 0 && vers != expect {
 				c.sendAlert(alertProtocolVersion)
 				return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, expect))
 			}
 		}
 	}
 	epoch := b.data[3:5]
 	seq := b.data[5:11]
 	// For test purposes, require the sequence number be monotonically
 	// increasing, so c.in includes the minimum next sequence number. Gaps
 	// may occur if packets failed to be sent out. A real implementation
 	// would maintain a replay window and such.
 	if !bytes.Equal(epoch, c.in.seq[:2]) {
 		c.sendAlert(alertIllegalParameter)
 		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad epoch"))
 	}
 	if bytes.Compare(seq, c.in.seq[2:]) < 0 {
 		c.sendAlert(alertIllegalParameter)
 		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad sequence number"))
 	}
 	copy(c.in.seq[2:], seq)
 	n := int(b.data[11])<<8 | int(b.data[12])
 	if n > maxCiphertext || len(b.data) < recordHeaderLen+n {
 		c.sendAlert(alertRecordOverflow)
 		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: oversized record received with length %d", n))
 	}

 	// Process message.
 	b, c.rawInput = c.in.splitBlock(b, recordHeaderLen+n)
 	ok, off, _, alertValue := c.in.decrypt(b)
 	if !ok {
 		// A real DTLS implementation would silently ignore bad records,
 		// but we want to notice errors from the implementation under
 		// test.
 		return 0, nil, c.in.setErrorLocked(c.sendAlert(alertValue))
 	}
 	b.off = off

 	// TODO(nharper): Once DTLS 1.3 is defined, handle the extra
 	// parameter from decrypt.

 	// Require that ChangeCipherSpec always share a packet with either the
 	// previous or next handshake message.
 	if newPacket && typ == recordTypeChangeCipherSpec && c.rawInput == nil {
 		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: ChangeCipherSpec not packed together with Finished"))
 	}

 	return typ, b, nil
 }

 func (c *Conn) makeFragment(header, data []byte, fragOffset, fragLen int) []byte {
 	fragment := make([]byte, 0, 12+fragLen)
 	fragment = append(fragment, header...)
 	fragment = append(fragment, byte(c.sendHandshakeSeq>>8), byte(c.sendHandshakeSeq))
 	fragment = append(fragment, byte(fragOffset>>16), byte(fragOffset>>8), byte(fragOffset))
 	fragment = append(fragment, byte(fragLen>>16), byte(fragLen>>8), byte(fragLen))
 	fragment = append(fragment, data[fragOffset:fragOffset+fragLen]...)
 	return fragment
 }

 func (c *Conn) dtlsWriteRecord(typ recordType, data []byte) (n int, err error) {
 	// Only handshake messages are fragmented.
 	if typ != recordTypeHandshake {
 		reorder := typ == recordTypeChangeCipherSpec && c.config.Bugs.ReorderChangeCipherSpec

 		// Flush pending handshake messages before encrypting a new record.
 		if !reorder {
 			err = c.dtlsPackHandshake()
 			if err != nil {
 				return
 			}
 		}

 		if typ == recordTypeApplicationData && len(data) > 1 && c.config.Bugs.SplitAndPackAppData {
 			_, err = c.dtlsPackRecord(typ, data[:len(data)/2], false)
 			if err != nil {
 				return
 			}
 			_, err = c.dtlsPackRecord(typ, data[len(data)/2:], true)
 			if err != nil {
 				return
 			}
 			n = len(data)
 		} else {
 			n, err = c.dtlsPackRecord(typ, data, false)
 			if err != nil {
 				return
 			}
 		}

 		if reorder {
 			err = c.dtlsPackHandshake()
 			if err != nil {
 				return
 			}
 		}

 		if typ == recordTypeChangeCipherSpec {
 			err = c.out.changeCipherSpec(c.config)
 			if err != nil {
 				return n, c.sendAlertLocked(alertLevelError, err.(alert))
 			}
 		} else {
 			// ChangeCipherSpec is part of the handshake and not
 			// flushed until dtlsFlushPacket.
 			err = c.dtlsFlushPacket()
 			if err != nil {
 				return
 			}
 		}
 		return
 	}

 	if c.out.cipher == nil && c.config.Bugs.StrayChangeCipherSpec {
 		_, err = c.dtlsPackRecord(recordTypeChangeCipherSpec, []byte{1}, false)
 		if err != nil {
 			return
 		}
 	}

 	maxLen := c.config.Bugs.MaxHandshakeRecordLength
 	if maxLen <= 0 {
 		maxLen = 1024
 	}

 	// Handshake messages have to be modified to include fragment
 	// offset and length and with the header replicated. Save the
 	// TLS header here.
 	//
 	// TODO(davidben): This assumes that data contains exactly one
 	// handshake message. This is incompatible with
 	// FragmentAcrossChangeCipherSpec. (Which is unfortunate
 	// because OpenSSL's DTLS implementation will probably accept
 	// such fragmentation and could do with a fix + tests.)
 	header := data[:4]
 	data = data[4:]

 	isFinished := header[0] == typeFinished

 	if c.config.Bugs.SendEmptyFragments {
 		c.pendingFragments = append(c.pendingFragments, c.makeFragment(header, data, 0, 0))
 		c.pendingFragments = append(c.pendingFragments, c.makeFragment(header, data, len(data), 0))
 	}

 	firstRun := true
 	fragOffset := 0
 	for firstRun || fragOffset < len(data) {
 		firstRun = false
 		fragLen := len(data) - fragOffset
 		if fragLen > maxLen {
 			fragLen = maxLen
 		}

 		fragment := c.makeFragment(header, data, fragOffset, fragLen)
 		if c.config.Bugs.FragmentMessageTypeMismatch && fragOffset > 0 {
 			fragment[0]++
 		}
 		if c.config.Bugs.FragmentMessageLengthMismatch && fragOffset > 0 {
 			fragment[3]++
 		}

 		// Buffer the fragment for later. They will be sent (and
 		// reordered) on flush.
 		c.pendingFragments = append(c.pendingFragments, fragment)
 		if c.config.Bugs.ReorderHandshakeFragments {
 			// Don't duplicate Finished to avoid the peer
 			// interpreting it as a retransmit request.
 			if !isFinished {
 				c.pendingFragments = append(c.pendingFragments, fragment)
 			}

 			if fragLen > (maxLen+1)/2 {
 				// Overlap each fragment by half.
 				fragLen = (maxLen + 1) / 2
 			}
 		}
 		fragOffset += fragLen
 		n += fragLen
 	}
 	shouldSendTwice := c.config.Bugs.MixCompleteMessageWithFragments
 	if isFinished {
 		shouldSendTwice = c.config.Bugs.RetransmitFinished
 	}
 	if shouldSendTwice {
 		fragment := c.makeFragment(header, data, 0, len(data))
 		c.pendingFragments = append(c.pendingFragments, fragment)
 	}

 	// Increment the handshake sequence number for the next
 	// handshake message.
 	c.sendHandshakeSeq++
 	return
 }

 // dtlsPackHandshake packs the pending handshake flight into the pending
 // record. Callers should follow up with dtlsFlushPacket to write the packets.
 func (c *Conn) dtlsPackHandshake() error {
 	// This is a test-only DTLS implementation, so there is no need to
 	// retain |c.pendingFragments| for a future retransmit.
 	var fragments [][]byte
 	fragments, c.pendingFragments = c.pendingFragments, fragments

 	if c.config.Bugs.ReorderHandshakeFragments {
 		perm := rand.New(rand.NewSource(0)).Perm(len(fragments))
 		tmp := make([][]byte, len(fragments))
 		for i := range tmp {
 			tmp[i] = fragments[perm[i]]
 		}
 		fragments = tmp
 	} else if c.config.Bugs.ReverseHandshakeFragments {
 		tmp := make([][]byte, len(fragments))
 		for i := range tmp {
 			tmp[i] = fragments[len(fragments)-i-1]
 		}
 		fragments = tmp
 	}

 	maxRecordLen := c.config.Bugs.PackHandshakeFragments

 	// Pack handshake fragments into records.
 	var records [][]byte
 	for _, fragment := range fragments {
 		if n := c.config.Bugs.SplitFragments; n > 0 {
 			if len(fragment) > n {
 				records = append(records, fragment[:n])
 				records = append(records, fragment[n:])
 			} else {
 				records = append(records, fragment)
 			}
 		} else if i := len(records) - 1; len(records) > 0 && len(records[i])+len(fragment) <= maxRecordLen {
 			records[i] = append(records[i], fragment...)
 		} else {
 			// The fragment will be appended to, so copy it.
 			records = append(records, append([]byte{}, fragment...))
 		}
 	}

 	// Send the records.
 	for _, record := range records {
 		_, err := c.dtlsPackRecord(recordTypeHandshake, record, false)
 		if err != nil {
 			return err
 		}
 	}

 	return nil
 }

 func (c *Conn) dtlsFlushHandshake() error {
 	if err := c.dtlsPackHandshake(); err != nil {
 		return err
 	}
 	if err := c.dtlsFlushPacket(); err != nil {
 		return err
 	}

 	return nil
 }

 // dtlsPackRecord packs a single record to the pending packet, flushing it
 // if necessary. The caller should call dtlsFlushPacket to flush the current
 // pending packet afterwards.
 func (c *Conn) dtlsPackRecord(typ recordType, data []byte, mustPack bool) (n int, err error) {
 	recordHeaderLen := dtlsRecordHeaderLen
 	maxLen := c.config.Bugs.MaxHandshakeRecordLength
 	if maxLen <= 0 {
 		maxLen = 1024
 	}

 	b := c.out.newBlock()

 	explicitIVLen := 0
 	explicitIVIsSeq := false

 	if cbc, ok := c.out.cipher.(cbcMode); ok {
 		// Block cipher modes have an explicit IV.
 		explicitIVLen = cbc.BlockSize()
 	} else if aead, ok := c.out.cipher.(*tlsAead); ok {
 		if aead.explicitNonce {
 			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
 		}
 	} else if _, ok := c.out.cipher.(nullCipher); !ok && c.out.cipher != nil {
 		panic("Unknown cipher")
 	}
 	b.resize(recordHeaderLen + explicitIVLen + len(data))
 	// TODO(nharper): DTLS 1.3 will likely need to set this to
 	// recordTypeApplicationData if c.out.cipher != nil.
 	b.data[0] = byte(typ)
 	vers := c.wireVersion
 	if vers == 0 {
 		// Some TLS servers fail if the record version is greater than
 		// TLS 1.0 for the initial ClientHello.
 		if c.isDTLS {
 			vers = VersionDTLS10
 		} else {
 			vers = VersionTLS10
 		}
 	}
 	b.data[1] = byte(vers >> 8)
 	b.data[2] = byte(vers)
 	// DTLS records include an explicit sequence number.
 	copy(b.data[3:11], c.out.outSeq[0:])
 	b.data[11] = byte(len(data) >> 8)
 	b.data[12] = byte(len(data))
 	if explicitIVLen > 0 {
 		explicitIV := b.data[recordHeaderLen : recordHeaderLen+explicitIVLen]
 		if explicitIVIsSeq {
 			copy(explicitIV, c.out.outSeq[:])
 		} else {
 			if _, err = io.ReadFull(c.config.rand(), explicitIV); err != nil {
 				return
 			}
 		}
 	}
 	copy(b.data[recordHeaderLen+explicitIVLen:], data)
 	c.out.encrypt(b, explicitIVLen, typ)

 	// Flush the current pending packet if necessary.
 	if !mustPack && len(b.data)+len(c.pendingPacket) > c.config.Bugs.PackHandshakeRecords {
 		err = c.dtlsFlushPacket()
 		if err != nil {
 			c.out.freeBlock(b)
 			return
 		}
 	}

 	// Add the record to the pending packet.
 	c.pendingPacket = append(c.pendingPacket, b.data...)
 	c.out.freeBlock(b)
 	n = len(data)
 	return
 }

 func (c *Conn) dtlsFlushPacket() error {
 	if len(c.pendingPacket) == 0 {
 		return nil
 	}
 	_, err := c.conn.Write(c.pendingPacket)
 	c.pendingPacket = nil
 	return err
 }

 func (c *Conn) dtlsDoReadHandshake() ([]byte, error) {
 	// Assemble a full handshake message.  For test purposes, this
 	// implementation assumes fragments arrive in order. It may
 	// need to be cleverer if we ever test BoringSSL's retransmit
 	// behavior.
 	for len(c.handMsg) < 4+c.handMsgLen {
 		// Get a new handshake record if the previous has been
 		// exhausted.
 		if c.hand.Len() == 0 {
 			if err := c.in.err; err != nil {
 				return nil, err
 			}
 			if err := c.readRecord(recordTypeHandshake); err != nil {
 				return nil, err
 			}
 		}

 		// Read the next fragment. It must fit entirely within
 		// the record.
 		if c.hand.Len() < 12 {
 			return nil, errors.New("dtls: bad handshake record")
 		}
 		header := c.hand.Next(12)
 		fragN := int(header[1])<<16 | int(header[2])<<8 | int(header[3])
 		fragSeq := uint16(header[4])<<8 | uint16(header[5])
 		fragOff := int(header[6])<<16 | int(header[7])<<8 | int(header[8])
 		fragLen := int(header[9])<<16 | int(header[10])<<8 | int(header[11])

 		if c.hand.Len() < fragLen {
 			return nil, errors.New("dtls: fragment length too long")
 		}
 		fragment := c.hand.Next(fragLen)

 		// Check it's a fragment for the right message.
 		if fragSeq != c.recvHandshakeSeq {
 			return nil, errors.New("dtls: bad handshake sequence number")
 		}

 		// Check that the length is consistent.
 		if c.handMsg == nil {
 			c.handMsgLen = fragN
 			if c.handMsgLen > maxHandshake {
 				return nil, c.in.setErrorLocked(c.sendAlert(alertInternalError))
 			}
 			// Start with the TLS handshake header,
 			// without the DTLS bits.
 			c.handMsg = append([]byte{}, header[:4]...)
 		} else if fragN != c.handMsgLen {
 			return nil, errors.New("dtls: bad handshake length")
 		}

 		// Add the fragment to the pending message.
 		if 4+fragOff != len(c.handMsg) {
 			return nil, errors.New("dtls: bad fragment offset")
 		}
 		if fragOff+fragLen > c.handMsgLen {
 			return nil, errors.New("dtls: bad fragment length")
 		}
 		c.handMsg = append(c.handMsg, fragment...)
 	}
 	c.recvHandshakeSeq++
 	ret := c.handMsg
 	c.handMsg, c.handMsgLen = nil, 0
 	return ret, nil
 }

 // DTLSServer returns a new DTLS server side connection
 // using conn as the underlying transport.
 // The configuration config must be non-nil and must have
 // at least one certificate.
 func DTLSServer(conn net.Conn, config *Config) *Conn {
 	c := &Conn{config: config, isDTLS: true, conn: conn}
 	c.init()
 	return c
 }

 // DTLSClient returns a new DTLS client side connection
 // using conn as the underlying transport.
 // The config cannot be nil: users must set either ServerHostname or
 // InsecureSkipVerify in the config.
 func DTLSClient(conn net.Conn, config *Config) *Conn {
 	c := &Conn{config: config, isClient: true, isDTLS: true, conn: conn}
 	c.init()
 	return c
 }
	// Copyright 2014 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.

	// DTLS implementation.
	//
	// NOTE: This is a not even a remotely production-quality DTLS
	// implementation. It is the bare minimum necessary to be able to
	// achieve coverage on BoringSSL's implementation. Of note is that
	// this implementation assumes the underlying net.PacketConn is not
	// only reliable but also ordered. BoringSSL will be expected to deal
	// with simulated loss, but there is no point in forcing the test
	// driver to.

	package runner

	import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"math/rand"
	"net"
	)

	func (c Conn) dtlsDoReadRecord(want recordType) (recordType, block, error) {
	recordHeaderLen := dtlsRecordHeaderLen

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

	// Read a new packet only if the current one is empty.
	var newPacket bool
	if len(b.data) == 0 {
	// Pick some absurdly large buffer size.
	b.resize(maxCiphertext + recordHeaderLen)
	n, err := c.conn.Read(c.rawInput.data)
	if err != nil {
	return 0, nil, err
	}
	if c.config.Bugs.MaxPacketLength != 0 && n > c.config.Bugs.MaxPacketLength {
	return 0, nil, fmt.Errorf("dtls: exceeded maximum packet length")
	}
	c.rawInput.resize(n)
	newPacket = true
	}

	// Read out one record.
	//
	// A real DTLS implementation should be tolerant of errors,
	// but this is test code. We should not be tolerant of our
	// peer sending garbage.
	if len(b.data) < recordHeaderLen {
	return 0, nil, errors.New("dtls: failed to read record header")
	}
	typ := recordType(b.data[0])
	vers := uint16(b.data[1])<<8 \| uint16(b.data[2])
	// Alerts sent near version negotiation do not have a well-defined
	// record-layer version prior to TLS 1.3. (In TLS 1.3, the record-layer
	// version is irrelevant.)
	if typ != recordTypeAlert {
	if c.haveVers {
	if vers != c.wireVersion {
	c.sendAlert(alertProtocolVersion)
	return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, c.wireVersion))
	}
	} else {
	// Pre-version-negotiation alerts may be sent with any version.
	if expect := c.config.Bugs.ExpectInitialRecordVersion; expect != 0 && vers != expect {
	c.sendAlert(alertProtocolVersion)
	return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, expect))
	}
	}
	}
	epoch := b.data[3:5]
	seq := b.data[5:11]
	// For test purposes, require the sequence number be monotonically
	// increasing, so c.in includes the minimum next sequence number. Gaps
	// may occur if packets failed to be sent out. A real implementation
	// would maintain a replay window and such.
	if !bytes.Equal(epoch, c.in.seq[:2]) {
	c.sendAlert(alertIllegalParameter)
	return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad epoch"))
	}
	if bytes.Compare(seq, c.in.seq[2:]) < 0 {
	c.sendAlert(alertIllegalParameter)
	return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad sequence number"))
	}
	copy(c.in.seq[2:], seq)
	n := int(b.data[11])<<8 \| int(b.data[12])
	if n > maxCiphertext \|\| len(b.data) < recordHeaderLen+n {
	c.sendAlert(alertRecordOverflow)
	return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: oversized record received with length %d", n))
	}

	// Process message.
	b, c.rawInput = c.in.splitBlock(b, recordHeaderLen+n)
	ok, off, _, alertValue := c.in.decrypt(b)
	if !ok {
	// A real DTLS implementation would silently ignore bad records,
	// but we want to notice errors from the implementation under
	// test.
	return 0, nil, c.in.setErrorLocked(c.sendAlert(alertValue))
	}
	b.off = off

	// TODO(nharper): Once DTLS 1.3 is defined, handle the extra
	// parameter from decrypt.

	// Require that ChangeCipherSpec always share a packet with either the
	// previous or next handshake message.
	if newPacket && typ == recordTypeChangeCipherSpec && c.rawInput == nil {
	return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: ChangeCipherSpec not packed together with Finished"))
	}

	return typ, b, nil
	}

	func (c *Conn) makeFragment(header, data []byte, fragOffset, fragLen int) []byte {
	fragment := make([]byte, 0, 12+fragLen)
	fragment = append(fragment, header...)
	fragment = append(fragment, byte(c.sendHandshakeSeq>>8), byte(c.sendHandshakeSeq))
	fragment = append(fragment, byte(fragOffset>>16), byte(fragOffset>>8), byte(fragOffset))
	fragment = append(fragment, byte(fragLen>>16), byte(fragLen>>8), byte(fragLen))
	fragment = append(fragment, data[fragOffset:fragOffset+fragLen]...)
	return fragment
	}

	func (c *Conn) dtlsWriteRecord(typ recordType, data []byte) (n int, err error) {
	// Only handshake messages are fragmented.
	if typ != recordTypeHandshake {
	reorder := typ == recordTypeChangeCipherSpec && c.config.Bugs.ReorderChangeCipherSpec

	// Flush pending handshake messages before encrypting a new record.
	if !reorder {
	err = c.dtlsPackHandshake()
	if err != nil {
	return
	}
	}

	if typ == recordTypeApplicationData && len(data) > 1 && c.config.Bugs.SplitAndPackAppData {
	_, err = c.dtlsPackRecord(typ, data[:len(data)/2], false)
	if err != nil {
	return
	}
	_, err = c.dtlsPackRecord(typ, data[len(data)/2:], true)
	if err != nil {
	return
	}
	n = len(data)
	} else {
	n, err = c.dtlsPackRecord(typ, data, false)
	if err != nil {
	return
	}
	}

	if reorder {
	err = c.dtlsPackHandshake()
	if err != nil {
	return
	}
	}

	if typ == recordTypeChangeCipherSpec {
	err = c.out.changeCipherSpec(c.config)
	if err != nil {
	return n, c.sendAlertLocked(alertLevelError, err.(alert))
	}
	} else {
	// ChangeCipherSpec is part of the handshake and not
	// flushed until dtlsFlushPacket.
	err = c.dtlsFlushPacket()
	if err != nil {
	return
	}
	}
	return
	}

	if c.out.cipher == nil && c.config.Bugs.StrayChangeCipherSpec {
	_, err = c.dtlsPackRecord(recordTypeChangeCipherSpec, []byte{1}, false)
	if err != nil {
	return
	}
	}

	maxLen := c.config.Bugs.MaxHandshakeRecordLength
	if maxLen <= 0 {
	maxLen = 1024
	}

	// Handshake messages have to be modified to include fragment
	// offset and length and with the header replicated. Save the
	// TLS header here.
	//
	// TODO(davidben): This assumes that data contains exactly one
	// handshake message. This is incompatible with
	// FragmentAcrossChangeCipherSpec. (Which is unfortunate
	// because OpenSSL's DTLS implementation will probably accept
	// such fragmentation and could do with a fix + tests.)
	header := data[:4]
	data = data[4:]

	isFinished := header[0] == typeFinished

	if c.config.Bugs.SendEmptyFragments {
	c.pendingFragments = append(c.pendingFragments, c.makeFragment(header, data, 0, 0))
	c.pendingFragments = append(c.pendingFragments, c.makeFragment(header, data, len(data), 0))
	}

	firstRun := true
	fragOffset := 0
	for firstRun \|\| fragOffset < len(data) {
	firstRun = false
	fragLen := len(data) - fragOffset
	if fragLen > maxLen {
	fragLen = maxLen
	}

	fragment := c.makeFragment(header, data, fragOffset, fragLen)
	if c.config.Bugs.FragmentMessageTypeMismatch && fragOffset > 0 {
	fragment[0]++
	}
	if c.config.Bugs.FragmentMessageLengthMismatch && fragOffset > 0 {
	fragment[3]++
	}

	// Buffer the fragment for later. They will be sent (and
	// reordered) on flush.
	c.pendingFragments = append(c.pendingFragments, fragment)
	if c.config.Bugs.ReorderHandshakeFragments {
	// Don't duplicate Finished to avoid the peer
	// interpreting it as a retransmit request.
	if !isFinished {
	c.pendingFragments = append(c.pendingFragments, fragment)
	}

	if fragLen > (maxLen+1)/2 {
	// Overlap each fragment by half.
	fragLen = (maxLen + 1) / 2
	}
	}
	fragOffset += fragLen
	n += fragLen
	}
	shouldSendTwice := c.config.Bugs.MixCompleteMessageWithFragments
	if isFinished {
	shouldSendTwice = c.config.Bugs.RetransmitFinished
	}
	if shouldSendTwice {
	fragment := c.makeFragment(header, data, 0, len(data))
	c.pendingFragments = append(c.pendingFragments, fragment)
	}

	// Increment the handshake sequence number for the next
	// handshake message.
	c.sendHandshakeSeq++
	return
	}

	// dtlsPackHandshake packs the pending handshake flight into the pending
	// record. Callers should follow up with dtlsFlushPacket to write the packets.
	func (c *Conn) dtlsPackHandshake() error {
	// This is a test-only DTLS implementation, so there is no need to
	// retain \|c.pendingFragments\| for a future retransmit.
	var fragments [][]byte
	fragments, c.pendingFragments = c.pendingFragments, fragments

	if c.config.Bugs.ReorderHandshakeFragments {
	perm := rand.New(rand.NewSource(0)).Perm(len(fragments))
	tmp := make([][]byte, len(fragments))
	for i := range tmp {
	tmp[i] = fragments[perm[i]]
	}
	fragments = tmp
	} else if c.config.Bugs.ReverseHandshakeFragments {
	tmp := make([][]byte, len(fragments))
	for i := range tmp {
	tmp[i] = fragments[len(fragments)-i-1]
	}
	fragments = tmp
	}

	maxRecordLen := c.config.Bugs.PackHandshakeFragments

	// Pack handshake fragments into records.
	var records [][]byte
	for _, fragment := range fragments {
	if n := c.config.Bugs.SplitFragments; n > 0 {
	if len(fragment) > n {
	records = append(records, fragment[:n])
	records = append(records, fragment[n:])
	} else {
	records = append(records, fragment)
	}
	} else if i := len(records) - 1; len(records) > 0 && len(records[i])+len(fragment) <= maxRecordLen {
	records[i] = append(records[i], fragment...)
	} else {
	// The fragment will be appended to, so copy it.
	records = append(records, append([]byte{}, fragment...))
	}
	}

	// Send the records.
	for _, record := range records {
	_, err := c.dtlsPackRecord(recordTypeHandshake, record, false)
	if err != nil {
	return err
	}
	}

	return nil
	}

	func (c *Conn) dtlsFlushHandshake() error {
	if err := c.dtlsPackHandshake(); err != nil {
	return err
	}
	if err := c.dtlsFlushPacket(); err != nil {
	return err
	}

	return nil
	}

	// dtlsPackRecord packs a single record to the pending packet, flushing it
	// if necessary. The caller should call dtlsFlushPacket to flush the current
	// pending packet afterwards.
	func (c *Conn) dtlsPackRecord(typ recordType, data []byte, mustPack bool) (n int, err error) {
	recordHeaderLen := dtlsRecordHeaderLen
	maxLen := c.config.Bugs.MaxHandshakeRecordLength
	if maxLen <= 0 {
	maxLen = 1024
	}

	b := c.out.newBlock()

	explicitIVLen := 0
	explicitIVIsSeq := false

	if cbc, ok := c.out.cipher.(cbcMode); ok {
	// Block cipher modes have an explicit IV.
	explicitIVLen = cbc.BlockSize()
	} else if aead, ok := c.out.cipher.(*tlsAead); ok {
	if aead.explicitNonce {
	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
	}
	} else if _, ok := c.out.cipher.(nullCipher); !ok && c.out.cipher != nil {
	panic("Unknown cipher")
	}
	b.resize(recordHeaderLen + explicitIVLen + len(data))
	// TODO(nharper): DTLS 1.3 will likely need to set this to
	// recordTypeApplicationData if c.out.cipher != nil.
	b.data[0] = byte(typ)
	vers := c.wireVersion
	if vers == 0 {
	// Some TLS servers fail if the record version is greater than
	// TLS 1.0 for the initial ClientHello.
	if c.isDTLS {
	vers = VersionDTLS10
	} else {
	vers = VersionTLS10
	}
	}
	b.data[1] = byte(vers >> 8)
	b.data[2] = byte(vers)
	// DTLS records include an explicit sequence number.
	copy(b.data[3:11], c.out.outSeq[0:])
	b.data[11] = byte(len(data) >> 8)
	b.data[12] = byte(len(data))
	if explicitIVLen > 0 {
	explicitIV := b.data[recordHeaderLen : recordHeaderLen+explicitIVLen]
	if explicitIVIsSeq {
	copy(explicitIV, c.out.outSeq[:])
	} else {
	if _, err = io.ReadFull(c.config.rand(), explicitIV); err != nil {
	return
	}
	}
	}
	copy(b.data[recordHeaderLen+explicitIVLen:], data)
	c.out.encrypt(b, explicitIVLen, typ)

	// Flush the current pending packet if necessary.
	if !mustPack && len(b.data)+len(c.pendingPacket) > c.config.Bugs.PackHandshakeRecords {
	err = c.dtlsFlushPacket()
	if err != nil {
	c.out.freeBlock(b)
	return
	}
	}

	// Add the record to the pending packet.
	c.pendingPacket = append(c.pendingPacket, b.data...)
	c.out.freeBlock(b)
	n = len(data)
	return
	}

	func (c *Conn) dtlsFlushPacket() error {
	if len(c.pendingPacket) == 0 {
	return nil
	}
	_, err := c.conn.Write(c.pendingPacket)
	c.pendingPacket = nil
	return err
	}

	func (c *Conn) dtlsDoReadHandshake() ([]byte, error) {
	// Assemble a full handshake message. For test purposes, this
	// implementation assumes fragments arrive in order. It may
	// need to be cleverer if we ever test BoringSSL's retransmit
	// behavior.
	for len(c.handMsg) < 4+c.handMsgLen {
	// Get a new handshake record if the previous has been
	// exhausted.
	if c.hand.Len() == 0 {
	if err := c.in.err; err != nil {
	return nil, err
	}
	if err := c.readRecord(recordTypeHandshake); err != nil {
	return nil, err
	}
	}

	// Read the next fragment. It must fit entirely within
	// the record.
	if c.hand.Len() < 12 {
	return nil, errors.New("dtls: bad handshake record")
	}
	header := c.hand.Next(12)
	fragN := int(header[1])<<16 \| int(header[2])<<8 \| int(header[3])
	fragSeq := uint16(header[4])<<8 \| uint16(header[5])
	fragOff := int(header[6])<<16 \| int(header[7])<<8 \| int(header[8])
	fragLen := int(header[9])<<16 \| int(header[10])<<8 \| int(header[11])

	if c.hand.Len() < fragLen {
	return nil, errors.New("dtls: fragment length too long")
	}
	fragment := c.hand.Next(fragLen)

	// Check it's a fragment for the right message.
	if fragSeq != c.recvHandshakeSeq {
	return nil, errors.New("dtls: bad handshake sequence number")
	}

	// Check that the length is consistent.
	if c.handMsg == nil {
	c.handMsgLen = fragN
	if c.handMsgLen > maxHandshake {
	return nil, c.in.setErrorLocked(c.sendAlert(alertInternalError))
	}
	// Start with the TLS handshake header,
	// without the DTLS bits.
	c.handMsg = append([]byte{}, header[:4]...)
	} else if fragN != c.handMsgLen {
	return nil, errors.New("dtls: bad handshake length")
	}

	// Add the fragment to the pending message.
	if 4+fragOff != len(c.handMsg) {
	return nil, errors.New("dtls: bad fragment offset")
	}
	if fragOff+fragLen > c.handMsgLen {
	return nil, errors.New("dtls: bad fragment length")
	}
	c.handMsg = append(c.handMsg, fragment...)
	}
	c.recvHandshakeSeq++
	ret := c.handMsg
	c.handMsg, c.handMsgLen = nil, 0
	return ret, nil
	}

	// DTLSServer returns a new DTLS server side connection
	// using conn as the underlying transport.
	// The configuration config must be non-nil and must have
	// at least one certificate.
	func DTLSServer(conn net.Conn, config Config) Conn {
	c := &Conn{config: config, isDTLS: true, conn: conn}
	c.init()
	return c
	}

	// DTLSClient returns a new DTLS client side connection
	// using conn as the underlying transport.
	// The config cannot be nil: users must set either ServerHostname or
	// InsecureSkipVerify in the config.
	func DTLSClient(conn net.Conn, config Config) Conn {
	c := &Conn{config: config, isClient: true, isDTLS: true, conn: conn}
	c.init()
	return c
	}