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tailscale/control/controlbase/handshake.go

444 lines
13 KiB
Go

// Copyright (c) 2021 Tailscale Inc & 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 controlbase
import (
"context"
"crypto/cipher"
"encoding/binary"
"errors"
"fmt"
"hash"
"io"
"net"
"strconv"
"time"
"go4.org/mem"
"golang.org/x/crypto/blake2s"
chp "golang.org/x/crypto/chacha20poly1305"
"golang.org/x/crypto/curve25519"
"golang.org/x/crypto/hkdf"
"tailscale.com/types/key"
)
const (
// protocolName is the name of the specific instantiation of Noise
// that the control protocol uses. This string's value is fixed by
// the Noise spec, and shouldn't be changed unless we're updating
// the control protocol to use a different Noise instance.
protocolName = "Noise_IK_25519_ChaChaPoly_BLAKE2s"
// protocolVersion is the version of the control protocol that
// Client will use when initiating a handshake.
protocolVersion uint16 = 1
// protocolVersionPrefix is the name portion of the protocol
// name+version string that gets mixed into the handshake as a
// prologue.
//
// This mixing verifies that both clients agree that they're
// executing the control protocol at a specific version that
// matches the advertised version in the cleartext packet header.
protocolVersionPrefix = "Tailscale Control Protocol v"
invalidNonce = ^uint64(0)
)
func protocolVersionPrologue(version uint16) []byte {
ret := make([]byte, 0, len(protocolVersionPrefix)+5) // 5 bytes is enough to encode all possible version numbers.
ret = append(ret, protocolVersionPrefix...)
return strconv.AppendUint(ret, uint64(version), 10)
}
// Client initiates a control client handshake, returning the resulting
// control connection.
//
// The context deadline, if any, covers the entire handshaking
// process. Any preexisting Conn deadline is removed.
func Client(ctx context.Context, conn net.Conn, machineKey key.MachinePrivate, controlKey key.MachinePublic) (*Conn, error) {
if deadline, ok := ctx.Deadline(); ok {
if err := conn.SetDeadline(deadline); err != nil {
return nil, fmt.Errorf("setting conn deadline: %w", err)
}
defer func() {
conn.SetDeadline(time.Time{})
}()
}
var s symmetricState
s.Initialize()
// prologue
s.MixHash(protocolVersionPrologue(protocolVersion))
// <- s
// ...
s.MixHash(controlKey.UntypedBytes())
// -> e, es, s, ss
init := mkInitiationMessage()
machineEphemeral := key.NewMachine()
machineEphemeralPub := machineEphemeral.Public()
copy(init.EphemeralPub(), machineEphemeralPub.UntypedBytes())
s.MixHash(machineEphemeralPub.UntypedBytes())
cipher, err := s.MixDH(machineEphemeral, controlKey)
if err != nil {
return nil, fmt.Errorf("computing es: %w", err)
}
machineKeyPub := machineKey.Public()
s.EncryptAndHash(cipher, init.MachinePub(), machineKeyPub.UntypedBytes())
cipher, err = s.MixDH(machineKey, controlKey)
if err != nil {
return nil, fmt.Errorf("computing ss: %w", err)
}
s.EncryptAndHash(cipher, init.Tag(), nil) // empty message payload
if _, err := conn.Write(init[:]); err != nil {
return nil, fmt.Errorf("writing initiation: %w", err)
}
// Read in the payload and look for errors/protocol violations from the server.
var resp responseMessage
if _, err := io.ReadFull(conn, resp.Header()); err != nil {
return nil, fmt.Errorf("reading response header: %w", err)
}
if resp.Type() != msgTypeResponse {
if resp.Type() != msgTypeError {
return nil, fmt.Errorf("unexpected response message type %d", resp.Type())
}
msg := make([]byte, resp.Length())
if _, err := io.ReadFull(conn, msg); err != nil {
return nil, err
}
return nil, fmt.Errorf("server error: %q", msg)
}
if resp.Length() != len(resp.Payload()) {
return nil, fmt.Errorf("wrong length %d received for handshake response", resp.Length())
}
if _, err := io.ReadFull(conn, resp.Payload()); err != nil {
return nil, err
}
// <- e, ee, se
controlEphemeralPub := key.MachinePublicFromRaw32(mem.B(resp.EphemeralPub()))
s.MixHash(controlEphemeralPub.UntypedBytes())
if _, err = s.MixDH(machineEphemeral, controlEphemeralPub); err != nil {
return nil, fmt.Errorf("computing ee: %w", err)
}
cipher, err = s.MixDH(machineKey, controlEphemeralPub)
if err != nil {
return nil, fmt.Errorf("computing se: %w", err)
}
if err := s.DecryptAndHash(cipher, nil, resp.Tag()); err != nil {
return nil, fmt.Errorf("decrypting payload: %w", err)
}
c1, c2, err := s.Split()
if err != nil {
return nil, fmt.Errorf("finalizing handshake: %w", err)
}
c := &Conn{
conn: conn,
version: protocolVersion,
peer: controlKey,
handshakeHash: s.h,
tx: txState{
cipher: c1,
},
rx: rxState{
cipher: c2,
},
}
return c, nil
}
// Server initiates a control server handshake, returning the resulting
// control connection.
//
// The context deadline, if any, covers the entire handshaking
// process.
func Server(ctx context.Context, conn net.Conn, controlKey key.MachinePrivate) (*Conn, error) {
if deadline, ok := ctx.Deadline(); ok {
if err := conn.SetDeadline(deadline); err != nil {
return nil, fmt.Errorf("setting conn deadline: %w", err)
}
defer func() {
conn.SetDeadline(time.Time{})
}()
}
// Deliberately does not support formatting, so that we don't echo
// attacker-controlled input back to them.
sendErr := func(msg string) error {
if len(msg) >= 1<<16 {
msg = msg[:1<<16]
}
var hdr [headerLen]byte
hdr[0] = msgTypeError
binary.BigEndian.PutUint16(hdr[1:3], uint16(len(msg)))
if _, err := conn.Write(hdr[:]); err != nil {
return fmt.Errorf("sending %q error to client: %w", msg, err)
}
if _, err := io.WriteString(conn, msg); err != nil {
return fmt.Errorf("sending %q error to client: %w", msg, err)
}
return fmt.Errorf("refused client handshake: %q", msg)
}
var s symmetricState
s.Initialize()
var init initiationMessage
if _, err := io.ReadFull(conn, init.Header()); err != nil {
return nil, err
}
if init.Version() != protocolVersion {
return nil, sendErr("unsupported protocol version")
}
if init.Type() != msgTypeInitiation {
return nil, sendErr("unexpected handshake message type")
}
if init.Length() != len(init.Payload()) {
return nil, sendErr("wrong handshake initiation length")
}
if _, err := io.ReadFull(conn, init.Payload()); err != nil {
return nil, err
}
// prologue. Can only do this once we at least think the client is
// handshaking using a supported version.
s.MixHash(protocolVersionPrologue(protocolVersion))
// <- s
// ...
controlKeyPub := controlKey.Public()
s.MixHash(controlKeyPub.UntypedBytes())
// -> e, es, s, ss
machineEphemeralPub := key.MachinePublicFromRaw32(mem.B(init.EphemeralPub()))
s.MixHash(machineEphemeralPub.UntypedBytes())
cipher, err := s.MixDH(controlKey, machineEphemeralPub)
if err != nil {
return nil, fmt.Errorf("computing es: %w", err)
}
var machineKeyBytes [32]byte
if err := s.DecryptAndHash(cipher, machineKeyBytes[:], init.MachinePub()); err != nil {
return nil, fmt.Errorf("decrypting machine key: %w", err)
}
machineKey := key.MachinePublicFromRaw32(mem.B(machineKeyBytes[:]))
cipher, err = s.MixDH(controlKey, machineKey)
if err != nil {
return nil, fmt.Errorf("computing ss: %w", err)
}
if err := s.DecryptAndHash(cipher, nil, init.Tag()); err != nil {
return nil, fmt.Errorf("decrypting initiation tag: %w", err)
}
// <- e, ee, se
resp := mkResponseMessage()
controlEphemeral := key.NewMachine()
controlEphemeralPub := controlEphemeral.Public()
copy(resp.EphemeralPub(), controlEphemeralPub.UntypedBytes())
s.MixHash(controlEphemeralPub.UntypedBytes())
if _, err := s.MixDH(controlEphemeral, machineEphemeralPub); err != nil {
return nil, fmt.Errorf("computing ee: %w", err)
}
cipher, err = s.MixDH(controlEphemeral, machineKey)
if err != nil {
return nil, fmt.Errorf("computing se: %w", err)
}
s.EncryptAndHash(cipher, resp.Tag(), nil) // empty message payload
c1, c2, err := s.Split()
if err != nil {
return nil, fmt.Errorf("finalizing handshake: %w", err)
}
if _, err := conn.Write(resp[:]); err != nil {
return nil, err
}
c := &Conn{
conn: conn,
version: protocolVersion,
peer: machineKey,
handshakeHash: s.h,
tx: txState{
cipher: c2,
},
rx: rxState{
cipher: c1,
},
}
return c, nil
}
// symmetricState contains the state of an in-flight handshake.
type symmetricState struct {
finished bool
h [blake2s.Size]byte // hash of currently-processed handshake state
ck [blake2s.Size]byte // chaining key used to construct session keys at the end of the handshake
}
func (s *symmetricState) checkFinished() {
if s.finished {
panic("attempted to use symmetricState after Split was called")
}
}
// Initialize sets s to the initial handshake state, prior to
// processing any handshake messages.
func (s *symmetricState) Initialize() {
s.checkFinished()
s.h = blake2s.Sum256([]byte(protocolName))
s.ck = s.h
}
// MixHash updates s.h to be BLAKE2s(s.h || data), where || is
// concatenation.
func (s *symmetricState) MixHash(data []byte) {
s.checkFinished()
h := newBLAKE2s()
h.Write(s.h[:])
h.Write(data)
h.Sum(s.h[:0])
}
// MixDH updates s.ck with the result of X25519(priv, pub) and returns
// a singleUseCHP that can be used to encrypt or decrypt handshake
// data.
//
// MixDH corresponds to MixKey(X25519(...))) in the spec. Implementing
// it as a single function allows for strongly-typed arguments that
// reduce the risk of error in the caller (e.g. invoking X25519 with
// two private keys, or two public keys), and thus producing the wrong
// calculation.
func (s *symmetricState) MixDH(priv key.MachinePrivate, pub key.MachinePublic) (*singleUseCHP, error) {
s.checkFinished()
keyData, err := curve25519.X25519(priv.UntypedBytes(), pub.UntypedBytes())
if err != nil {
return nil, fmt.Errorf("computing X25519: %w", err)
}
r := hkdf.New(newBLAKE2s, keyData, s.ck[:], nil)
if _, err := io.ReadFull(r, s.ck[:]); err != nil {
return nil, fmt.Errorf("extracting ck: %w", err)
}
var k [chp.KeySize]byte
if _, err := io.ReadFull(r, k[:]); err != nil {
return nil, fmt.Errorf("extracting k: %w", err)
}
return newSingleUseCHP(k), nil
}
// EncryptAndHash encrypts plaintext into ciphertext (which must be
// the correct size to hold the encrypted plaintext) using cipher,
// mixes the ciphertext into s.h, and returns the ciphertext.
func (s *symmetricState) EncryptAndHash(cipher *singleUseCHP, ciphertext, plaintext []byte) {
s.checkFinished()
if len(ciphertext) != len(plaintext)+chp.Overhead {
panic("ciphertext is wrong size for given plaintext")
}
ret := cipher.Seal(ciphertext[:0], plaintext, s.h[:])
s.MixHash(ret)
}
// DecryptAndHash decrypts the given ciphertext into plaintext (which
// must be the correct size to hold the decrypted ciphertext) using
// cipher. If decryption is successful, it mixes the ciphertext into
// s.h.
func (s *symmetricState) DecryptAndHash(cipher *singleUseCHP, plaintext, ciphertext []byte) error {
s.checkFinished()
if len(ciphertext) != len(plaintext)+chp.Overhead {
return errors.New("plaintext is wrong size for given ciphertext")
}
if _, err := cipher.Open(plaintext[:0], ciphertext, s.h[:]); err != nil {
return err
}
s.MixHash(ciphertext)
return nil
}
// Split returns two ChaCha20Poly1305 ciphers with keys derived from
// the current handshake state. Methods on s cannot be used again
// after calling Split.
func (s *symmetricState) Split() (c1, c2 cipher.AEAD, err error) {
s.finished = true
var k1, k2 [chp.KeySize]byte
r := hkdf.New(newBLAKE2s, nil, s.ck[:], nil)
if _, err := io.ReadFull(r, k1[:]); err != nil {
return nil, nil, fmt.Errorf("extracting k1: %w", err)
}
if _, err := io.ReadFull(r, k2[:]); err != nil {
return nil, nil, fmt.Errorf("extracting k2: %w", err)
}
c1, err = chp.New(k1[:])
if err != nil {
return nil, nil, fmt.Errorf("constructing AEAD c1: %w", err)
}
c2, err = chp.New(k2[:])
if err != nil {
return nil, nil, fmt.Errorf("constructing AEAD c2: %w", err)
}
return c1, c2, nil
}
// newBLAKE2s returns a hash.Hash implementing BLAKE2s, or panics on
// error.
func newBLAKE2s() hash.Hash {
h, err := blake2s.New256(nil)
if err != nil {
// Should never happen, errors only happen when using BLAKE2s
// in MAC mode with a key.
panic(err)
}
return h
}
// newCHP returns a cipher.AEAD implementing ChaCha20Poly1305, or
// panics on error.
func newCHP(key [chp.KeySize]byte) cipher.AEAD {
aead, err := chp.New(key[:])
if err != nil {
// Can only happen if we passed a key of the wrong length. The
// function signature prevents that.
panic(err)
}
return aead
}
// singleUseCHP is an instance of ChaCha20Poly1305 that can be used
// only once, either for encrypting or decrypting, but not both. The
// chosen operation is always executed with an all-zeros
// nonce. Subsequent calls to either Seal or Open panic.
type singleUseCHP struct {
c cipher.AEAD
}
func newSingleUseCHP(key [chp.KeySize]byte) *singleUseCHP {
return &singleUseCHP{newCHP(key)}
}
func (c *singleUseCHP) Seal(dst, plaintext, additionalData []byte) []byte {
if c.c == nil {
panic("Attempted reuse of singleUseAEAD")
}
cipher := c.c
c.c = nil
var nonce [chp.NonceSize]byte
return cipher.Seal(dst, nonce[:], plaintext, additionalData)
}
func (c *singleUseCHP) Open(dst, ciphertext, additionalData []byte) ([]byte, error) {
if c.c == nil {
panic("Attempted reuse of singleUseAEAD")
}
cipher := c.c
c.c = nil
var nonce [chp.NonceSize]byte
return cipher.Open(dst, nonce[:], ciphertext, additionalData)
}