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tailscale/tka/state.go

263 lines
7.3 KiB
Go

// Copyright (c) 2022 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 tka
import (
"bytes"
"errors"
"fmt"
"golang.org/x/crypto/argon2"
"tailscale.com/types/tkatype"
)
// ErrNoSuchKey is returned if the key referenced by a KeyID does not exist.
var ErrNoSuchKey = errors.New("key not found")
// State describes Tailnet Key Authority state at an instant in time.
//
// State is mutated by applying Authority Update Messages (AUMs), resulting
// in a new State.
type State struct {
// LastAUMHash is the blake2s digest of the last-applied AUM.
// Because AUMs are strictly ordered and form a hash chain, we
// check the previous AUM hash in an update we are applying
// is the same as the LastAUMHash.
LastAUMHash *AUMHash `cbor:"1,keyasint"`
// DisablementSecrets are KDF-derived values which can be used
// to turn off the TKA in the event of a consensus-breaking bug.
//
// TODO(tom): This is an alpha feature, remove this mechanism once
// we have confidence in our implementation.
DisablementSecrets [][]byte `cbor:"2,keyasint"`
// Keys are the public keys currently trusted by the TKA.
Keys []Key `cbor:"3,keyasint"`
}
// GetKey returns the trusted key with the specified KeyID.
func (s State) GetKey(key tkatype.KeyID) (Key, error) {
for _, k := range s.Keys {
if bytes.Equal(k.ID(), key) {
return k, nil
}
}
return Key{}, ErrNoSuchKey
}
// Clone makes an independent copy of State.
//
// NOTE: There is a difference between a nil slice and an empty
// slice for encoding purposes, so an implementation of Clone()
// must take care to preserve this.
func (s State) Clone() State {
out := State{}
if s.LastAUMHash != nil {
dupe := *s.LastAUMHash
out.LastAUMHash = &dupe
}
if s.DisablementSecrets != nil {
out.DisablementSecrets = make([][]byte, len(s.DisablementSecrets))
for i := range s.DisablementSecrets {
out.DisablementSecrets[i] = make([]byte, len(s.DisablementSecrets[i]))
copy(out.DisablementSecrets[i], s.DisablementSecrets[i])
}
}
if s.Keys != nil {
out.Keys = make([]Key, len(s.Keys))
for i := range s.Keys {
out.Keys[i] = s.Keys[i].Clone()
}
}
return out
}
// cloneForUpdate is like Clone, except LastAUMHash is set based
// on the hash of the given update.
func (s State) cloneForUpdate(update *AUM) State {
out := s.Clone()
aumHash := update.Hash()
out.LastAUMHash = &aumHash
return out
}
const disablementLength = 32
var disablementSalt = []byte("tailscale network-lock disablement salt")
// DisablementKDF computes a public value which can be stored in a
// key authority, but cannot be reversed to find the input secret.
//
// When the output of this function is stored in tka state (i.e. in
// tka.State.DisablementSecrets) a call to Authority.ValidDisablement()
// with the input of this function as the argument will return true.
func DisablementKDF(secret []byte) []byte {
// time = 4 (3 recommended, booped to 4 to compensate for less memory)
// memory = 16 (32 recommended)
// threads = 4
// keyLen = 32 (256 bits)
return argon2.Key(secret, disablementSalt, 4, 16*1024, 4, disablementLength)
}
// checkDisablement returns true for a valid disablement secret.
func (s State) checkDisablement(secret []byte) bool {
derived := DisablementKDF(secret)
for _, candidate := range s.DisablementSecrets {
if bytes.Equal(derived, candidate) {
return true
}
}
return false
}
// parentMatches returns true if an AUM can chain to (be applied)
// to the current state.
//
// Specifically, the rules are:
// - The last AUM hash must match (transitively, this implies that this
// update follows the last update message applied to the state machine)
// - Or, the state machine knows no parent (its brand new).
func (s State) parentMatches(update AUM) bool {
if s.LastAUMHash == nil {
return true
}
return bytes.Equal(s.LastAUMHash[:], update.PrevAUMHash)
}
// applyVerifiedAUM computes a new state based on the update provided.
//
// The provided update MUST be verified: That is, the AUM must be well-formed
// (as defined by StaticValidate()), and signatures over the AUM must have
// been verified.
func (s State) applyVerifiedAUM(update AUM) (State, error) {
// Validate that the update message has the right parent.
if !s.parentMatches(update) {
return State{}, errors.New("parent AUMHash mismatch")
}
switch update.MessageKind {
case AUMNoOp:
out := s.cloneForUpdate(&update)
return out, nil
case AUMCheckpoint:
return update.State.cloneForUpdate(&update), nil
case AUMAddKey:
if update.Key == nil {
return State{}, errors.New("no key to add provided")
}
if _, err := s.GetKey(update.Key.ID()); err == nil {
return State{}, errors.New("key already exists")
}
out := s.cloneForUpdate(&update)
out.Keys = append(out.Keys, *update.Key)
return out, nil
case AUMUpdateKey:
k, err := s.GetKey(update.KeyID)
if err != nil {
return State{}, err
}
if update.Votes != nil {
k.Votes = *update.Votes
}
if update.Meta != nil {
k.Meta = update.Meta
}
if err := k.StaticValidate(); err != nil {
return State{}, fmt.Errorf("updated key fails validation: %v", err)
}
out := s.cloneForUpdate(&update)
for i := range out.Keys {
if bytes.Equal(out.Keys[i].ID(), update.KeyID) {
out.Keys[i] = k
}
}
return out, nil
case AUMRemoveKey:
idx := -1
for i := range s.Keys {
if bytes.Equal(update.KeyID, s.Keys[i].ID()) {
idx = i
break
}
}
if idx < 0 {
return State{}, ErrNoSuchKey
}
out := s.cloneForUpdate(&update)
out.Keys = append(out.Keys[:idx], out.Keys[idx+1:]...)
return out, nil
default:
// TODO(tom): Instead of erroring, update lastHash and
// continue (to preserve future compatibility).
return State{}, fmt.Errorf("unhandled message: %v", update.MessageKind)
}
}
// Upper bound on checkpoint elements, chosen arbitrarily. Intended to
// cap out insanely large AUMs.
const (
maxDisablementSecrets = 32
maxKeys = 512
)
// staticValidateCheckpoint validates that the state is well-formed for
// inclusion in a checkpoint AUM.
func (s *State) staticValidateCheckpoint() error {
if s.LastAUMHash != nil {
return errors.New("cannot specify a parent AUM")
}
if len(s.DisablementSecrets) == 0 {
return errors.New("at least one disablement secret required")
}
if numDS := len(s.DisablementSecrets); numDS > maxDisablementSecrets {
return fmt.Errorf("too many disablement secrets (%d, max %d)", numDS, maxDisablementSecrets)
}
for i, ds := range s.DisablementSecrets {
if len(ds) != disablementLength {
return fmt.Errorf("disablement[%d]: invalid length (got %d, want %d)", i, len(ds), disablementLength)
}
for j, ds2 := range s.DisablementSecrets {
if i == j {
continue
}
if bytes.Equal(ds, ds2) {
return fmt.Errorf("disablement[%d]: duplicates disablement[%d]", i, j)
}
}
}
if len(s.Keys) == 0 {
return errors.New("at least one key is required")
}
if numKeys := len(s.Keys); numKeys > maxKeys {
return fmt.Errorf("too many keys (%d, max %d)", numKeys, maxKeys)
}
for i, k := range s.Keys {
if err := k.StaticValidate(); err != nil {
return fmt.Errorf("key[%d]: %v", i, err)
}
for j, k2 := range s.Keys {
if i == j {
continue
}
if bytes.Equal(k.ID(), k2.ID()) {
return fmt.Errorf("key[%d]: duplicates key[%d]", i, j)
}
}
}
return nil
}