mg4/server/pairing.go

package main

import (
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"strings"
	"sync"
	"time"
)

// Pairing lets a car get its own ingest token without anyone typing it: the car asks for a
// pairing request and shows its code as a QR code, a phone opens the approval page, the
// owner signs in with the approval password and approves, and the car — polling meanwhile —
// receives a token once. Modelled on the OAuth device authorization grant (RFC 8628).
//
// Pending requests live in memory only: they last ten minutes, and the server runs as one
// replica. Approved devices are persisted (see Device) so their tokens survive a restart.

const (
	pairingTTL        = 10 * time.Minute
	pairingInterval   = 5 * time.Second
	maxPendingPairing = 50
	// After this many wrong approval passwords in a window, approvals pause for the window.
	maxFailedLogins    = 5
	failedLoginsWindow = time.Minute
	// Unambiguous characters only: no 0/O, 1/I/L, so a code reads right off a car screen.
	userCodeAlphabet = "ABCDEFGHJKMNPQRSTUVWXYZ23456789"
)

var (
	errUnknownCode   = errors.New("unknown or expired code")
	errNotPending    = errors.New("request already decided")
	errTooManyFailed = errors.New("too many wrong passwords, try again in a minute")
	errWrongPassword = errors.New("wrong password")
	errTooMany       = errors.New("too many pending pairing requests")
)

// Device is a paired car. Only the SHA-256 of its token is kept.
type Device struct {
	ID             string
	InstallationID string
	TokenHash      string
	CreatedAt      time.Time
	Revoked        bool
}

type pairingState int

const (
	pairingPending pairingState = iota
	pairingApproved
	pairingDenied
)

type pairingRequest struct {
	deviceCode     string
	userCode       string
	installationID string
	expires        time.Time
	state          pairingState
	token          string // set on approval, handed out once
	deviceID       string
}

// Pairing holds pending requests and the known devices. The clock is injectable for tests.
type Pairing struct {
	ApprovalPassword string
	Now              func() time.Time

	mu       sync.Mutex
	byDevice map[string]*pairingRequest
	byUser   map[string]*pairingRequest
	devices  map[string]Device // by token hash
	failures []time.Time
}

func NewPairing(password string, devices []Device) *Pairing {
	p := &Pairing{
		ApprovalPassword: password,
		Now:              time.Now,
		byDevice:         map[string]*pairingRequest{},
		byUser:           map[string]*pairingRequest{},
		devices:          map[string]Device{},
	}
	for _, d := range devices {
		p.devices[d.TokenHash] = d
	}
	return p
}

func (p *Pairing) Enabled() bool { return p != nil && p.ApprovalPassword != "" }

func randomString(n int) string {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		panic(err) // crypto/rand does not fail on supported platforms
	}
	return base64.RawURLEncoding.EncodeToString(b)
}

func newUserCode() string {
	b := make([]byte, 6)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}
	code := make([]byte, 0, 7)
	for i, v := range b {
		if i == 3 {
			code = append(code, '-')
		}
		// 256 % 31 leaves a slight bias; irrelevant for a 10-minute code that also needs
		// the approval password.
		code = append(code, userCodeAlphabet[int(v)%len(userCodeAlphabet)])
	}
	return string(code)
}

// normalizeUserCode accepts what a person types: any case, with or without the dash.
func normalizeUserCode(code string) string {
	code = strings.ToUpper(strings.TrimSpace(code))
	code = strings.ReplaceAll(code, "-", "")
	code = strings.ReplaceAll(code, " ", "")
	if len(code) != 6 {
		return ""
	}
	return code[:3] + "-" + code[3:]
}

func hashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:])
}

func (p *Pairing) expireLocked(now time.Time) {
	for code, r := range p.byDevice {
		if now.After(r.expires) {
			delete(p.byDevice, code)
			delete(p.byUser, r.userCode)
		}
	}
}

// Start opens a pairing request for a car.
func (p *Pairing) Start(installationID string) (*pairingRequest, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	now := p.Now()
	p.expireLocked(now)
	if len(p.byDevice) >= maxPendingPairing {
		return nil, errTooMany
	}
	r := &pairingRequest{
		deviceCode:     randomString(32),
		installationID: installationID,
		expires:        now.Add(pairingTTL),
	}
	for {
		r.userCode = newUserCode()
		if _, taken := p.byUser[r.userCode]; !taken {
			break
		}
	}
	p.byDevice[r.deviceCode] = r
	p.byUser[r.userCode] = r
	out := *r
	return &out, nil
}

// Lookup returns the pending request for a user code, for the approval page.
func (p *Pairing) Lookup(userCode string) (pairingRequest, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	p.expireLocked(p.Now())
	r, ok := p.byUser[normalizeUserCode(userCode)]
	if !ok {
		return pairingRequest{}, errUnknownCode
	}
	return *r, nil
}

// checkPassword counts failures and refuses everything while too many are recent.
func (p *Pairing) checkPasswordLocked(password string, now time.Time) error {
	recent := p.failures[:0]
	for _, t := range p.failures {
		if now.Sub(t) < failedLoginsWindow {
			recent = append(recent, t)
		}
	}
	p.failures = recent
	if len(p.failures) >= maxFailedLogins {
		return errTooManyFailed
	}
	if subtle.ConstantTimeCompare([]byte(password), []byte(p.ApprovalPassword)) != 1 {
		p.failures = append(p.failures, now)
		return errWrongPassword
	}
	return nil
}

// Decide approves or denies a pending request. On approval it returns the new device, which
// the caller must persist before the car can collect the token.
func (p *Pairing) Decide(userCode, password string, approve bool) (Device, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	now := p.Now()
	if err := p.checkPasswordLocked(password, now); err != nil {
		return Device{}, err
	}
	p.expireLocked(now)
	r, ok := p.byUser[normalizeUserCode(userCode)]
	if !ok {
		return Device{}, errUnknownCode
	}
	if r.state != pairingPending {
		return Device{}, errNotPending
	}
	if !approve {
		r.state = pairingDenied
		return Device{}, nil
	}
	token := randomString(32) // 43 characters
	d := Device{
		ID:             randomString(9),
		InstallationID: r.installationID,
		TokenHash:      hashToken(token),
		CreatedAt:      now,
	}
	r.state = pairingApproved
	r.token = token
	r.deviceID = d.ID
	p.devices[d.TokenHash] = d
	return d, nil
}

// Forget undoes an approval whose device could not be persisted, so no token is handed out
// that would stop working at the next restart.
func (p *Pairing) Forget(userCode string, d Device) {
	p.mu.Lock()
	defer p.mu.Unlock()
	delete(p.devices, d.TokenHash)
	if r, ok := p.byUser[normalizeUserCode(userCode)]; ok {
		r.state = pairingPending
		r.token = ""
		r.deviceID = ""
	}
}

// Poll is what the car calls: pending, the token (exactly once), denied, or unknown.
func (p *Pairing) Poll(deviceCode string) (state pairingState, token, deviceID string, err error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	p.expireLocked(p.Now())
	r, ok := p.byDevice[deviceCode]
	if !ok {
		return 0, "", "", errUnknownCode
	}
	switch r.state {
	case pairingApproved, pairingDenied:
		// Decided: hand the outcome out once, then forget the request.
		delete(p.byDevice, r.deviceCode)
		delete(p.byUser, r.userCode)
	}
	return r.state, r.token, r.deviceID, nil
}

// Authorized reports whether a token belongs to a paired, unrevoked device.
func (p *Pairing) Authorized(token string) bool {
	if p == nil || token == "" {
		return false
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	d, ok := p.devices[hashToken(token)]
	return ok && !d.Revoked
}

func (p *Pairing) Devices() []Device {
	p.mu.Lock()
	defer p.mu.Unlock()
	out := make([]Device, 0, len(p.devices))
	for _, d := range p.devices {
		out = append(out, d)
	}
	return out
}

// Revoke marks a device revoked and returns it for persisting.
func (p *Pairing) Revoke(id string) (Device, bool) {
	p.mu.Lock()
	defer p.mu.Unlock()
	for hash, d := range p.devices {
		if d.ID == id {
			d.Revoked = true
			p.devices[hash] = d
			return d, true
		}
	}
	return Device{}, false
}

// CheckAdmin verifies the approval password for the device-management API, with the same
// failure counting as the approval page.
func (p *Pairing) CheckAdmin(password string) error {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.checkPasswordLocked(password, p.Now())
}