diora-web/accounts/keyshare.py

"""Passing the book key from an unlocked device to a new one (models.KeyShare).

The book key never reaches the server in the clear: the new device posts an ECDH P-256
public key, an unlocked device of the same account answers with its own public key and
the book key AES-GCM-encrypted under the shared secret. Both devices show a code made
from the request's public key (static/js/app/33-unlock.js), so the user approves the
right device. Requests run out after a few minutes; an answer is handed out once.

  POST /accounts/keyshare/                  {pub, device}      -> {token}
  GET  /accounts/keyshare/<token>/                             -> {state, pub, iv, ct}
  GET  /accounts/keyshare/pending/                             -> {requests: [...]}
  POST /accounts/keyshare/<token>/answer/   {pub, iv, ct} | {decline: true}
"""
import base64
import binascii
import json
from datetime import timedelta

from django.http import JsonResponse
from django.utils import timezone
from django.views.decorators.http import require_http_methods

from .models import KeyShare

LIFETIME = timedelta(minutes=10)
# Only requests whose device asked lately are offered: a closed tab drops out quickly.
ALIVE = timedelta(seconds=20)
MAX_OPEN = 5


def _auth(request):
    if not request.user.is_authenticated:
        return JsonResponse({'error': 'authentication required'}, status=401)
    return None


def _body(request):
    try:
        body = json.loads(request.body or b'{}')
    except (json.JSONDecodeError, ValueError):
        return None
    return body if isinstance(body, dict) else None


def _b64(value, low, high):
    """A base64 string whose decoded length lies in [low, high], else None."""
    if not isinstance(value, str) or len(value) > 4 * high:
        return None
    try:
        raw = base64.b64decode(value, validate=True)
    except (binascii.Error, ValueError):
        return None
    return value if low <= len(raw) <= high else None


def _purge(user):
    KeyShare.objects.filter(user=user, created_at__lt=timezone.now() - LIFETIME).delete()


@require_http_methods(['POST'])
def create(request):
    if (r := _auth(request)):
        return r
    body = _body(request)
    pub = _b64((body or {}).get('pub'), 65, 65)  # uncompressed P-256 point
    if pub is None:
        return JsonResponse({'error': 'pub required'}, status=400)
    _purge(request.user)
    # A device that reloads asks anew: the oldest requests make room instead of blocking it.
    stale = KeyShare.objects.filter(user=request.user).order_by('-created_at').values_list('pk', flat=True)[MAX_OPEN - 1:]
    KeyShare.objects.filter(pk__in=list(stale)).delete()
    device = str(body.get('device') or '')[:100]
    share = KeyShare.objects.create(user=request.user, request_pub=pub, device=device,
                                    polled_at=timezone.now())
    return JsonResponse({'token': share.token})


@require_http_methods(['GET'])
def status(request, token):
    if (r := _auth(request)):
        return r
    _purge(request.user)
    share = KeyShare.objects.filter(user=request.user, token=token).first()
    if share is None:
        return JsonResponse({'state': 'expired'})
    if share.declined:
        share.delete()
        return JsonResponse({'state': 'declined'})
    if not share.answer_ct:
        share.polled_at = timezone.now()
        share.save(update_fields=['polled_at'])
        return JsonResponse({'state': 'pending'})
    data = {'state': 'answered', 'pub': share.answer_pub, 'iv': share.answer_iv, 'ct': share.answer_ct}
    share.delete()  # handed out once
    return JsonResponse(data)


@require_http_methods(['GET'])
def pending(request):
    if (r := _auth(request)):
        return r
    _purge(request.user)
    open_ = KeyShare.objects.filter(user=request.user, declined=False, answer_ct='',
                                    polled_at__gte=timezone.now() - ALIVE).order_by('-created_at')
    return JsonResponse({'requests': [
        {'token': s.token, 'pub': s.request_pub, 'device': s.device,
         'created_at': s.created_at.isoformat()} for s in open_]})


@require_http_methods(['POST'])
def answer(request, token):
    if (r := _auth(request)):
        return r
    _purge(request.user)
    share = KeyShare.objects.filter(user=request.user, token=token).first()
    if share is None:
        return JsonResponse({'error': 'expired'}, status=404)
    if share.answer_ct or share.declined:
        return JsonResponse({'error': 'already answered'}, status=409)
    body = _body(request) or {}
    if body.get('decline'):
        share.declined = True
        share.answered_at = timezone.now()
        share.save(update_fields=['declined', 'answered_at'])
        return JsonResponse({'ok': True})
    pub = _b64(body.get('pub'), 65, 65)
    iv = _b64(body.get('iv'), 12, 12)
    ct = _b64(body.get('ct'), 32 + 16, 32 + 16)  # 256-bit key + GCM tag
    if not (pub and iv and ct):
        return JsonResponse({'error': 'pub, iv and ct required'}, status=400)
    share.answer_pub, share.answer_iv, share.answer_ct = pub, iv, ct
    share.answered_at = timezone.now()
    share.save(update_fields=['answer_pub', 'answer_iv', 'answer_ct', 'answered_at'])
    return JsonResponse({'ok': True})