shelldeck/shelldeck/relay.py at main · codejunction/shelldeck

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"""Tell trusted iOS devices where the share is: the tunnel URL changes on every `sd share`.

Each device has a random ntfy.sh topic and an AES-256-GCM key, both made at enrolment and held only by

this server and the device's Keychain. The message is base64(nonce(12) | ciphertext | tag(16)) of

{"url", "at"}, the layout CryptoKit's AES.GCM.SealedBox(combined:) reads. ntfy.sh sees only ciphertext

and timing; GCM also stops anyone who guesses a topic from steering the app to a page of their own.

"""

import base64

import json

import logging

import os

import secrets

import time

import urllib.request

from cryptography.hazmat.primitives.ciphers.aead import AESGCM

log = logging.getLogger(__name__)

NTFY = "https://ntfy.sh"

REPEAT_S = 6 * 3600 # ntfy.sh keeps messages 12h: repeat while sharing so a phone opened later still finds it

def new_device() -> tuple[str, str]:

"""(topic, base64 key) for a new device."""

return secrets.token_urlsafe(24), base64.b64encode(AESGCM.generate_key(bit_length=256)).decode()

def seal(key: str, payload: dict) -> str:

nonce = os.urandom(12)

return base64.b64encode(nonce + AESGCM(base64.b64decode(key)).encrypt(nonce, json.dumps(payload).encode(), None)).decode()

def open_(key: str, message: str) -> dict:

raw = base64.b64decode(message)

return json.loads(AESGCM(base64.b64decode(key)).decrypt(raw[:12], raw[12:], None))

def publish(devices: list[dict], url: str) -> None:

"""Post the share URL to every device's topic. Blocking: run it on a thread. ponytail: no retry, the next

repeat (REPEAT_S) or share start tries again."""

for d in devices:

body = seal(d["key"], {"url": url, "at": time.time()}).encode()

req = urllib.request.Request(f"{NTFY}/{d['topic']}", data=body, method="POST")

try:

urllib.request.urlopen(req, timeout=10).close()

except OSError as e:

log.warning("relay to device %s failed: %s", d["id"], e)