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nips/146.md
artur c23ec0672c Add nip146 Iframe-based Nostr Connect
2024-10-25 17:10:37 +02:00

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NIP-146

Iframe-based Nostr Connect (INC)

draft optional

This NIP defines a way for web apps to communicate with web signers embedded as iframes using browser APIs. It is applicable to self-custodial signers that store keys in the browser's local storage. Basically, client app does nip-46 RPC with an iframe using postMessage.

Due to privacy-related restrictions, local storage of iframes is partitioned from top-frame's storage - iframe of signer.com can't see the storage of top-level signer.com and can't access user's private key. Solution to this challenge is the biggest part of this NIP.

Advantage: minimal latency, no relay involved, no reliance on unreliable webpush, fully cross-platform. User private key is as safe in iframe's local storage as in top-level storage due to browsers' cross-origin restrictions.

Disadvantage: potentially more frequent "confirmations", higher surface for web-based hacks.

Terms

  • client: a web app trying to access user's keys
  • signer: non-custodial signer storing keys in it's local storage in the browser
  • top-level signer: signer opened in a separate tab/popup
  • iframe signer: signer opened as an iframe inside the client
  • worker iframe: iframe signer that handles nip46 calls from the client
  • starter iframe: iframe signer that launches the connection flow to get user confirmation and import keys from top-level signer to iframe signer
  • rebinder iframe: iframe signer that launches re-connection flow to check that client has access to keys at the top-level signer and to re-import the keys to iframe signer

Overview

When nip46 connection is established, client embeds invisible worker iframe and exchanges nip46 messages with it using postMessage.

To overcome partitioned storage, when client initiates nip46 connection and receives auth_url, it shows the starter iframe with "Continue" button. After user clicks, starter iframe launches auth_url popup to access top-level signer. When user confirms, top-level signer sends the user private key back to starter iframe using postMessage. User private key can then be used by worker iframe.

To overcome ephemeral partitioned storage (Brave, Webkit), when worker iframe receives a request but has no target keys, it pauses the call and sends an error code. Client shows visible rebinder iframe with "Continue" button. When user clicks, rebinder iframe launches top-level signer in a popup, which checks if client pubkey has access to the user pubkey and if so sends user private key back to rebinder iframe, without user having to confirm anything.

Worker iframe

Signer may signal support for this NIP by adding nip46.iframe_url field in their nostr.json file. If client detects a signer that supports iframes, after the connection has already been established, the client MAY create an invisible worker iframe with src set to nip46.iframe_url.

When worker iframe has started and is ready to process requests it must notify the client by calling window.parent.postMessage("workerReady", "*"). In turn, the client must subscribe to worker's messages with window.addEventListener("message", handler) before creating an iframe, and must wait for the message event with data set to workerReady and origin set to origin of nip46.iframe_url before sending requests.

When workerReady is received, the client MAY start sending nip-46 requests to the worker by calling workerIframe.postMessage(signedNip46RequestEvent, originOf(nip46.iframe_url)) - this will send the request event JS object to the worker iframe and will ensure it is only delivered if iframe still has proper origin (signer).

Worker iframe processes the request and sends back the reply event by using requestEvent.source.postMessage(signedNip46ResponseEvent, requestEvent.origin), this will deliver the reply to the client.

Starter iframe

Due to local storage partitioning, an iframe must import the needed key from the top-level storage of the signer. To do that, iframe must obtain a reference to a top-level signer and exchange messages. This fits well with the need to open top-level signer popup with auth_url of the nip-46 connection flow. However, this time the popup opener must be the signer iframe, and it must follow a user gesture in iframe.

To achieve that, a starter iframe MAY be embedded by the client when auth_url is received while the client is connecting. Starter iframe MUST be served when nip46.iframe_url has ?auth_url=<auth_url> parameter, and MUST display a button suggesting the users to Continue. Recommended button dimentions SHOULD be up to 180px width and 80px height.

When user clicks on Continue button in starter iframe, it MUST open the popup with auth_url and MUST accept the user's private key if user confirms in the popup. Details are implementation-defined, see recommendations below.

When starter iframe has finished importing the user private key from the popup, it MUST notify the app by calling window.parent.postMessage("starterDone", "*"). When client receives the starterDone from iframe_url's origin it MAY destroy the starter iframe and MAY create the worker iframe to start processing requests.

Rebinder iframe

When worker iframe receives a nip46 request targeting user pubkey that it doesn't have (local storage deleted) it MAY pause the call and then MUST reply with a string errorNoKey:<requestEvent.id> to notify the client.

If client receives errorNoKey:<requestEvent.id> reply matching one of pending requests, it MAY display a rebinder iframe. Rebinder iframe MUST be served when nip46.iframe_url has &rebind=<local_pubkey>&pubkey=<remote_user_pubkey> parameters, and MUST display a button similar to starter iframe suggesting the users to Continue.

When user clicks on Continue button in rebinder iframe, it MUST open a top-level signer popup. The popup MUST check that provided local_pubkey has permissions to access remote_user_pubkey and then MUST pass the user private key to rebinder iframe. Details are implementation-defined, see appendix for recommendations.

After rebinder iframe has finished importing the user private key, it MUST notify the client by calling window.parent.postMessage("rebinderDone", "*"). When client receives the rebinerDone from iframe_url's origin it MAY destroy the rebinder iframe.

When worker iframe that has paused the request notices that user private key was imported by rebinder into the local store, it MUST restart the paused call and deliver the reply to the client.

Recommendations

  • Popups SHOULD be opened using window.open(url, "<random_target>", "<options>"), <random_target> is required (instead of _blank) to make sure popup has access to opener, noopener SHOULD NOT be provided for the same reason.
  • Iframes SHOULD NOT launch a popup until their service worker has started, it seems like Safari pauses iframe's SW initialization if the tab loses focus
  • Starter/rebinder iframes MAY be sandboxed, but then MUST at least have allow-scripts,allow-same-origin,allow-popups-to-escape-sandbox
  • Starter iframe SHOULD be provided access to referrer to let the signer use it as client name/url.
  • Starter iframe SHOULD check that auth_url has valid origin.
  • Starter/rebinder MAY use MessageChannel to create a port that will be transferred to top-level signer and to it's service-worker so that it could then export the user private key to the iframe signer.
  • Client MAY save the iframe_url to local storage and reuse it until logout.
  • Client MAY send normal nip-46 requests over relay in paralles to iframe requests, in which case it SHOULD deduplicate the replies across both channels.
  • Signer SHOULD use random auxiliary sub-domains to serve iframe_url - Chrome desktop allows users to delete signer's storage on client's tab, which (mistakenly?) removes top-level signer storage too and users' keys might be lost.
  • Top-level signer MAY export local client pubkey to iframe along with user private key to scope the iframe signer to this particular connection.