Organizing a secure channel between a Web app and a Native app

Question

This question is kinda complimentary to \"Share credentials between native app and web site\", as we aim to share secrets in the opposite direction.

TL;TR:

Answer 1

You could try driving the synchronization the other way:

1. Once the user is authenticated into the web app, launch the native app from the web app via the custom URL scheme.
2. If the native app is not authenticated, connect securely to the backend over HTTPS, create a record for the native app, retrieve a one time token associated with that record and then launch the web app in the user's browser with the token as a URL parameter.
3. Since the user is authenticated in the browser, when the server sees the token it can bind the native app's record with the user account.
4. Have the native app poll (or use some other realtime channel like push notifications or a TCP connection) the server to see if the token has been bound to a user account: once that happens you can pass a persistent auth token that the native app can store.

Answer 2

The best solution : Single Sign On (SSO) using Custom URL Scheme

When I was checking your question, I remembered the Zoom app that I am using in my office. How it works ?

I have my Gmail account linked to a Zoom account (this is account linkage, which is outside the scope of implementation). When I open Zoom app, I can choose the option to login with Gmail. This opens my browser and take me to Gmail. If I am logged in to Gmail, I am redirected back to a page that asking me to launch Zoom app. How this app launch happen ? The application register a custom URL scheme when app get installed and the final redirect in browser targets this URL. And this URL passes a temporary secret, which Zoom application uses to obtain OAuth tokens. And token obtaining is done independent of the browser, a direct call with SSL to token endpoint of OAuth server.

Well this is Authorization code flow for native applications. And this is how Mobile applications use OAuth. Your main issue, not allowing user to re-login is solved. This is SSO in action.

There is a specification which define best practices around this mechanism. I welcome you to go through RFC8252 - OAuth 2.0 for Native Apps.

Challenge

You need to implement OS specific native code for each application distribution. Windows, Mac and Linux have different implementation support for custom URL scheme.

Advice

PKCE is mandatory (in IETF words SHOULD) for all OAuth grant types. There is this ongoing draft which talks about this. So include PKCE for your implementation too.

With PKCE, the redirect/callback response is protected from stealing. Even some other application intercept the callback, the token request cannot be recreated as the PKCE code_verifer is there.

Also, do not use a custom solution like passing secret through another channel. This will make things complicated when it comes to maintenance. Since this flow already exists in OAuth, you can benefit with libraries and guidance.

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Update : Protecting Token Request

While the custom URL scheme solves the problem of launching the native application, protecting token request can be challenging. There are several options to consider.

- Bind native application launch with a secret shared from browser

When browser based client launch the native client, it can invoke a custom API to generate a secret. This secret acts like a one time password (OTP). User has to enter this value in native app before it obtain tokens. This is a customization on top of Authorization code flow.

- Dynamic client registration & Dynamic client authentication

Embedding secrets into public clients is discouraged by OAuth specification. But as question owner points out, some malicious app may register itself to receive custom URL response and obtain tokens. In such occasion, PKCE can provide an added layer of security.

But still in an extreme case, if malicious app registers the URL plus use PKCE as the original application, then there can be potential threats.

One option is to allow dynamic client registration at the first time of application launch. Here, installer/distribution can include a secret that used along with DCR.

Also, it is possible to use dynamic client authentication through a dedicated service. Here, the application's token request contains a temporary token issued by a custom service. Custom service obtain a challenge from native application. This may be done through totp or a cryptographic binding based on an embedded secret. Also it is possible to utilize OTP (as mentioned in first note) issued through browser, which needs to be copy pasted manually by end user. Once validated, this service issue a token which correlate to the secret. In the token request, native client sends this token along with call back values. This way we reduce threat vectors even though we increase implementation complexity.

Summary

• Use custom URL scheme to launch the native application
• Browser app generate a temporary secret shared with a custom service
• At native app launch, user should copy the secret to native app UI
• Native app exchange this secret with custom service to obtain a token
• This second token combined with call back authorization code (issued through custom url scheme) is used to authenticate to token endpoint
• Above can be considered as a dynamic client authentication
• Value exposed to user can be a hashed secret, hence original value is never exposed to end user or another client
• DCR is also an option but embedded secrets are discouraged in OAuth world

Answer 3

Just got the following idea. It's simple and while it doesn't allow to fully automate the setup of a secure channel between Web Browser app and the Native app, it may significantly improve the user experience.

We can use Time-based One-Time Password algorithm (TOTP). In a way, it's similar to how we pair a Bluetooth keyboard to a computer or a phone.

The Web Browser app (where the user is already authenticated) could display a time-based code to the user, and the Native app should ask the user to enter that code as a confirmation. It would then use the code to authenticate against the Web API. That should be enough to establish a back-end channel between the two. The life time of the channel should be limited to that of the session within the Web Browser app. This approach might even eliminate the need for a custom protocol communication in the first place.

Still open to other ideas.

Answer 4

Did you think about using LDAP or Active Directory?

Also OAuth2 could be combined, here are a related question:
- Oauth service for LDAP authentication
- Oauth 2 token for Active Directory accounts

SSO should be easier then too, furthermore access-rights could be managed centralized.

Concerning general security considerations you could work with two servers and redirect form the one for the web-application to the other one after successful access check. That 2nd server can be protected so far that a redirect is required from the 1st server and an access check could be made independent again but without need to login another time, might be important to mention here the proposed usage of Oracle Access Manager in one linked answer for perimeter authentication.
This scenario with two servers could be also hidden by using a proxy-server in the frontend and making the redirects hidden, like that data-transfer between servers would be easier and secure too. The important point about my proposition is that access to the 2nd server is just not granted if anything is wrong and the data are still protected.

I read here some comments concerning 2FA and some other ideas like tokens, surely those things increase security and it would be good to implement them.

If you like the general idea, I'm willing to spend still some time on the details. Some questions might be helpful for me ;-)

EDIT:
Technically the design in detail might depend on the used external authentication provider like Oracle Access Manager or something else. So if the solution in general sounds reasonable for you it would be useful to elaborate some parameters for the choice of a external authentication provider, i.e. price, open-source, features, etc.
Nevertheless the general procedure then is that the provider issues a token and this token serves for authentication. The token is for unique one-time usage, the second link I posted above has some answers that explain token-usage very well related to security and OAuth.

EDIT2
The Difference between an own OAuth2 / OIDC server and a LDAP/AD server is that you need to program everything by yourself and can't use ready solutions. Nevertheless you're independent and if everything is programmed well perhaps even a bit more secure as your solution is not public available and therefore harder to hack - potential vulnerabilities just can't be known by others. Also you're more independent, never have to wait for updates and are free to change whatever you want at any time. Considering that several software-servers are involved and perhaps even hardware-servers the own solution might be limited scale-able, but that can't be know from outside and depends on your company / team. Your code base probably is slimmer than full-blown solutions as you've only to consider your own solution and requirements.
The weak point in your solution might be that you have to program interfaces to several things that exist ready for business-frameworks. Also it might be hard to consider every single point in a small team, large companies could have more overview and capacity to tackle every potential issue.

Answer 5

As you mentioned, using a custom protocol handler is not a safe way to pass secrets, since another app may handle your protocol and intercept that secret.

If you are imposing a strict constraint that the communication channel between the native app and the web app is initiated from the web app, and that the native app has not previously established a secure channel (e.g. shared secret which could encrypt other secrets), then it is not possible to safely transmit a secret to the native app.

Imagine if this were possible, then PKCE would be redundant in an OAuth 2.0 Code Flow, since the server could have safely transmitted the access token in response to the authorization request, instead of requiring the code_verifier to be provided with the grant when obtaining the access token.