Trusted Web Device Client

Question

I have a public web API for a SaaS. We do not have user accounts for the users except for the admin, simply because the users who are using the system are often non-permanent workers or staff.

Instead we have a client/customer number and a PIN which is used to authenticate and setup the browser client. The authenticated JWT is stored in the browser like any other access token and is sent back to the server together with every API request. This works fine, although we do understand that a malicious actor could potentially extract this JWT and use it to illicitly authenticate as well.

Now we want to add two extra layers of security: IP whitelist and trusted devices.

First we have introduced an IP whitelist. This is straight-forward, we simply check the remote IP against a whitelist created by the admin which is bound to the client number.

Now the problem is with the trusted list of devices. For each client number, there should be a way to trust a device for that specific client number. So the client/customer should be able to use the web API only on the devices it has in the office. The ideal scenario would be that the admin can issue a "trusted device" certificate with a digital signature which sign something like "client number+year+client pin". Then the local manager individual needs to import this certificate/secret manually on the device that should be trusted.

Then for each request our web client app (via javascript) also sends this signature with each request in a HTTP header. I understand this could potentially be spoofed as well but together with the IP whitelist we assess and believe it should be secure enough for this application.

The problem is now, how to implement that? As far as I understand:

• I cannot store such a secret in the browser because it might be cleared.
• I cannot read the filesystem with javascript. Store the secret in a text file?
• I cannot access the device certificate store.

How can I manage (import, store and read from javascript) this digital signature/certificate/secret in the client device? The web client can be anything from a iPad and Android phone to a Windows, Linux, iOS desktop. All web calls are done with SSL. We are not running any native app on the client device, only browser.