How to authenticate an Add-In on interprocess communication

We are considerung to build an Windows application that is split in 2 parts:
One part running as a Windows service and the other part as Add-Ins.
There could be different kinds of Add-Ins:
For Microsoft Office, for Microsoft Management Console (MMC) and for the PowerShell.

The service as well as the Add-Ins are digitally signed with a companies certificate and are all running on the same machine.
The service runs in a different user account than the Add-Ins.
The Add-Ins may run in various interactive user accounts.
The Add-Ins need to communicate with the Windows service to exchange a secret that is known to the service only, but is needed at the Add-Ins to get access to some sensitive data.
Therefor the Add-In would connect to the service (via e.g. named pipe) and requests that secret on demand.
But we need to avoid that another application/process does the same and gets the secret.
Only those Add-Ins that belong to our application (that are digitally signed with our certificate) may get the secret. In other words, the Add-Ins need some way to authenticate themself to the service.

So my question is:
How can the Add-Ins authenticate themself to the service while requesting the secret? Is it possible to use the digital signature (or someting else that is unique to them and to the service) for this?

It might get more difficult when considering, that the Add-Ins are hosted by processes that might be signed with different certificates (e.g. Word.exe, mmc.exe, …).

We are implementing on the .NET Framework.

Thanks for any kind of help.

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Author: MartinM