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Generate a self-signed certificate for .NET Core

A team member contacted me because he no longer could invoke a local service he was developing. I had a look at the error message the API returned:

One or more errors occurred.
An error occurred while sending the request.
The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
The remote certificate is invalid according to the validation procedure

This was a local service using a self-signed certificate and it turned out that the certificate was expired.

Time to create a new self-signed certificate…

Create a self-signed certificate using dotnet dev-certs

Generating a new self-signed certificate is easy in .NET Core thanks to the built-in support in the dotnet commandline tool.

Open a command prompt and execute the following command:

dotnet dev-certs https -ep c:\users\BaWu\localhost.pfx -p crypticpassword

Remark:  If the command returns the following response "A valid HTTPS certificate is already present.", a trusted certificate already exists in your store.

Import certificate in IIS

OK, almost there. A last step to get it working in IIS is to import our generated pfx in IIS:

  • Open IIS using inetmgr.
  • Go to Server Certificates.


  • Click on the Import… action on the right. The Import certificate screen is shown.


  • Select the pfx, specify the password and click OK.
  • Now that the certificate is available in IIS, you can change the bindings to use it. Click on the Default Web site(or any other site) on the left.
  • Click on the Bindings… action on the right. The Site Bindings screen is shown.


  • Click on the https item in the list and choose Edit… . The Edit Site Binding screen is shown.


  • Select the newly created SSL certificate from the list and click OK.

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