Code Signing Certificates and PowerShell

When creating PowerShell scripts for distribution, it can be useful to digitally sign the script. This ensures the person getting your code knows it has not changed since you sent it, and can verify you as the person signing the code. If you put code on GitHub, for example, a signature might be a great idea. To do this, you need to have a code signing certificate.

In terms of certificates, you have two main options. You can use your own CA to issue the certificate(s) – or use a public CA. The benefit of the public CA is that their root CAs tend to be fully trusted making it more useful. If you issue your own certs, you may have trouble with other people trusting your code signing certificates.

I have recently obtained a new code signing certificate from DigiCert (https://www.digicert.com). It was really very easy:

1. Open an account and order your cert.

2. Validate you are who you say you are. This involves sending DigiCert some documentation (eg your Passport) to prove you are who you way you are.

3. Do a Skype call, where they watch you sign their Identify Verification and Authorization document.

4. Generate, download, and install the certificate.

The validation process was easy although I had issues with the Skype call, initially. Mainly because I was flat out ill for weeks. Then when I was better, I had some difficulty getting the Skype call going. Entirely my issue, although it has to be said, Digicert support are really very, very, very busy. Between being ill and their overload, it took a bit longer to organise – but today it’s done. I did the call, they saw me sign the form and within an hour or so, the cert was working.

To use the cert to sign something is pretty easy. First you start with the script you want to sign:

# C:\Foo\Certs\Cert1.ps1
Write-Host "I got my cert from DigiCert"

A simple script saved as C:\Foo\Certs\cert1.ps1. To sign it it is simple:

$Cert = Get-ChildItem -Path Cert:\CurrentUser\My\ –CodeSigningCert
Set-AuthenticodeSignature -Certificate $Cert -FilePath C:\Foo\Certs\Cert1.ps1

Once signed, you can verify the signature by using Get-AuthenticodeSignature, like this:

Very simple and very straightforward. If you, for some reason, have multiple signing certificates then you’d need to adjust call to Get-ChildItem to ensure you get the right certificate.