Creating Code-Signing Certificates

by May 8, 2019

Windows 10 and Server 2016 (and better) ship with an enhanced New-SelfSignedCert cmdlet that finally can create code-signing certificates. With code-signing certificates, you can digitally sign PowerShell scripts and use the signature to detect when people tamper with your script content.

Here is a function that you can use to create a code-signing certificate:

function New-CodeSigningCert
        [Parameter(Mandatory, Position=0)]
        [Parameter(Mandatory, Position=1)]
    # Create a certificate:
    New-SelfSignedCertificate -KeyUsage DigitalSignature -KeySpec Signature -FriendlyName $FriendlyName -Subject "CN=$Name" -KeyExportPolicy ExportableEncrypted -CertStoreLocation Cert:\CurrentUser\My -NotAfter (Get-Date).AddYears(5) -TextExtension @('{text}')

To create a new certificate, run this command:

PS> New-CodeSigningCert -FriendlyName TobiasWeltner -Name TWeltner 

   PSParentPath: Microsoft.PowerShell.Security\Certificate::CurrentUser\My

Thumbprint                                Subject
----------                                -------                                                                                                                        
2350D77A4CACAF17136B94D297DEB1A5E413655D  CN=TWeltner  

With your new code-signing certificate, you can now digitally sign your scripts. Your code-signing certificate resides in your personal certificate store. To use it, you need to read it from there first:

$cert = Get-ChildItem -Path Cert:\CurrentUser\My -CodeSigningCert |
          Out-GridView -Title 'Select Certificate' -OutputMode Single 

To sign a single script, run this line:

$Path = "C:\path\to\your\script.ps1"

Set-AuthenticodeSignature -Certificate $cert -FilePath $Path 

If you want to sign with a timestamped signature, use this line instead:

Set-AuthenticodeSignature -Certificate $cert -TimestampServer -FilePath $Path  

Timestamped signatures remain valid even when the certificate used for signing has expired.

To sign many scripts, use Get-ChildItem and pipe the files into Set-AuthenticodeSignature. This line would sign all PowerShell scripts stored in your user profile:

Get-ChildItem -Path "$home\Documents" -Filter *.ps1 -Include *.ps1 -Recurse |
Set-AuthenticodeSignature -Certificate $cert 

Once you have signed scripts, use Get-AuthenticodeSignature anytime you want to check the integrity of the signature:

Get-ChildItem -Path "$home\Documents" -Filter *.ps1 -Include *.ps1 -Recurse |
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