The Imperative of Long-Term Signature Validity

Digitally signing PowerShell scripts is no longer just a best practice; it's a core security requirement. It provides guarantees of **authenticity** (who wrote the script) and **integrity** (the code hasn't changed). But a simple signature isn't enough. The ticking clock of certificate expiration poses a serious challenge. This section explores why cryptographic timestamping is the only sustainable solution.

Infographic: The TSA Countersignature Process

Certificate Authority Strategy: Public vs. Private

Choosing between a public CA (like DigiCert) and an internal, private CA (like ADCS) is a pivotal decision. It defines your trust boundaries, cost model, and security posture. Here's a breakdown to help you decide.

Decision Matrix: Public vs. Internal CA

End-to-End Implementation

Let's get practical. Here are the code snippets and verification steps you need to sign and timestamp your PowerShell scripts correctly.

Step 1: Find Your Certificate

# List code-signing certs in the current user's store
Get-ChildItem -Path Cert:CurrentUserMy -CodeSigningCert

Step 2: Sign and Timestamp the Script

# Assign the certificate to a variable
$cert = Get-ChildItem -Path Cert:CurrentUserMy -CodeSigningCert | Select-Object -First 1

# Define script path and TSA server
$scriptPath = "C:ScriptsMyImportantScript.ps1"
$timeStampServer = "http://timestamp.digicert.com"

# Apply the signature, explicitly using SHA256
try {
    Set-AuthenticodeSignature -FilePath $scriptPath -Certificate $cert -TimeStampServer $timeStampServer -HashAlgorithm 'SHA256' -ErrorAction Stop
    Write-Host "Successfully signed and timestamped '$scriptPath'."
}
catch {
    Write-Error "Failed to sign the script. Error: $_"
}

Step 3: Verify the Signature

After signing, programmatic verification is crucial. Use `Get-AuthenticodeSignature` to check the status.

$signature = Get-AuthenticodeSignature -FilePath $scriptPath
if ($signature.Status -eq 'Valid') {
    Write-Host "Verification successful: Signature is valid." -ForegroundColor Green
    if ($signature.TimeStamperCertificate) {
        Write-Host "Script is timestamped by: $($signature.TimeStamperCertificate.Subject)"
    } else {
        Write-Warning "Script is signed but NOT timestamped."
    }
} else {
    Write-Error "Verification FAILED. Status: $($signature.Status)"
}

Reliable Public TSA Servers

Choosing a reliable TSA server is crucial. Here's a filterable list of recommended providers.

Certificate Lifecycle Management

Code signing is not a one-time setup. It's a continuous lifecycle requiring proactive management of certificate renewals, automated re-signing, and emergency revocation to maintain security and avoid operational downtime.

Visualizing the Trust Process: A Flowchart

The process PowerShell uses to validate a script's signature involves a multi-step verification chain. This flowchart illustrates the decision-making logic, from the initial check for a signature to the final fallback on timestamping for expired certificates.