feat(ci): Authenticode-sign the bootstrap MSI and the runner it embeds (D-223)

MSI carries an Authenticode signature like any PE, so the same identity and the same
ordering rules apply: signing identity logs in first and only for the Key Vault token,
the storage login stays last, and no environment: on the job.

The embedded Invoke-BootstrapDownload.ps1 is signed BEFORE wix build -- wix embeds the
file as it finds it, so signing afterwards would leave the copy inside the MSI unsigned,
and a device under an AllSigned policy needs the runner to carry its own signature.

The MSI is signed after the build and before the hash and the upload, then both files are
verified (Valid, timestamped, optional publisher pin) while nothing is published yet.
The provenance attestation now also skips runs that upload nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/build-msi.yml

@@ -25,6 +25,13 @@ env:
   MSI_PROJECT_DIR: src/Agent/AutopilotMonitor.BootstrapMsi
   RUNNER_SCRIPT: src/Agent/AutopilotMonitor.BootstrapMsi/Invoke-BootstrapDownload.ps1
   STABLE_MSI: AutopilotMonitor-Bootstrap.msi
+  # Authenticode via Azure Key Vault; identity and vault come from repository variables.
+  # CODE_SIGN_EXPECTED_SUBJECT is optional and pins the publisher in the verify steps.
+  CODE_SIGN_CLIENT_ID: ${{ vars.CODE_SIGN_CLIENT_ID }}
+  CODE_SIGN_CERTIFICATE_NAME: ${{ vars.CODE_SIGN_CERTIFICATE_NAME }}
+  CODE_SIGN_KEY_VAULT_URL: ${{ vars.CODE_SIGN_KEY_VAULT_URL }}
+  CODE_SIGN_EXPECTED_SUBJECT: ${{ vars.CODE_SIGN_EXPECTED_SUBJECT }}
+  CODE_SIGN_TIMESTAMP_URL: http://timestamp.acs.microsoft.com
 
 jobs:
   build:
@@ -62,6 +69,38 @@ jobs:
           Write-Host "MSI version (from Package.wxs): $version"
           "version=$version" >> $env:GITHUB_OUTPUT
 
+      # Every published MSI is signed; a missing variable stops the run before the build.
+      - name: Check code-signing configuration
+        shell: pwsh
+        run: |
+          foreach ($name in 'CODE_SIGN_CLIENT_ID', 'CODE_SIGN_CERTIFICATE_NAME', 'CODE_SIGN_KEY_VAULT_URL') {
+            if ([string]::IsNullOrWhiteSpace([Environment]::GetEnvironmentVariable($name))) {
+              throw "Repository variable $name is not set. Every published MSI must be Authenticode-signed."
+            }
+          }
+
+      # Logs in FIRST and only to fetch the Key Vault token: azure/login sets the default az
+      # account, so the storage login below has to be the last one. Both federated credentials
+      # are bound to the ref, so this job must never be given an `environment:` -- that would
+      # change the OIDC subject claim and break both logins.
+      - name: Login to Azure (code-signing identity)
+        uses: azure/login@7ddb5af1ef8758cf1353cf3b42f940aee27ba21c # v3
+        with:
+          client-id: ${{ env.CODE_SIGN_CLIENT_ID }}
+          tenant-id: ${{ secrets.AZURE_TENANT_ID }}
+          allow-no-subscriptions: true
+
+      - name: Acquire Key Vault token for signing
+        shell: pwsh
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $token = az account get-access-token --scope https://vault.azure.net/.default --query accessToken --output tsv
+          if ($LASTEXITCODE -ne 0 -or [string]::IsNullOrWhiteSpace($token)) {
+            throw "Could not acquire a Key Vault token for the code-signing identity."
+          }
+          Write-Host "::add-mask::$token"
+          "CODE_SIGN_AKV_ACCESS_TOKEN=$token" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+
       # The blob upload below is authorized by this federated identity, not by a SAS.
       - name: Login to Azure (OIDC)
         uses: azure/login@7ddb5af1ef8758cf1353cf3b42f940aee27ba21c # v3
@@ -73,6 +112,32 @@ jobs:
       - name: Install WiX toolset
         run: dotnet tool install --global wix --version "5.*"
 
+      # Version pinned; roll-forward because the tool targets an older runtime than the SDK.
+      - name: Install AzureSignTool
+        env:
+          DOTNET_ROLL_FORWARD: LatestMajor
+        run: dotnet tool install --no-cache --global AzureSignTool --version 7.0.1
+
+      # BEFORE the build: wix embeds the file as it finds it, so a script signed afterwards
+      # would leave the copy inside the MSI unsigned. Devices under an AllSigned policy need
+      # the embedded runner to carry a signature of its own.
+      - name: Sign bootstrap runner script (Authenticode)
+        shell: pwsh
+        env:
+          DOTNET_ROLL_FORWARD: LatestMajor
+        run: |
+          $ErrorActionPreference = 'Stop'
+          & azuresigntool.exe sign --verbose `
+            --azure-key-vault-url "$env:CODE_SIGN_KEY_VAULT_URL" `
+            --azure-key-vault-accesstoken "$env:CODE_SIGN_AKV_ACCESS_TOKEN" `
+            --azure-key-vault-certificate "$env:CODE_SIGN_CERTIFICATE_NAME" `
+            --timestamp-rfc3161 "$env:CODE_SIGN_TIMESTAMP_URL" `
+            --file-digest sha256 `
+            --description 'Autopilot Monitor bootstrap runner' `
+            --description-url 'https://www.autopilotmonitor.com' `
+            $env:RUNNER_SCRIPT
+          if ($LASTEXITCODE -ne 0) { throw "azuresigntool failed with exit code $LASTEXITCODE" }
+
       - name: Build MSI
         shell: pwsh
         run: |
@@ -83,9 +148,39 @@ jobs:
           wix build Package.wxs -arch x64 -o $out
           Pop-Location
           if (-not (Test-Path $env:STABLE_MSI)) { Write-Error "wix build produced no MSI"; exit 1 }
-          $sha256 = (Get-FileHash $env:STABLE_MSI -Algorithm SHA256).Hash.ToLowerInvariant()
           Write-Host "Built $env:STABLE_MSI ($([math]::Round((Get-Item $env:STABLE_MSI).Length / 1KB)) KB)"
-          Write-Host "SHA-256: $sha256"
+
+      # Signing rewrites the MSI, so it runs before the hash and the upload. A missing
+      # timestamp fails the run: without it the signature dies with the certificate.
+      - name: Sign MSI (Authenticode)
+        shell: pwsh
+        env:
+          DOTNET_ROLL_FORWARD: LatestMajor
+        run: |
+          $ErrorActionPreference = 'Stop'
+          & azuresigntool.exe sign --verbose `
+            --azure-key-vault-url "$env:CODE_SIGN_KEY_VAULT_URL" `
+            --azure-key-vault-accesstoken "$env:CODE_SIGN_AKV_ACCESS_TOKEN" `
+            --azure-key-vault-certificate "$env:CODE_SIGN_CERTIFICATE_NAME" `
+            --timestamp-rfc3161 "$env:CODE_SIGN_TIMESTAMP_URL" `
+            --file-digest sha256 `
+            --description 'Autopilot Monitor Bootstrap' `
+            --description-url 'https://www.autopilotmonitor.com' `
+            $env:STABLE_MSI
+          if ($LASTEXITCODE -ne 0) { throw "azuresigntool failed with exit code $LASTEXITCODE" }
+
+          # The MSI and the runner it carries -- verified here, before anything is published.
+          foreach ($file in @($env:STABLE_MSI, $env:RUNNER_SCRIPT)) {
+            $sig = Get-AuthenticodeSignature -FilePath $file
+            if ($sig.Status -ne 'Valid') { throw "Signature on $file is '$($sig.Status)', expected 'Valid'." }
+            if (-not $sig.TimeStamperCertificate) { throw "Signature on $file carries no timestamp." }
+            if ($env:CODE_SIGN_EXPECTED_SUBJECT -and $sig.SignerCertificate.Subject -notlike $env:CODE_SIGN_EXPECTED_SUBJECT) {
+              throw "$file was signed by '$($sig.SignerCertificate.Subject)', expected '$env:CODE_SIGN_EXPECTED_SUBJECT'."
+            }
+            Write-Host "$file : Valid, timestamped, signer $($sig.SignerCertificate.Subject.Split(',')[0])"
+          }
+
+          Write-Host "SHA-256: $((Get-FileHash $env:STABLE_MSI -Algorithm SHA256).Hash.ToLowerInvariant())"
 
       - name: Upload to Azure Blob Storage
         if: inputs.upload_blob
@@ -133,9 +228,11 @@ jobs:
           }
           Write-Host "Alias verified: https://download.autopilotmonitor.com/agent/$env:STABLE_MSI ($remoteSize bytes)"
 
-      # Sigstore keyless provenance, same as the agent ZIP. Verify:
+      # Sigstore keyless provenance, same as the agent ZIP; covers the signed bytes. Skipped
+      # on runs that publish nothing -- no log entry for bytes nobody receives. Verify:
       # gh attestation verify AutopilotMonitor-Bootstrap.msi --repo okieselbach/AutopilotMonitor
       - name: Attest build provenance (MSI)
+        if: inputs.upload_blob
         uses: actions/attest-build-provenance@v4
         with:
           subject-path: ${{ env.STABLE_MSI }}