wiki:BuildingTapWindows6

Introduction

Generic build instructions for tap-windows6 are available in it's Git repo. This page contains additional information that is more generic and not really suitable for inclusion in the main documentation.

Requirements

Getting the Authenticode signatures right so that all Windows versions detect them can be quite tricky. This seems to be particularly true for kernel-mode driver packages. The Authenticode signatures have a few requirements:

  1. The Certificate path needs to be complete. This can be achieved by including cross-certificate of your CA (e.g. Digicert) in the signed files. At least for Digicert non-EV and EV code-signing certificates have different CAs.
  2. The signature needs to be timestamped, or the driver will stop functioning when the code-signing certificate expires.

It is not clear if signtool's digest algorithm (/fd SHA|SHA256) affects the acceptability of the signature in Windows 7 and beyond, or if the only important thing is the hash algorithm of the actual certificate.

Cross-signing is possible for Windows 7/8/8.1/Server 2012r2 as long as the certification authority's cross-certificate is valid. Beyond that point an actual Microsoft signature is required in all drivers. Windows 10 already requires these Microsoft signatures - they're called attestation signatures in MS jargon. These signatures can be created in Windows Dev Center once you've cleared all the bureaucratic obstacles like signing in to development programs and registering your EV hardware token with your account.

Note that while it is technically possible to put two signatures in one driver (attestation signed + cross-signed), the resulting driver will not install cleanly.

Here are the general prequisites for building and signing, regardless of signature type:

  • On build computer
  • On signing computer
    • An EV token is visible in the Windows Certificate Store
    • A correct cross-certificate from your CA is installed into the tap-windows6 directory
    • sign\Sign-Tap6.conf.ps1 is configured properly

On top of the generic requirements listed above there are a few extra requirements when doing attestation signing for Windows 10:

  • You need to register your EV dongle with your organization's account in the Windows Dev Center (direct link).
  • INF file syntax needs to be valid, as MS backend servers will check it at submission time. Use the latest version of InfVerif (see footer in Hardware Dev Center) to validate the syntax before submission.

In the documentation below it is assumed that all Windows commands are executed from within a Powershell session.

Setting up the SMB share

If the building and signing computers are separate you are strongly encouraged to share the build directory on the build computer with the signing computer, e.g. using SMB); this removes the need for doing error-prone file copying, archive extraction, etc. In the instructions below it is assumed that this is the case.

You can create new shares on the build computer in various ways. Here's an example on how to do it with Puppet using Powershell DSC resources:

dsc_windowsfeature { 'File server':
  dsc_ensure => 'present',
  dsc_name   => 'FS-FileServer',
}

dsc_xsmbshare { 'tap-windows6-build-directory':
  dsc_ensure                => 'present',
  dsc_name                  => 'tap6build',
  dsc_description           => 'tap-windows6 build directory',
  dsc_path                  => 'C:\\Users\\build\\opt\\tap-windows6',
  dsc_folderenumerationmode => 'AccessBased',
  dsc_fullaccess            => 'tapbuilder\build',
}

This code can be mapped almost 1:1 to Powershell DSC. Alternatively you can use raw Powershell commands Add-Windowsfeature and New-Smbshare to do the same. Or just create the share via Windows GUI somehow. Remember to open port 445 in the firewall.

Once the share is up, mount it on a Powershell session:

$ net use W: \\tapbuilder.example.org\tap6build /user:build 'password-here'

Note that this mount is session-specific, so you need to do it from the session you use to sign the files from.

Building

The building and signing process is automated to a large degree and documented in detail in tap-windows6's README.rst file. The instructions work equally well in two scenarios:

  • A single computer is use to build and sign
  • Build and signing computers are separate, but the build directory is shared with SMB

If tap-windows6 build directory is shared via SMB you will get warnings about running scripts from the Internet. Unfortunately it is not possible to make this problem go away, even with the Unblock-File CmdLet when working with SMB shares, unless you modify the global Powershell execution policy with Set-ExecutionPolicy CmdLet.

It is generally a bad idea to support Windows Vista. But if you must, please look here.

Hints

Modifying the Visual Studio project files

Many tap-windows6 build settings come from the Visual studio project file, src\tap-windows6.vcxproj. That file is generated by buildtap.py from src\tap-windows6.vcxproj.in.

There does not seem to be any easily available reference for vcxproj files. This means that the safest and possibly the only realistic way to modify them is by using Visual Studio itself. The Visual Studio + WDK installation process is fairly well documented here, but for sake of completeness the requirements are:

  • Visual Studio 2019 Community
    • Install workload "Desktop Development with C++"
    • Install component "Windows 10 SDK"
  • Windows Driver Kit (WDK)
    • Version must match that of "Windows 10 SDK"
  • Copy of tap-windows6.vcxproj under src
    • This file can be generated elsewhere with buildtap.py and copied over to the Visual Studio computer

After you've installed the above components launch Visual Studio 2019 and open tap-windows6.vcxproj file. Then you can modify the project properties as you want using the GUI. Your changes will not affect tap-windows6.vcxproj, but will create a new file called tap-windows6.vcxproj.user. Its contents look something like this:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
    <SignMode>TestSign</SignMode>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
    <SignMode>Off</SignMode>
  </PropertyGroup>
</Project>

Once you've figured out what the GUI does you can port those changes to tap-windows6.vcxproj.in and they will get applied to all subsequent builds.

Installing certificates

Installing a PFX file to the Currentuser certificate store using Powershell:

Import-PfxCertificate –FilePath <path-to-pfx> cert:\CurrentUser\My -Password (ConvertTo-SecureString -String <pfx-password> -Force –AsPlainText)

If you're not accustomed to Powershell you can just use mmc.exe and the certificate snap-ins to install the certificate.

Querying the certificate store

To list all certificates in Currentuser\My store using Powershell:

Get-ChildItem cert:\CurrentUser\My

Or alternatively:

Set-Location cert:\CurrentUser\My
dir

The dir command is just an alias for Get-ChildItem

Creating catalog files with inf2cat

To create a catalog file for a 32-bit driver:

Inf2Cat.exe /driver:<full-path-to-driver-directory> /os:Vista_x86,Server2008_X86,7_X86

To create a catalog file for a 64-bit driver:

Inf2Cat.exe /driver:<full-path-to-driver-directory> /os:Vista_X64,Server2008_X64,Server2008R2_X64,7_X64 

Example:

Inf2Cat.exe /driver:C:\Users\John\tap6\amd64 /os:Vista_X64,Server2008_X64,Server2008R2_X64,7_X64 

NOTE: According to Microsoft Inf2Cat requires a full path to the driver directory.

Signing files with signtool.exe

NOTE: signtool seems to expect absoluete paths to certificate files. Below only the filenames are given for clarity.

Sign a file using a (non-EV) certificate stored in a pfx file. Note that this process is not suitable for EV certificates, which are probably all stored in some sort of dongle and thus only visible through the Windows Certificate Store:

signtool.exe sign /v /ac <cross-certificate> /t <timestamp-url> /f <pfx-file> /p <pfx-password> <file>

Sign a driver with the "best" certificate found from the certificate store. This should work if there is only code-signing certificate in the store:

signtool.exe sign /v /ac <cross-certificate> /t <timestamp-url> /a <file>

Sign a driver using a certificate under Currentuser\My, selecting the right certificate based on a substring of the certificate's subjectname:

signtool.exe sign /v /ac <cross-certificate> /t <timestamp-url> /s My /n <subjectname> <file>

Example of adding two signatures and timestamps. This requires a relatively recent signtool.exe (e.g. from Windows Kit 10):

# Create primary (SHA1) signature (certificate in a pfx file)
signtool.exe sign /v /f digicert-sha1.pfx /p <pfx-password> /ac digicert-assured-id.crt /t http://timestamp.digicert.com /fd SHA1 tap6/amd64/tap0901.cat

# Add secondary (SHA2) signature (certificate in the certificate store)
signtool.exe sign /v /s My /n OpenVPN /ac digicert-high-assurance-ev.crt /as /fd SHA256 tap6/amd64/tap0901.cat
signtool.exe timestamp /tr http://timestamp.digicert.com /td SHA256 /tp 1 tap6/amd64/tap0901.cat

Signing a file (e.g. the installer) directly with Signtool using a certificate from local PFX file.

signtool.exe sign /v /ac digicert-assured-id.crt /f digicert-user-mode-2019.pfx /p password /t http://timestamp.digicert.com tap-windows-9.22.1-I601.exe

Validating signatures

Verifying the Authenticode signature of a file using Powershell:

Get-AuthenticodeSignature <path-to-file>

Note that even if the above command says that the file's certificate is valid, there is absolutely no guarantee that various Windows versions will accept it. It is unclear whether the Cmdlet checks the entire certificate path or not: it does hang for long periods of time occasionally doing something.

Using signtool.exe to verify a driver's signature probably gives more reliable results than the Get-AuthenticodeSignature Cmdlet:

signtool.exe verify /v /kp /c <drivername>.cat <drivername>.sys

Signatures can also be validated by looking at "File properties" of the tap0901.cat file. The publisher should show up correctly in some places (not necessarily all), there should be a timestamp counter-certificate, and an unbroken certification path should be present.

External links

General information

Practical guides

References

Last modified 6 months ago Last modified on 04/25/19 08:02:27