If you have ever installed some software or drivers in Windows, you have probably seen a dialog telling you the name of the company or person that published that software. This means that the publisher has cryptographically signed their work. Signing your software is important: by showing a nicer dialog to the end user, it gives end users more confidence that they are not installing malware. In the case of device drivers, signing is even required by certain versions of Windows in certain situations.
If you are a developer figuring out how to sign drivers or software, the aim of this guide is to tell you everything you need to know so that you can do it correctly.
My name is David Grayson and I work at Pololu Robotics & Electronics. In 2012 I went through the process of signing all of our company's USB drivers and most of our installers for Windows. I encountered so many problems along the way that could have been easily avoided if someone had told me about them ahead of time. If you are going through the same process, I sincerely hope that this document can clear up all of your confusion and save you a lot of time. I learned the hard way and now you can learn the easy way.
A lot of this information can be verified in official Microsoft documentation found on MSDN, and I will try to cite the official documentation when needed. The authoritative documents on kernel-mode code signing are kmsigning.doc and KMCS_walkthrough.doc. These are pretty good resources, but they are from 2007 and thus contain no information about Windows 7, Windows 8, or SHA-2. Also, their scope is more limited than the scope of this document because they don't talk about signing executables. Therefore, a lot of the things I say here are actually conclusions that I have drawn from my own experiments. When I am telling you something that I determined experimentally, I will use phrases like "it seems like" or "in my experience". When my experiments contradict the official documentation I will say so.
If you think any of the information I am providing here is wrong, please post a comment and let me know so we can figure it out.
This document only covers Windows XP 32-bit, Windows Vista, Windows 7, and Windows 8, Windows 8.1, and Windows 10.
The most useful part of this document is the signature requirements section.
This document was originally published in January 2013 and described many problems I had with certificates that use the SHA-2 hashing algorithm. Because of all these problems, I used to recommend sticking to SHA-1. Since then, Microsoft has announced the Deprecation of SHA-1 which will happen on January 1, 2016. Therefore, SHA-1 will not be a long term solution, and most people should probably use SHA-2 or start thinking about switching to it. In July 2015, I did a systematic set of experiments with different types of signatures. Using the data from those experiments, I have updated this document to better cover SHA-2 and the recent updates from Microsoft that allow it to be a viable option.
The RSA cryptosystem
Pretty much every secure thing you do with a computer, including code and driver signing, uses the RSA cryptosystem invented by Rivest, Shamir, and Adleman in the 1970s. I am not going to really explain the mathematics behind it, but I will give you an idea of what RSA lets us do. This will help you understand what a digital signature actually is and why it works.
The first thing RSA gives us is a way to generate a key pair, which consists of a public key and a private key. As the names suggest, the private key must be kept secret, but you can give the public key to anyone.
The second thing that RSA gives us is a pair of functions. The public key provides a function that we will call f. The private key provides a function that we will call g. Do not worry about what the exact inputs or outputs of these functions are. The important properties of these functions are:
- f and g are inverses of each other: f(g(x)) = x and g(f(x)) = x.
- It is very very hard to determine g from the public key (or f).
Encrypting and decrypting a message can be done with the functions f and g respectively. Basically, any sender can encrypt a message by passing it through the f function from receiver's public key. Then the receiver is the only one who can read the encrypted message, and he does so by applying g to it.
Signing and verifying a message can be done with the functions g and f respectively. The sender passes his message (or a cryptographic hash of it) through the g function from his private key to make a signature for the message. The sender is the only one who can do this because he is the only one with access to g. Anyone who receives the message and signature can verify the signature by passing it through the f function from the public key and making sure that everything matches up. This is exactly what Windows is doing for you behind the scenes whenever it verifies a signature on a piece of software and tells you who the publisher is.
At a deep level, the RSA cryptosystem works because it is very hard to factor large numbers into primes. The private key mainly consists of two very large primes, and the public key mainly consists of their product. Wikipedia has more details about how RSA works, of course.
Anatomy of a signature
Windows has a series of dialog boxes that allow you to view the details about a signature embedded in a file. It is important that you know your way around these dialogs because they will help you understand the nature of the signature you are applying to your software.
If you right-click on a signed file and go to Properties, you will see a Digital Signatures tab.
In the Digital Signatures tab, you can click on Details to open the Digital Signature Details dialog box. The digital signature is created by the publisher of the software.
You can click on View Certificate to view the certificate that is embedded in the file's signature. The certificate is purchased from a certification authority such as Verisign.
You can click on Certification Path to view most of the certificates in the chain of trust. The point of these certificates is to prove that your certificate was issued by some trustable company. You can double-click on any certificate visible in the certification path to get information about it.
Some of the certificates shown in the certification path come from the file whose signature your are inspecting. Other certificates might come from your computer's certificate store, which you can see by running certmgr.msc. In particular, Windows seems to use certificates from the Intermediate Certification Authorities list and the Trusted Root Certification Authorities list to build the certification path. Unfortunately, I do not know of a good way to look at a signed file and tell exactly what certificates are embedded in it.
The names shown in the Certification Path are the "Friendly Names" of the certificates, which you can configure in certmgr.msc. Sometimes, multiple certificates might have the same friendly name, which makes it confusing to see what is going on. To clear up the confusion, I like to double-click on the certificates and look at the "Subject Key Identifier" and "Authority Key Identifier". I think that the Subject Key Identifier represents an entity you might trust, and every certificate simply represents a transfer of trust from some authority to the subject. If the authority and subject identifiers are the same, that is called a "self-signed" or "root" certificate.
The same subject can be found in multiple different certificates. For example, the "GlobalSign Root CA - R3" (subject key identifier 8f f0 4b 7f a8 2e 45 24 ae 4d 50 fa 63 9a 8b de e2 dd 1b bc) has its own root certificate as well as a cross-certificate issued by the GlobalSign Root CA, which seems to be an older and better supported authority.
Some day I might expand this section to include details about the different fields you can see in these dialogs, and why those pieces of information are necessary. For now, you should click around yourself and explore some signatures. Feel free to download these various experimental files I have signed and examine their signatures:
You can also use a hex editor such as WinHex to examine the embedded signatures; you can easily see the names of the signer and the organizations in the certification path. One great feature of WinHex is that it lets you compare two files and highlights the differences in them, so you can see exactly which bytes in the header are modified and which bytes are appended to the end when the signature is added.
To successfully release your software, you should make sure that your digital signature meets all the necessary requirements documented below. The requirements are summarized in the tables below, and then the terms in the tables are defined and explained after the tables. Each data cell of each table contains a boolean expression that combines different requirements using and (&) and or operations. I strongly suspect that this list is incomplete, so please post a comment if there is anything to add to it.
If you just care about your software working and don't mind if the user sees a scary warning message, these are the requirements your signature needs to meet in my experience:
|Running an executable||Installing a driver package||Loading a kernel module|
|on Windows XP||none||none||none|
|on Windows Vista 32-bit||Unsigned or SHA‑1 or KB2763674||none||none|
|on Windows Vista 64-bit||none||MCVR & /t & SHA-1|
|on Windows 7||none||none||MCVR & (SHA‑1 or KB3033929)
& SHA-1 phase-out
|on Windows 8, 8.1||none||TRCA & SHA-1 phase-out||MCVR & SHA-1 phase-out|
|on Windows 10||none||TRCA & SHA-1 phase-out||MCVR & SHA-1 phase-out & Portal|
If you care about your software working AND you want to show up as the verified publisher in any warning dialog boxes and any Properties dialogs, these are the requirements your signature needs to meet in my experience:
|Running an executable||Installing a driver package||Loading a kernel module|
|on Windows XP||TRCA & /t & SHA-1 sig||WHQL||?|
|on Windows Vista 32-bit||TRCA & /t & SHA-1 sig
& (SHA‑1 or KB2763674)
|TRCA & /t||?|
|on Windows Vista 64-bit||TRCA & /t||?|
|on Windows 7||TRCA & SHA-1 phase-out||TRCA & SHA-1 phase-out||?|
|on Windows 8, 8.1||TRCA & SHA-1 phase-out||TRCA & SHA-1 phase-out||?|
|on Windows 10||TRCA & SHA-1 phase-out||TRCA & SHA-1 phase-out||?|
- SHA-1 phase-out
The article Windows Enforcement of Authenticode Code Signing and Timestamping from Microsoft describes how SHA-1 will stop being supported in various ways in Windows 7. On 2016-01-01, those versions of Windows will stop trusting code that was signed with a SHA-1 code-signing certificate and a timestamp of 2016-01-01 or later. On 2017-01-01, SHA-1 will be restricted further, but the article uses confusing language and it is hard to tell exactly what they are talking about. Also, once SHA-1 preimage attacks are possible, Microsoft will restrict its use even further. To avoid these problems, it might be best to start using SHA-2 for everything, including the file digest, main certificate, timestamp digest, and timestamp certificate.
Microsoft announced that all new Windows 10 kernel mode drivers must be submitted to and digitally signed by the Windows Hardware Developer Center Dashboard Portal. Also, 90 days after the release of Windows 10, the portal will only accept driver submissions from you if you sign them with an Extended Validation (EV) certificate, which is typically more expensive than a normal certificate.
However, there is a really nice loophole in the Microsoft announcement. For backwards compatibility, kernel-mode drivers signed with a valid cross-certificate that pre-dates Windows 10 will continue to pass signing checks in Windows 10. The cross-certificate for GlobalSign was issued long before Windows 10 and it expires on . Therefore, you should be able to sign kernel-mode drivers for Windows 10 with a regular GlobalSign code-signing certificate until then.
In the tables above, TRCA means the signature's chain of trust must go back to a certificate in the user's Trusted Root Certification Authorities (TRCA) list. As you can see in certmgr.msc, the TRCA list has certificates from several well-known companies such as a Verisign, Globalsign, Digicert, and Go Daddy. Many certificates are not present in the list initially, but Window will attempt to automatically install them from the various sources when they are needed to verify a signature.
One way Windows can download root certificates is by connecting to Windows Update using the Internet. This is called the Microsoft Root Cerificate Program. However, I would not rely on the auto-update. Logically, it shouldn't work if the computer is disconnected from the internet. In my experience, even with an internet connection it does not always work reliably.
Windows can also get the certificates it needs from crypt32.dll, and I think that method is much faster and more reliable.
Therefore, your best bet is to make sure your chain of trust goes back to a certificate that is included in fresh installs of Windows, either in the TRCA or in crypt32.dll. Unfortunately, I don't have an authoritative list of those certificates. The TRCA requirement is documented in kmsigning.doc. I suspect that the "Trusted Publishers" or "Trusted People" lists would work just as well, if you convince your users to install a certificate there. You typically don't need a cross-certificate (specified with the
/acoption to signtool) to meet this requirement. However, an intermediate certificate (which gets automatically used during signing if it is installed in your certificate store) could help by extending your chain of trust back to an older and better supported certificate.
In the tables above, MCVR means the signature's chain of trust must go back to the Microsoft Code Verification Root certificate, or some other certificate that is trusted by the kernel. Any signature that you get through the WHQL process should already satisfy this requirement. This is documented very clearly in kmsigning.doc, which explains that the kernel does not have access to the Trusted Root Certification Authorities list. A cross-certificate is typically needed to satisfy this requirement. Microsoft publishes a complete list of the Cross-Certificates for Kernel Mode Code Signing.
The signature must come from the WHQL program. My understanding is that you can submit your driver to Microsoft or some third party to be tested. If your driver is OK, they will sign your driver and give you legal permission to use the Windows Logo to sell your product. WHQL is never actually required for your software or drivers to work and probably harder than just using a standard code signing certificate. I have never gotten a driver WHQL-signed, so my experience with it is limited.
This requirement is true if the file simply has no signature. Keep in mind that the table above uses boolean expressions, so when I write "X or Y or Z" it means that if any of those three are true, then your signature will work.
A signature must be present and it must not use SHA-2 in any way, only SHA-1. This applies to the signature of the file itself and the signatures that secure the chain of trust to your certificate. This probably also applies to the timestamp and its chain of trust. Note that Microsoft is retiring SHA-1, so sticking to SHA-1 will not be a long term solution.
- SHA-1 sig
The signature must be made using SHA-1 as the digest algorithm, but it is OK if parts of the certificate's chain of trust use SHA-2. This might also apply to digest algorithm used by the timestamp.
In the tables above, /t means that the signature should be timestamped using the
/toption of signtool instead of
In the tables above, KB2763674 means that KB2763674 must be installed, which in turn requires Windows Vista SP2.
In the tables above, KB3033929 means that KB3033929 must be installed.
Don't use spaces in the INF file name. This is an additional requirement for driver package installation that was reported by Jimmy Kaz. I have not tested it myself, but he says that the driver package will appear to be unsigned in Windows 7 if the INF file has spaces in the name.
The content above is a concise summary of all the code and driver-signing requirements I know about. In the next three sections, I will explain each of the requirements and what you can expect if your software does not meet them.
Running an executable
Digital signatures can be directly embedded inside executables (EXE files). They can also be embedded inside Windows Installer files (MSI files), and everything that I will say about executables also applies to Windows Installer files. Windows verifies the signature inside an executable file in two situations:
- If the file was downloaded from the internet (including network drives), Windows will show a "Open File - Security Warning" message when the user tries to run the file. The publisher information in the warning comes from the signature embedded in the file. Usually the warning is a simple dialog box, but in Windows 8 and later the warning is sometimes a SmartScreen dialog that takes over the whole screen. Two examples are shown below:
- If the executable requests administrator privileges, which is also known as elevating, Windows will display a UAC prompt. The publisher information in the prompt comes from the signature embedded in the file. Two examples are shown below:
Windows has never required signatures on executables. However, it is nice to sign your executable so that whenever the user sees a warning message about it, they will see your name as the publisher instead of being told that the publisher is unknown.
In my experience, in order for your signature to work properly on an executable, it should have a chain of trust that goes back to a certificate in the user's Trusted Root Certification Authorities list, which you can see by running certmgr.msc.
Use SHA-1 to sign an executable if Windows Vista matters
Even if your certificate is signed with SHA-2 and has SHA-2 certificates in its chain of trust, you have a choice about what digest to use when making a signature: SHA-1 or SHA-2.
To choose the algorithm, you should pass either
/fd sha1 or
/fd sha256 to signtool.
I recommend using SHA-1 as the digest algorithm when signing an executable. In my experience, SHA-2 signatures will be deemed invalid on Windows Vista, and Windows Vista will say "This digital signature is not valid." when you view the signature details. I suspect that Windows XP behaves the same way, but I have not tested it. I have not found any downside to using SHA-1 as the digest algorithm on all types of signatures, so I recommend doing that.
To make sure that your signature uses SHA-1 instead of SHA-2, be sure to check the "Digest algorithm" of the signature in its properties page.
Use /t to timestamp an executable if Windows Vista matters
When signing with signtool, you have a choice about whether to specify the timestamp server using the
/t option or the
If you specify it with
/t, signtool gets a timestamp from the server using a custom Microsoft protocol.
If you specify it with
/tr, signtool gets a timestamp from the server using RFC3161.
But these aren't just different protocols, they also seem to affect something about the timestamp itself.
I have found through experimentation that timestamps made with
/tr are not recognized on Windows Vista, for either executables or drivers.
If you open the properties for your signature in Windows Vista, you will see that there is no timestamp listed.
I suspect that Windows XP behaves the same way, but I have not tested it, but someone else has.
When I first wrote this document in 2013, I was convinced that you should use
I had done an experiment where I made both types of timestamps, and I found that Internet Explorer 10.0.9200.16686 on Windows 7 64-bit SP1 and Internet Explorer 10.0.9200.16688 on Windows 8 64-bit flagged the
/t executable as being corrupt or invalid.
I was able to reproduce these results in 2015 if I used the exact same file and browser, but I was not able to reproduce them using IE11 or with a newly-signed file on IE10.
The results I got earlier might be explained by a subtle bug in the Starfield timestamp server's implementation of
/t, which for some reason was only detected by IE 10.
Be aware of KB2763674 for Windows Vista SP2
If your certificate uses SHA-2 or has SHA-2 certificates in its chain of trust, then you should be aware of KB2763674, an update for Windows Vista SP2 distributed through Windows Update. On versions of Windows Vista without this update, when the end user double-clicks on a downloaded executable with a signature whose chain of trust uses SHA-2, nothing happens! There is no error message or activity of any kind.
I believe that there is some code in Windows Vista that checks the signature of the executable in order to show the publisher in the warning dialog for downloaded executables. Before the update, that code apparently could not handle SHA-2, and would silently exit.
There are several ways to get around this problem. Since the number of people using Windows Vista is pretty small these days, you can simply put a note in your documentation that tells Windows Vista users to make sure they have that update installed.
One workaround that the user can do is to run the executable from the Command Prompt, thereby bypassing the warning dialog and the signature checking that goes along with it.
Another workaround for the user to do would be to remove the special flag in the file system that marks the file as coming from the internet. I have not tested that but I expect it to work. Alternatively, you could distribute the executable unsigned. You could also try to get a SHA-1 certificate and use that to sign the executable, but SHA-1 certificates are going away.
It seems like this problem doesn't affect installers created with NSIS, and I think I know why. NSIS installers are always pre-emptively elevated by Windows, so when you run an NSIS installer you will always see the standard UAC warning and never see the special warning that results from running a downloaded executable. In that way, the buggy code in Windows Vista is bypassed.
Installing a driver package
A driver package consists of a single INF file and the files that it references. You can have multiple INF files in the same directory, but in my experience Windows treats each INF file as a separate and independent driver package. A driver package can be signed by first generating a security catalog (CAT) file with cryptographic hashes of all the files, and then embedding a signature in the security catalog. The security catalog contains a list of file names and a hash of the contents of each file; you can simply double-click on it to inspect the information it contains and see its signature.
There are at least five ways to install a driver package. First, the user can right-click on the INF file and select "Install" if the INF file has a DefaultInstall section. (Actually, this method seems to work in Windows 8 and above even if the INF files does not have a DefaultInstall section.) Second, a program can call the SetupCopyOEMInf function. Third, a program or user can run PnPUtil. Fourth, a user can right-click on a matching device in the Device Manager, select "Update driver software...", and then tell Windows the directory where the driver package is stored. Fifth, the driver package can be shipped with DPInst executables that install it.
When the driver package installation is initiated, Windows will check for a signature and behave differently depending on what it finds; different versions of Windows behave differently.
Driver package installation in Windows XP
Back in Windows XP, it seems that the only kind of driver package signature Microsoft cared about was a WHQL signature. You can have a driver package that displays a nice install prompt in Windows Vista and up that indicates who the publisher is, but installing the same driver package in Windows XP results in the following warning:
Driver package installation in Windows Vista and 7
Starting in Windows Vista, Microsoft changed their tune. Instead of warning users about whether or not the drivers have passed WHQL testing, Windows Vista and 7 warn the user about whether the publisher is verified or unverified. To show up as the verified publisher, you need to provide a CAT file with a proper signature. The requirements are documented in kmsigning.doc and in the signature requirements section above.
If you sign your driver package properly, users will see a friendly prompt when they install it in Windows Vista, 7, or 8:
The name in the prompt comes from from the INF file's DriverPackageDisplayName directive and the publisher comes from the verified signature on the CAT file.
However, if you don't sign your driver package, users will see a big red warning when they install it in Windows Vista or 7:
I think it was a good idea for Microsoft to make that change. WHQL testing is inflexible. If you change one byte of your driver, you would have to re-submit it to be tested again. Regular code signing is easier and cheaper: you can get a certificate for a couple hundred dollars per year that lets you sign as many driver packages as you want. This probably resulted in more companies making signed drivers, so the malware stood out more.
Driver package installation in Windows 8 and above
Starting in Windows 8, all driver packages have to be signed. Unfortunately, I have not seen any official document from Microsoft about this change, even though I asked about it on StackOverflow.
If you try to install an unsigned driver package which previously worked on older versions of Windows, you will get a simple error message:
This wasn't necessarily a bad decision on Microsoft's part, but it was bad news for a lot of small companies such as ours and individuals making USB devices on a small scale. For years, we had successfully distributed unsigned driver packages that worked fine in Windows XP, Vista, and 7 because they relied on kernel modules (SYS files) that are signed by Microsoft, in particular WinUSB and usbser.sys. Starting with Windows 8, we had to figure out the driver signing process or tell our Windows 8 customers to follow the complicated procedure for disabling driver signature enforcement.
If you are new to the industry and want to start making USB devices, the vendor ID from the USB-IF will cost you $5000 and the code signing certificate will probably cost a few hundred dollars per year. However, your signatures should keep working after the certificate expires if you make sure to use a timestamp when signing.
The friendly driver installation prompt for signed driver packages in Windows 8 looks pretty much the same as it did in Windows Vista and 7.
Loading a kernel module
Some driver packages contain kernel-mode code (SYS files) that need to get loaded into the kernel at some point, typically when a matching device is plugged into the computer.
Starting with Windows Vista 64-bit, kernel modules must come with a properly-signed security catalog (CAT file) or else they cannot be loaded into the kernel. In July 2007, six months after the release of Windows Vista, Microsoft published two documents about the new signing requirements: kmsigning.doc and KMCS_walkthrough.doc.
If your driver only uses WinUSB or usbser.sys, all you need to worry about is getting your driver package installed, as described in the Installing a driver package section. The kernel modules you are using have already been signed by Microsoft and you will have no trouble getting them loaded into the kernel after the driver package is installed.
The signatures for kernel-mode code are typically kept in the security catalog (CAT file) for the driver package, but in the case of a boot-start driver you are supposed to embed the signature inside the SYS file itself, according to kmsigning.doc.
The signature's chain of trust must go back to the Microsoft Code Verification Root certificate, or some other certificate that is trusted by the kernel. Any signature that you get through the WHQL process should already satisfy this requirement. This is documented very clearly in kmsigning.doc, which explains that the kernel does not have access to the Trusted Root Certification Authorities list. A cross-certificate is typically needed to satisfy this requirement. Microsoft publishes a complete list of the Cross-Certificates for Kernel Mode Code Signing.
It is important to note that a given signature might be a good enough to get a driver package installed, but not good enough to get the kernel modules (SYS files) loaded into the kernel. The part of Windows that checks signatures for driver package installation is different from the part that checks signatures for loading kernel modules, and they each impose different requirements on the signature.
One important detail is that the signature can come from any security catalog installed on the system; the signature does not actually have to be in the security catalog for the INF file that is matching the device. In other words, if Corporation X makes a kernel module and properly signs it for Windows Vista/7, Corporation Y can make a driver package for their own product that uses the kernel module and Corporation Y does not have to provide any kind of signature for it to work in Windows Vista/7. This is not particularly surprising if you think about it: the dangers of loading code into the kernel depend only the code itself, not the device or INF file it is being used with. In fact, this detail is what allows our CP2102 USB-to-Serial Bridge Driver to work on Windows XP/Vista/7/8. The signature that I put on our catalog file (plluvcp.cat) does not meet all the requirements to get silabser.sys loaded into the kernel, but the signature that Silicon Laboratories put on their catalog file (slabvcp.cat) does, and that's all that matters.
When it is time load a SYS file into the kernel, it seems that Windows will scan all the files in the security catalog store (C:\Windows\System32\catroot) to see if one of them contains a hash for the SYS file and an adequate signature. If it finds what it is looking for, the loading succeeds.
If Windows cannot find a properly-signed security catalog for your kernel module, the Properties dialog for your device will show a Code 52 error:
I have also sometimes seen a Code 39 error happen in this situation:
Sometimes Windows can detect that the kernel module is not signed as soon as you try to install the driver package instead of later when you plug in a matching device. Here are some error messages you might see if that happens:
The digital signature for a kernel module also affects what users see in the Device Manager. Just double-click on a device using the module, select the Details tab, and click Driver details. Ideally you would want your company's name to show up in this dialog box, but I have not done enough research to really know what the requirements are. That is why I put question marks in the "Loading a kernel module" column in the table above entitled "Signature requirements for it to look good".
Windows 10 kernel modules must be signed by the portal
Microsoft announced that all new Windows 10 kernel mode drivers must be submitted to and digitally signed by the Windows Hardware Developer Center Dashboard Portal, which is a web service provided by Microsoft. Also, 90 days after the release of Windows 10, the portal will only accept driver submissions from you if you sign them with an Extended Validation (EV) certificate, which is typically more expensive than a normal certificate.
However, there is a really nice loophole. For backwards compatibility, kernel-mode drivers signed with a valid cross-certificate that pre-dates Windows 10 will continue to pass signing checks in Windows 10. The cross-certificate for GlobalSign was issued long before Windows 10 and it expires on . Therefore, you should be able to sign kernel-mode drivers for Windows 10 with a regular GlobalSign code-signing certificate until then. I successfully tested this on a Windows 10 machine on 2015-11-10.
SHA-2 certificates require KB3033929 on Windows 7
If your certificate uses SHA-2 or has SHA-2 certificates in its chain of trust and you are using it to sign kernel modules, then you should be aware of KB3033929, an update for Windows 7 distributed through Windows Update. On versions of Windows 7 without this update, the kernel will reject signatures made with certificates that use SHA-2, so they cannot be used to get a kernel module to load.
SHA-2 certificates do not work for Vista kernel modules
If your certificate uses SHA-2 or has SHA-2 certificates in its chain of trust, then you will not be able to use it to get kernel modules loaded into the Windows Vista 64-bit kernel. I tried to make this work on multiple occasions but I was never able to. Unlike Windows 7, there is no update to fix this. If you really need to make new kernel-mode drivers for Windows Vista 64-bit, you might try getting a SHA-1 code signing certificate or instructing your users on how to disable driver signature enforcement.
Use /t for timestamps if Windows Vista matters
I have not tested it, but I suspect Windows Vista 64-bit will not accept timestamps made with the
/tr option when it is loading kernel modules, because that is how it behaves when checking a signature on an executable or driver package.
Therefore, you should use
Choosing a certificate
You will need to choose a certificate issuer and purchase a code signing certificate from them.
Code Signing Certificate from GlobalSign
I recommend choosing the code signing certificate offered by Globalsign. We purchased a SHA-2 certificate from them in July 2015, and it seems to work fine for almost everything we need. It only costs about $219 per year. The staff at GlobalSign handled our order very quickly: I was able to download and use the certificate withing 24 hours of placing the order.
At the time of this writing (July 2015), GlobalSign offers SHA-1 and SHA-2 options. If you choose SHA-2, you should be aware of some limitations that apply to all SHA-2 certificates: There will be no way for you to successfully sign kernel modules (SYS files) for Windows Vista 64-bit. Also, you might need to advise all of your customers who use Windows Vista to get KB2763674 (assuming you are signing executables or installers) and advise all of your customers who use Windows 7 to get KB3033929 (assuming you are signing kernel modules).
Be sure to install GlobalSign's R1-R3 cross-certificate on the computer that will be making signatures. You can find that certificate on this page of their website, and there is also a copy of it here. This certificate gets installed to the "Intermediate Certification Authorities" list, which is shown in certmgr.msc. My experiments have shown that by installing the certificate there, signtool will find it and include it in signatures that you make. The result is that any computer checking the signature will look for the GlobalSign root R1 certificate instead of looking for the GlobalSign root R3 certificate. The R1 certificate is much older, so it is likely to be available on more computers. For example, I found that on an internet-disconnected Windows 7 machine, the R1 certificate is available while the R3 certificate is not. On an internet-disconnected Windows Vista computer, unfortunately neither of the certificates were available. Regardless of what type of signature you are making, the R1-R3 cross-certificate will help your signature be recognized on more computers.
To learn more about how to do these experiments yourself and verify these results, see the testing section.
Code Signing Certificate from Go Daddy
We used a SHA-1 code signing certificate from GoDaddy from 2012 to 2015, and then switched to GlobalSign. The GoDaddy certificate worked for signing executables and driver packages, but did not work for kernel-mode drivers (SYS files) because there was no crosscertificate available to extend the chain of trust back to the Microsoft Code Verification Root. If you get a SHA-2 code signing certificate from GoDaddy, it might be just as good as the GlobalSign certificate I mentioned above, but I have not tested it.
Finding a good certificate product
To find a good all-purpose certificate, here is what I would do:
- Download all the cross-certificates available on Microsoft's Cross-Certificates for Kernel Mode Code Signing web page. The companies listed on that page are the only ones that can provide a certificate with a chain of trust that goes back to the Microsoft Code Verification Root, so they are the only ones that can help you sign kernel modules.
- You can narrow down your list by looking at a few different Windows computers and seeing which certificates are already installed in the Trusted Root Certification Authorities list, which is visible from certmgr.msc. You might also want to look at the certificates embedded as resources inside C:\Windows\System32\crypt32.dll, because those certificates can be automatically installed on demand. Unfortunately, I have not found or developed a good way to look at the certificates in crypt32.dll. For example, on my Windows 8 computer I see "GlobalSign Root CA" in my Trusted Root Certification Authorities, and that matches the "Subject" of one of the cross-certificates from the Microsoft. The "Subject Key Identifier" values of the two certificates also match. Your goal is to buy a certificate whose chain of trust is rooted in a certificate that will already be a Trusted Root Certification Authority (or be inside crypt32.dll) on all of your users' computers so you don't have to rely on Windows Update.
- Each of the certificates you have left represents a company that could possibly sell you a good certificate. Look at their websites and compare the prices of the different certificate products they offer. Try to make sure you are buying a product that specifically says it can be used for driver signing, because some companies have more than one root certificate and it's hard to be sure which one will be in your certificate's chain of trust. If possible, try to get a sample file that was signed with the certificate they are selling you, so you can inspect its signature and make sure it meets all the signature requirements you care about. Make sure they have a decent return policy.
How to sign
This section will explain what to do after you have purchased the code signing certificate in order to actually use it. This information can mostly be found from official sources, but some of those sources (e.g. kmsigning.doc) are out of date.
First of all, you should follow the installation instructions from your certificate provider. These will include some sort of procedure to generate a private key and certificate and install them on your computer. This process will probably involve installing one or more intermediate certificates on your computer so that you have a complete chain of trust from your certificate to a root certificate of your provider. After you have followed those instructions, you should open up certmgr.msc and look at your certificate to make sure everything looks good. The certificate should be in the "Personal" category, and the properties dialog for it should say "You have a private key that corresponds to this certificate."
You might need to download an appropriate cross-certificate in order to extend your chain of trust and meet all the desired signature requirements.
All of the standard cross-certificates that go back to the Microsoft Code Verification Root are available for download from Microsoft.
Your certificate provider might have some other useful cross-certificates available for download on their website.
To use a cross-certificate, you will have to include an argument of the form
/ac "path-to-your-cross-cert.ct" when you invoke signtool.
It might be beneficial to download an intermediate certificate and install it on the computer making signatures. For example, as I explained above, the GlobalSign R1-R3 intermediate certificate extends the chain of trust from their new R3 certificate (which uses SHA-2) back to their older, better supported R1 certificate. My experiments have shown that this kind of intermediate certificate will get automatically included in signatures that you make if it is installed on the signing computer. If the authority of the intermediate certificate corresponds to a root certificate that is older and better supported than your normal root certificate, then using the intermediate certificate could allow your signature to be recognized by more computers. However, if that authority's certificate is poorly supported, then using the intermediate certificate could probably hurt you, so watch out for that.
The digest algorithm (or file digest) is the hash function used on your file before it is signed. You have to choose whether to use SHA-1 or SHA-2. (If you do not want to choose, it is possible to apply two signatures to most types of files, but this does not work for MSI files.)
In general, I recommend that you use SHA-1 as the digest algorithm, even though your certificate will probably be based on SHA-2. This is the setup that is recommended in an article from Microsoft, because it will allow you to make a single executable or driver package that works fine on Windows Vista and up.
However, there is also an argument to use SHA-2 as the digest algorithm. Although the language in that Microsoft article is confusing, it talks about how they might disallow SHA-1 digests once a preimage attack on SHA-1 becomes possible. If you don't mind having a degraded user experience for Windows Vista users (who are a small minority these days), and you would like your software signatures to keep working for many years, it seems to me that SHA-2 is a better choice for the digest algorithm.
To use SHA-1 as the digest algorithm, include the arguments
/fd sha1 when you invoke signtool.
To use SHA-2 as the digest algorithm, include the arguments
/fd sha256 when you invoke signtool.
Note that there is no way to specify the digest algorithm when running inf2cat; it seems like CAT files always use SHA-1.
Timestamp server, protocol, and digest algorithm
Make sure to timestamp your signatures so they will continue to work after your certificate expires. Your certificate provider should provide the URL of a timestamp server in their documentation, but you can probably use the timestamp server from any provider for free. Here is a list of timestamp servers I have heard about:
If you are using a GlobalSign certificate, I recommend using the GlobalSign timestamping server. That way, both your main signature and your timestamp signature can chain back to the same root certificate. Every root certificate that your signature relies on is a liability because it might be missing or unavailable on the user's system. If possible, it is better to rely on just one root certificate instead of two.
You should decide which algorithm to use for the timestamp: SHA-1 or SHA-2. I recommend using SHA-2 because this article from Microsoft makes it sound like Windows 10 and up will start ignoring timestamps that use SHA-1 on . However, SHA-2 timestamps do not work in Windows Vista, so if you want to keep supporting Windows Vista as much as possible, you should probably use SHA-1.
If you choose SHA-1 for the timestamp digest, you have a choice to either use the Authenticode protocol or RFC3161. I recommend using Authenticode, because RFC3161 timestamps are not recognized by Windows Vista.
If you choose to use SHA-2 for the timestamp digest, you must use RFC3161 also.
To timestamp your signature using the RFC3161 protocol and SHA-2 (recommended), include the arguments
/tr http://timestampserver.com /td sha256 when you invoke signtool.
To timestamp your signature using the RFC3161 protocol and SHA-1, include the arguments
/tr http://timestampserver.com /td sha1 when you invoke signtool.
To timestamp your signature using the Authenticode protocol and SHA-1, include the arguments
/t http://timestampserver.com when you invoke signtool.
Some time-stamping servers will disobey your
/td argument, so be sure to inspect your signature to make sure it uses the right digest algorithm for the timestamp.
/td option doesn't just control the digest algorithm used for the timestamp, but usually timestamp servers also use it to select a proper certificate whose chain of trust uses that same algorithm.
Signtool and inf2cat
To sign anything, you will need the Signtool.exe (Sign Tool) utility from Microsoft. To obtain signtool.exe, I installed the latest version of the Windows SDK.
To sign driver packages, you first need to use another tool called Inf2Cat (Inf2Cat.exe) to create the security catalog (CAT) file, which you can then sign with signtool. To obtain inf2cat.exe, I installed the latest version of the Windows Driver Kit (WDK).
You should probably get the latest versions of both signtool and inf2cat to ensure that your drivers will support the latest versions of Windows.
Links to the Microsoft website for downloading the Windows SDK and Windows WDK tend to break, so I do not have any here. I recommend using a search engine to search for "Windows SDK download" and "Windows WDK download" in order to find the latest versions. I think the SDK should be installed first. The WDK will install itself into the same folder as the SDK, which will be something like "C:\Program Files (x86)\Windows Kits\10" by default.
You can probably figure out how to use inf2cat and signtool from the documentation, but here are some examples of how to use them.
This is an example batch (.bat) script. You can simply drag an executable or MSI file onto it, and it will sign the file for you:
"C:\Program Files (x86)\Windows Kits\10\bin\x86\signtool" sign /v /ac "your-cross-cert.crt" /n "Your company name" /fd sha1 /tr http://timestamp.globalsign.com/?signature=sha2 /td sha256 %1 pause
This is another example batch script. You can put it in the same directory as your driver package and then double-click on it to create the security catalog and sign it.
"C:\Program Files (x86)\Windows Kits\10\bin\x86\inf2cat" /v /driver:%~dp0 /os:XP_X86,Vista_X86,Vista_X64,7_X86,7_X64,8_X86,8_X64,6_3_X86,6_3_X64,10_X86,10_X64 "C:\Program Files (x86)\Windows Kits\10\bin\x86\signtool" sign /v /ac "your-cross-cert.crt" /n "Your company name" /tr http://timestamp.globalsign.com/?signature=sha2 /td sha256 *.cat pause
For both of these batch files, if you are using a cross-certificate, I recommend just putting the cross-certificate in the same directory as the batch file to make the
/ac parameter simpler.
You should use the verify option of signtool.exe to check your signatures while you are still learning the process. The documentation of the options for signtool verify is pretty confusing, so I will tell you what you need to know:
- To test a signature for the purpose of running an executable or installing a driver package, the correct option is
/pa. I infer this from KMCS_Walkthrough.doc.
- To test a signature for the purpose of loading kernel-mode code, the correct option is
Here is an example batch script that verifies the signature of a file you drop onto it, using
"C:\Program Files (x86)\Windows Kits\10\bin\x86\signtool" verify /v /pa %1 pause
Tip: Run signtool verify without the
/v option to see whether a signature's timestamp is an Authenticode timestamp (signed with
/t) or an RFC3161 timestamp (signed with
The distinction between these two types of timestamps is sometimes important and this is the only way I know to verify that the correct type was used.
Unfortunately, signtool verify has limited usefulness. It will make sure that your chain of trust extends back to the right place, but it will not tell you about most of the other signature requirements that I have documented above. The chain of trust reported by signtool verify is probably affected by the set of trusted root certificates and intermediate certificates that are installed on your computer. To actually be confident in your signatures, you need to properly test them, so keep reading.
It's pretty obvious that it would be ideal to test your signed drivers/executables on every different version of Windows you are targeting.
What isn't obvious is that when you are testing executables or MSI files, you should run them right after downloading them from the internet. As I explained in the Installing a driver package section, there is a bug in unpatched versions of Windows Vista that only manifests itself if the file was downloaded from the internet, and there could easily be more bugs like that. Just throw your executables into a zip file at a secret URL and download them onto the test computer. Generally, you will know that you are testing executables correctly if Windows displays an extra warning when you try to run the executable.
You should test your downloadable file (e.g. your ZIP file or installer) by downloading it in Internet Explorer to make sure there are no problems when Internet Explorer checks your signature. In my experience, Internet Explorer checks the signatures on EXE downloads (and probably MSI too), but in future versions it might reach inside ZIP files and check the signatures on the executable files inside.
You should conduct these tests on a machine that does not have any intermediate certificates from your certificate provider or timestamp provider installed. You can find them and delete them using the "Intermediate Certification Authorities" list in certmgr.msc. Windows will use those intermediate certificates to help build a chain of trust back to a trusted root certificate, so having them installed on your testing computer could effect the results of your tests. Deleting those certificates forces Windows find them in the signed file itself.
You should also try deleting the root certificates that your main signature and your timestamp rely on. You can find them and delete them using the "Trusted Root Certification Authorities" list in certmgr.msc. Ideally, you would be able to delete those certificates, disable the computer's internet connection, and then verify that your signature still works. Windows will attempt to automatically install the root certificates it needs to verify your signature. In my experience, the automatic installation happens whenever Windows shows a dialog box that contains the publisher information of your signed file, and it also happens whenever you open the "Digital Signature Details" window in the properties of the file. If the signature gets successfully verified by Windows, you should see the certificates that you deleted reappear in certmgr.msc after you refresh it.
If your certificate provider is like Go Daddy, they do not actually store a backup of that private key for you. The private key only exists on your machine, and if you lose it you will have to "re-key" your certificate. Re-keying probably isn't a big deal; with Go Daddy you can just do it for free through their web interface.
However, I recommend backing up your certificate and private key. To do this, run certmgr.msc, select your certificate, and then select Action > All Tasks > Export.... This brings up the Certificate Export Wizard. Be sure to select "Yes, export the private key", select PFX format, select "Include all certificates in the certification path if possible", do NOT select "Delete the private key if the export is successful", and select "Export all extended properties". You will then be asked to choose a password and choose the name of the output file. When you are finished, you will have a password-protected Personal Information Exchange (PFX) file that contains your certificate, your private key, and relevant certificates from the chain of trust. You can install the contents of the file on other computers simply by double-clicking on it and entering the password.
It's annoying when you ask for help and the good people trying to help you end up telling you things that are untrue or half-truths. Here are some of the myths I have encountered:
Myth: Kernel-mode drivers require WHQL testing
Let's say that the booby girls at GoDaddy don't give the user much confidence in the a certificate issued by them, promising that the driver always works. Drivers for the 64-bit version of Windows have to be qualified by Microsoft, not them girls. Google "whql labs certifications". Hans Passant, who has 300,000+ reputation on StackOverflow, in response to my question
A customized installation [generated by our software] does not contain certified drivers for Windows XP/2003/Vista/7. Certification must be performed by Microsoft for the new driver installation. An uncertified installation will not cause any other problems other than the warning message displayed by Windows XP/2003/Vista when installing uncertified drivers. Uncertified drivers cannot be installed in Windows 7 unless they are installed with a testing certificate or the Ignore Serial Signing option is enabled by pressing F8 on start up and selecting the corresponding option. Silicon Labs, makers of the CP2102 serial bridge, in AN220
This is a myth. Microsoft isn't trying to assert total control over what gets loaded into the kernel. They just want to make it easier to figure out what went wrong when the user experiences a blue screen of death or some other problem.
So starting with Windows Vista 64-bit, Windows requires signatures for loading kernel-mode code. Starting with Windows 8, they also require driver packages to be signed before they can be installed. The code could still contain infinite loops and viruses, but at least it can be tracked to its source when problems arise! See the signing requirements section for a complete explanation.
Myth: DefaultInstall doesn't work with signed drivers
The INF file of a driver package must not contain an INF DefaultInstall section if the driver package is to be digitally signed. Microsoft, in the INF Default Install Section documentation
The documentation is incorrect. I have distributed signed drivers with DefaultInstall sections to our customers since November 2012 and the DefaultInstall section has caused no problems. I don't see any reason why there should be a problem. While I was figuring out the signing process in 2012, I used the DefaultInstall section almost exclusively as my method for testing driver package installation.
The DefaultInstall section allows a user to install your INF file simply by right-clicking on it and selecting "Install". Really you should use DPInst, SetupCopyOEMInf, or PnPUtil to install drivers, but the DefaultInstall section is easy to add and it could be useful to some customers, so you should have it.
For example, if your driver is named
foo_driver.inf, you should add the following lines:
You can even reference multiple INF files in the CopyINF directive if you want.
Myth: The INF version number indicates OS support
Create an INF file in your driver package directory and edit it for Windows Vista. Specifically, change the build date to 4/1/2006 or greater and the version to 6. For example:
DriverVer=04/01/2006, 22.214.171.124Microsoft, in kmsigning.doc
Generally, kmsigning.doc is pretty good, but that line is wrong.
I don't know if their claim about the date is correct, because I have never tried dating one of my drivers before 2006, but they are definitely wrong about the version number. I have successfully distributed drivers to thousands of customers who run Windows XP/Vista/7, and our driver versions are typically in the 126.96.36.199 to 188.8.131.52 range.
The INF DriverVer Directive is documented here on MSDN. If the DriverVer version number were important in some way, that should be documented on that page, not buried on page 11 of kmsigning.doc. In fact, the DriverVer version is optional according to that page.
I think the best practice for the version number is to start it at 1.0.0, and whenever you edit the file for any reason you should increase the version number and update the driver date. I recommend using semantic versioning.
Half-truth: Windows 7 doesn't support SHA-2
In some cases, you might want to sign a driver package with two different signatures. For example, suppose you want your driver to run on Windows 7 and Windows 8. Windows 8 supports signatures created with the SHA256 hashing algorithm, but Windows 7 does not. For Windows 7, you need a signature created with the SHA1 hashing algorithm. Microsoft, from Signing a Driver for Public Release on MSDN
This is a half truth. In my experience, SHA-2 signatures on driver packages (i.e. CAT files) work just fine in Windows Vista and Windows 7 for the purpose of driver package installation. However, they don't work for the purpose of loading kernel modules (SYS files) into the kernel. If you use SHA-2 to sign a driver-package that has kernel-mode code, you will get a Code 52 error when you plug in your device and actually try to use the driver. The part of Windows Vista/7 that checks to see if a file can be loaded into the kernel apparently does not recognize SHA-2 signatures. If your driver package doesn't contain any new kernel modules (e.g. you use WinUSB or usbser.sys), a SHA-2 signature will work fine. For more details about this, see the signature requirements section above.
Sometimes telling your customer a half-truth can be worse than just telling a myth. The first time I read the paragraph from the MSDN documentation quoted above, I just assumed it was totally wrong because in my experience the SHA-2 signature was working fine for my WinUSB and usbser.sys-based driver packages. There is a kernel of truth to that paragraph, but unfortunately I could not receive that truth because it was veiled in inaccuracy. Keep that in mind the next time you try to write documentation or explain something to someone: if you say something that goes against their experience of the world, they will discount what you are saying.
On the other hand, someone once told me:
Signing is perhaps the least suitable area to show off creativity and independent thinking. Just the opposite. Try to follow the instructions precisely. ... No matter what they scribble at Stack Overflow – the WDK documentations says the ultimate truth (when updated, of course). Pavel A., in response to my question on MSDN
Well, Pavel was right in this case. If I had turned off all of my creativity and independent thinking, I would have accepted that paragraph as the truth (even though it contradicts all available evidence) and it would have saved me some pain later.
I can't just turn off my brain, but I think it is important that we compromise with Pavel. We should take the documentation seriously, and when it says something that contradicts our experience, we should consider the possibility that the documentation could be correct in some other domain that we haven't tested yet. Therefore, two of the myths I listed above might actually be half-truths.
- SHA1 Certificate Signature Check (SHA1SigCertCheck.ps1). Microsoft. . Script for checking if your signature uses SHA-1 and whether the SHA-1 deprecation applies.
- How to Sign Windows Drivers & Executables. Adafruit. . Explicit tutorial with plenty of screenshots.
- Renew your Windows Code Signing Certificates by December 31, 2015. DCSoft blog. .
- Questions and Answers: Windows 10 Driver Signing. OSR poses questions to James Murray of Microsoft. .
- Windows Enforcement of Authenticode Code Signing and Timestamping. Microsoft. Changes to Windows in 2016 and beyond.
- Windows Code Signing Hash Algorithm Support. GlobalSign. . Nice chart about SHA-1 and SHA-256.
- Authenticode in 2015. Eric Law, ex-Microsoft employee. .
- Driver signing changes in Windows 10. Windows Hardware Certification Blog. .
- KB2763674. Microsoft. SHA-2 bug fix for Vista.
- KB3033929. Microsoft. SHA-2 for Windows 7.
- KB2921916. Microsoft. Specific SHA-2 bug fix for Windows 7.
- Digital Signatures for Kernel Modules on Windows (kmsigning.doc). Microsoft. .
- Kernel-Mode Code Signing Walkthrough (KMCS_walkthrough.doc). Microsoft. .
- Windows root certificate program members. Microsoft.
- Cross-Certificates for Kernel Mode Code Signing. Microsoft.
- Signtool.exe (Sign Tool). Microsoft.
- Inf2Cat. Microsoft.
- Automatic root certificate update problems when verifying my signed INF driver package. David Grayson. .
- How does Windows automatically install certificates in the Trusted Root Certification Authorities list? David Grayson. .
- Certificate Chaining Engine (CCE). Microsoft. Last updated .
- Signing Windows 8 Drivers. PiXCL Automation Technologies. 2013-03-09.
- Microsoft Security Advisory (2880823). Microsoft. .
- Korean translation of this article. Heejune Kim. .
- Windows Authenticode Portable Executable Signature Format. Microsoft. .
- : Removed links for downloading the WDK and SDK. Added references to new resources from Microsoft and Adafruit. Apparently WHQL testing is free now, so adjusted some mentions of "money" and "expensive".
- : Added the really nice portal loophole. Added discussion in "How to sign" about how to pick digest algorithms.
- : Added "SHA-1 phase-out" to to the signature requirements section.
- : Added inf2cat OS options 6_3_X86 and 6_3_X64.
- : Added information about the new driver signing portal for Windows 10.
- : Major changes. Updated the document for SHA-2 and Windows 10. Also changed it recommend /t over /tr. Also added information about how intermediate certificates work and how they can be useful.
- : Added information about KB3033929 in the note at the top.
- : Added tip from Jimmy Kaz about avoiding spaces in INF file names.
- : Added links to PnPUtil.
- : Added a note at the top about how SHA-1 is going away so parts of this document will need to be updated.
- : Added a mention of KB2763674, which should make SHA-2 executables usable on Windows Vista.
- : Added a mentions of Microsoft Security Advisory (2880823).
- : Added section Use /tr to specify the timestamp server and changed all examples to use /tr.
- : Added new info from PiXCL about how to double-sign with SHA1 and SHA2 to the Use SHA-1 for kernel modules prior to Windows 8 section.
- : Initial release.