Category Archives: Standards

Hard revocation checking and why it’s not here yet.

Leave a reply

If you follow discussions around x.509 and SSL you have likely heard that “Revocation Checking is Broken”, you might even hear it will never work therefore we should start over with a technology that isn’t dependent on this concept.

There are some merits to these arguments but I don’t agree with the conclusion, I thought I would summarize what the problems are in this post.

Fundamentally the largest problem is that, as-deployed, all x.509 revocation technologies introduce a communication with a third-party (the Certificate Authority).

This isn’t necessarily a deal breaker but it does have consequences, for example in the case of SSL:

  1. It can slow down the user’s experience.
  2. It introduces a new point of failure in a transaction.

These issues can be mitigated through intelligent deployments and engineering but unfortunately this really has not happened, as a result Browsers have implemented what is called “Soft-fail revocation checking”.

With soft-fail revocation checking browsers ignore all conditions other than an authoritative “revoked” message, in the case of OCSP that means if they reach the responder and it says “I don’t know the status” or if it fails to reach the responder it assumes it is “good”.

This behavior is of course fundamentally flawed, the Browsers say they have no choice (I disagree with this conclusion but that’s a topic for another post) other than to behave this way, but why?

The rational is as follows:

  1. Revocation repositories are not reliable.
  2. Revocation repositories are slow.
  3. Revocation repositories are not always available (captive portals).
  4. Revocation messages are too large to be returned in time.
  5. There are too many revocation messages to be returned in time.

These are all legitimate concerns, ones that are unfortunately as true today as they were almost a decade ago.

They are not however insurmountable and I think it’s time we as an industry did something about it.

Least Privilege and Subordinate Certificate Authorities

3 Replies

One of the most fundamental design principals when designing a secure system is that of least privilege, in the case of CAs one scenario where this can be applied is the subordination of another CA.

The application of this concept in this scenario is referred to as qualified subordination,  it was first formalized in the IETF standards for X.509 in 1999 in RFC 2459 through the introduction of the Basic Constraints, (see section 4.2.1.10), Name Constraints (see section 4.2.1.11) and Policy Constraints (see section 4.2.1.11).

Unfortunately broad product support did not begin to emerge until the RFC 3280 was released in 2002.

The development and deployment of these concepts was primarily driven by the US Federal Government’s deployment of PKI as a foundational technology for their security infrastructure. One of the many benefits of the government adopting these concepts was that NIST published a robust Test Suite to validate conformance with their interpretations of RFC 3280 which included extensive coverage of Qualified Subordination.

When these concepts are used together a Root CA is able to delegate the right to issue certificates to another CA while restricting them from creating other CAs or issuing certificates for names they are not authoritative for.

The Federal Bridge made extensive use of these concepts; they were able to do so through the mandate to use software that met the published guidelines. Adoption on the Internet however took much longer given the historically slow adoption rates for browsers, that gladly has changed and there is now sufficient browser support to deploy these restrictions.

In addition Microsoft introduced another mechanism to restrict the scope in which a CA is trusted for, they did this by treating the Extended Key Usage (see section 4.2.1.13) extension as a means to delegate only certain issuance capabilities to a Certificate Authority.

It accomplishes this by using the same logic specified in RFC 3280 for Certificate Policies (see section  4.2.1.5), more specifically it assumes when an issuer lists an Extended Key Usage (such as the one for S/MIME encryption) in a CA certificate that its issuer intended to restrict the usage of that CA to the EKUs present in the certificate. A simplified version of this logic was also adopted by OpenSSL for SSL certificates.

Given the Microsoft behavior is more restrictive than the behavior specified in RFC 3280 it does not break applications that do not support it and allows a CA to restrict behavior even further for clients that use the Windows certificate validation logic (nearly 70% of the deployed browsers today).

 

Client Compatibility

Most browsers and email clients support these concepts, however unfortunately not all of them support Name Constraints.

Despite that that they all do support honoring the RFC 3280 behavior for critical extensions (see section 4.2), which states:

A certificate using system MUST reject the certificate if it encounters a critical extension it does not recognize

This means by marking the Name Constraints extension Critical those implementations that do not support the concept will “fail-closed”.  This means it can be used as an effective way to technically enforce that CAs are not trusted for names they are not authoritative for, it also means that there will be cases where they may be authoritative but clients cant trust the certificates they issue.

This issue can be addressed by not marking the extension Critical, when this is done the clients that understand Name Constraints will continue to honor the policies expressed in it and those that do not will simply ignore the extension.

This is of course a trade-off of security in exchange for compatibility, with that said one with far more positive trade-offs than negative ones.

Specifically this approach means users of clients that do not support the extension are no-worse off than they are without its use and those with support get the additional protection from cases where a subordinate CA has been compromised or is willfully issuing certificates that it is not authoritative for.

With that said, support for Name Constraints is actually quite good as the following table illustrates.

 

Honor Criticality Support Basic Constraints Supports DNS Name Constraints Supports RFC 822 Name Constraints Supports Policy Constraints Supports constrained EKU Successfully enforces
IE [1] Yes Yes Yes N/A Yes Yes Yes (Open)
Outlook [1] Yes Yes Yes Yes Yes Yes Yes (Open)
Firefox [1] Yes Yes Yes Yes Yes No Yes (Open)
Thunderbird [1] Yes Yes Yes Yes Yes Yes Yes (Open)
Opera [1] Yes Yes No[2] No[2] No[2] Yes (SSL only) [3] Yes (Closed)
Windows / Safari [1] Yes Yes Yes Yes Yes Yes Yes (Open)
OSX / Safari[4] Yes Yes No[5] No[5] No[5] No Yes (Closed)

 

What this table shows is:

  1. It is possible to rely on the Name Constraints extension as an effective enforcement technique if the extension is marked as critical.
  2. It is possible to rely on the Basic Constraints extension as an effective enforcement technique.
  3. In the case of Safari and Opera that this success is due to these browsers support of honoring the semantics for critical extensions vs. understanding the Name Constraints extension.

For customers this means if you must interoperate with Opera or Safari (yes even on iPad and iPhone) the use of a certificate with a “Critical” “Name Constraints extension” in it will result in the certificate chain looking invalid.

Thankfully according to StatCounter these represent less than 6% of all browsers on the Internet and antidotal evidence shows almost no use in the enterprise.

With that said most environments business requirements will not allow them to fail even for such a small number, in these environments deploying Name Constraints as a non-critical extension will be required, not 100% of the security benefits are realized with this approach but it does significantly reduce the risk.

In such cases it is recommended that once the remaining legacy clients that do not support Name Constraints have been replaced with more recent versions that do the CAs be re-issued with the extension marked as critical.

 

[1] Tests on Windows were completed with Windows 7, IE 9.0, Outlook 2007, Safari 5.05, Opera 11.61, Firefox/Thunderbird 10.0.2.

[2] OpenSSL supports name constraints for both name forms as well as policy constraints, Opera has chosen not to enable thee capabilities until demand was present. This work was done in OpenSSL in 2008 as part of a contract to Google.

[3] Opera uses OpenSSL which supports restricting a CA from issuing valid SSL server certificates if it’s parent did not place the SSL EKU  in it’s certificate.

[4] Tests on OSX were completed with Lion and Safari 5.05

[5] Safari on the Mac uses the PKITS tests so they are aware of the deficiency in their validation logic, they have not publically stated they will support them but we expect support in the future.

 

Server Compatibility

If you have server that accepts or validates client certificates you will also care about their support for validating certificates that have these constraints.

Each environment is a little different and the number of server choices one sees in these cases feels limitless at times, as such we are only able to provide more abstract guidance here.

In the case of Windows servers such as IIS the important factor is what version of Windows you are running on as the support for PKI is built into the Windows platform. Applications are most commonly built on this platform when they are designed for Windows and is always the case for Microsoft applications.

The concepts discussed here were all supported since Windows 2003, though there were significant improvements in the 2008 release.

The net of the above is that if your server platform is built on this API you gain support for these concepts, on other platforms it of course depends on which libraries they chose to use for support for certificate validation.

 

Generic Identity Device Specification Published

Leave a reply

In the PC ecosystem, when a new device (say mass storage) technology is introduced, commonly there is little standardization, vendors produce proprietary software stacks for interacting with that device, they have custom hardware interfaces for interacting with the device, custom software for managing those devices, etc.

As a device picks up in popularity common platform programing interfaces typically emerge, sometimes these are commercial in nature, other times they are standards based; in either case the goals of these interfaces are simple: abstract out the variety in the hardware ecosystem for the application developer allowing them to write software that can run on any machine regardless of which vendor manufactured a given device. These abstractions also commonly allow the sharing of devices so that multiple applications can use them at the same time.

The next phase in a devices maturity is normally the definition of a class interface for interacting with hardware, it’s this last phase that allows the “no driver needed” story that users like so much; we all reap the rewards of this with flash drives today, plug in the device and it just works (the same is true for display technologies like VGA).

These class drivers commonly cater to the lowest common denominator when it comes to functionality, but vendors are always able to add additional capabilities that are exposed when their drivers and custom software are present (again think about display technologies here as a good example).

There is one device in particular that has not entirely followed this flow that I wanted to talk about and that is Smart Cards; as a concept was they emerged in the 1970s, the first cards went into production in the late 70s. Here we are 40 years later and there is no clear “class driver” for these devices, that is not to say there have not been attempts, some even with success, but those that have had success have been closed system solutions, for example the PIV interfaces used within the US Federal Government.

In the commercial space however, no class specification that has been attempted really was viable, there are lots of reasons for this but I am cautiously optimistic that there is now a candidate.

One of the projects I was working on over the last few years was the specification of the Generic Identity Device Specification, this attempts to build on the success of the government based card specifications and extend it to commercial applications as well.

I had opportunities to work with some great folks on this effort, we all had the same goal make smart cards as reliable, cost effective and accessible as possible; I believe this work does just that.

This specification has now been released by Microsoft under the Microsoft Community Promise, that means it is available royalty free for anyone to adopt; this is a big win for our partners and above all the customers who will benefit the most from it.

So what does this mean for you? Well if you’re a customer looking to deploy smart cards you should seriously look for vendors who produce cards that are compliant with this specification, it means lower cost of deployment, makes it easier for you to multi-source cards and in the end it will likely reduce the overall cost of cards as volumes go up based on function of scale.

For a card manufacturer there are a number of benefits as well, it is possible to develop a GIDS card that is compatible with the PIV card-edge, this means you can develop a single card stock get it evaluated for FIPS (or whatever other standard) that can be sold into commercial or government applications (reducing cost) and these cards will have a great experience in Windows.

If you are a platform or operating system developer you now have a specification you can use as a baseline for testing card scenarios, a way to (hopefully) support a large number of “real” cards that will exist on the market (soon I hope), if this happens we can experience driver coverage numbers similar to other device classes.

For those of you not in this segment, this last point is super important, there is so much fragmentation in the market no solution has over a couple percent of card coverage in-box, if this specification gets adopted that number can start to look more like other device classes where the number is in the 90 percentile range.

In any event, I am pleased to see this out there, here’s hoping it gets adopted broadly…