Monthly Archives: May 2017

Let’s talk about revocation checking, let’s talk about you and me.

4 Replies

I have been having a conversation with Melhi at Comodo, this is the most recent post in that series.

First of all, the author is unaware that my company has built the most sophisticated Certificate Management system that can automatically request, issue, renew and manage the whole lifecycle of the certificate. Many Fortune 500 companies rely on this amazing technology to manage their PKI infrastructure.

I am aware.

So it is with that expertise and insight I must insist that the author does not appreciate the nuance between “high frequency” renewal vs “low frequency” renewals. Short lived certs will require “high frequency” renewal system. To an IT admin this is a scary prospect! They tell us that!

Having been responsible for and or worked heavly on:

  • The Valicert OCSP responder and clients,
  • The Windows CA and the enrollment client at Microsoft, which is the most used CA software ever,
  • The Network Access Protection solution that used IPSEC and ~12-hour certificates to do segmentation of hosts at the IP layer and their health. based network isolation solution that was deployed into many of the Fortune 500,
  • All of GlobalSign’s technology offerings, in particular, the technology enabling their expansion into the Enterprise,
  • Helping the ISRG build out Let’s Encrypt,
  • Designing, building and operating numerous other high volume products and services in finance, healthcare, and government.

Above and beyond that I helped secure Bing Ads and Live ID and now work at Google on other very high scale systems.

Needless to say, I too have familiarity with “high frequency”,  “low frequency”, “disaster recovery” and “availability” problems.

As for fear, it is a natural part of life, I believe it was Nelson Mandela who famously said: Courage is not the absence of fear but the triumph over it.

The reality is that it is manual certificate lifecycle management that is the thing to be afraid of. That is why COMODO and other CAs have been building out cloud-based management and automation solutions over the last few years. For customers this reduces failure, reduces costs, gives more control, and nets higher customer satisfaction.

Importantly, one needs to remember it is manual processes that lead to the majority of outages [see 1,2,3, and 4]. The scale of this problem has even led to an entire market segment dedicated to managing the lifecycle of WebPKI certificates.

So to the question of fear, I would say the same thing my father told me, though change can be scary and uncomfortable in the short term, it usually turns out okay in the end, and often better.

I believe there can be a scalable revocation infrastructure that can serve status for all certs from all CAs that is backward compatible with existing issued certificates that can be called from a browser. As I said before I do believe in the ingenuity of our scientist and engineers to bring us this solution……soon….

You misunderstand me. I did not say it was impossible to have a scalable revocation infrastructure that is backward compatible. I said the creation of a new system that could deliver on the small message size promise we were discussing would take 10 years to design, build and deploy.

To understand my rational take a look at this post. It is a little old now, and update rates are a faster now but not massively. When you review it you will see that if you exclude IE/Edge it takes just under a year for a new version of the most common browsers to reach 90% market share.

To put that in a more concrete way, if today, Chrome, Firefox, and Safari decided to simultaneously release a new version with a new revocation scheme it would take a bit less than a year for us to see 90% deployment. That is, of course, an unrealistic goal. Apple, for example only releases new versions a few times a year and does not even support WebRTC yet. Additionally, browsers have a pretty deep-seated position on revocation checking at this point given all the problems of the past so convince them will take time.

A more realistic, but still optimistic period assuming pre-existing consensus and a will to solve the problem is 5 to 6 years. If you question this figure, just look at how long it took for TLS 1.2 to get deployed. TLS 1.2 was published in 2008 and was not enabled by default in Windows until Windows 8.1 in 2014.

Web servers are even worse, as an example consider Apache version 2 was released in 2002 and these later versions still have less than 50% deployment.

Lets call it DCSP 😉

DCSP isn’t a bad idea, but it has its own challenges. For example, some that come at the top of mind are:

  • Most browsers do not ship their own DNS clients, this is one of the problems DNSSEC had in deployment. If the operating system DNS APIs they use do not provide the information they need then they can not adopt any technology depend on it.
  • Middleboxes and captive networks make DNS-based distribution problematic. Again DNSSEC suffered from this.
  • If DCSP requires a custom record type, for example, DNS servers and tooling will need to be updated. DNS servers also do not get updated regularly, this is another thing that has held back DNSSEC as well as CAA. It is fair to say that it is fair for the WebPKI CAs to update their DNS but based on the glacial pace in which CAs adopt technology that is 3-4 years before you could see deployment even with that (see CT as an example).

To be clear, I think something like this is worth exploring but I don’t think it will see meaningful deployment in the near term.

If we were in Vegas, I would say the most expedient path is to build on what is there, while in parallel building the new thing. This can shave as much as 5 years off the time it takes to solve the problem. If nothing else this moves the CA ecosystem closer to an operational maturity capable of supporting the new system.

Ryan

My response, to his response, to my response? or short-lived certificates part 3

4 Replies

The conversation on short-lived certificates and their value continues. In the most recent conversation, we have started to shift from an either or position to one where we explore what is needed to make revocation checking a viable technology, this is a topic I am passionate about.

That said, there still seems to be some confusion on short-lived certificates, specifically the author states:

Of course, the cost of short-lived certs is very high as change the whole computing infrastructure so that certificates are renewed on a daily basis (daily for it to be secure enough vs 90 day certificates in my view) and introduce this new moving part that might cause vulnerability and operational issues.

Let’s explore this “cost” argument for a moment. First, when I issue an end-entity certificate I minimally have to perform two cryptographic signatures, one on the certificate and one on the OCSP response (it’s actually more than this in some cases but to keep the conversation simple I have omitted the others).

If we look at the performance optimization that Firefox has implemented where they do not require revocation checking when the certificate is within a subset the resolution of the revocation period short-lived certificates can, in fact, reduce the cost for a CA. This is because you no longer need to sign two things during the first few days/hours of the life of the certificate and you do not need to distribute that response.

The only way I think short lived certificates are more expensive for the CA is if you compare a model of certificates issued for a period better measured in hours to a model with certificate validity measured in years providing weekly revocation updates. This, however, is a bad model, and something in-between is needed.

The author also believes the use of automation represents a security vulnerability, so this deserves a response. It is true that complexity is the enemy of security, it is even true that automation can if poorly implemented can hurt security. The inverse is also true, however.  It is also generally accepted that availability is a component of security and one of the more common problems in the WebPKI is poor manual management practices resulting in the lack of understanding what is deployed and those certificates that are deployed expiring [see 1,2,3, and 4] and taking down services.

It is also important to look at the big picture when evaluating the net-security benefits of automation, for example, does anyone honestly believe we would ever get to a world where the majority of the web is encrypted if organizations have to staff people to manually generate certificate requests and hand carry them to CAs? Is the net-benefit of automation of reliability and scope of deployment worth its secondary effects?

The author also suggests that a certificate must have a validity period of only a day to be “secure enough”. This seems both arbitrary and wrong, as stated previously the User Agents and WebTrust allows an OCSP response or CRL can be a week old and still be trusted.

One of the largest reasons for this is that clock skew is a big problem in the real world and as a result, you need to keep validity periods of certificates and revocation messages outside this skew period to prevent skew related failures.

The decision to define “secure enough” at a day, both defines the problem in an intractable way and furthermore ignores the fact that it establishes a double standard that does nothing to address the issue if stale revocation information.

If we were to bring this conversation back to how we improve certificate revocation I would say there should be one standard for how recent the client’s understanding of the certificate’s validity would need to be.

On that topic, the author goes on to discuss how 32 bytes is better than 470 (the size of the smallest OCSP response). I could not agree more about this, in fact in the 90s’ when I was at Valicert we implemented a proposal from Paul Kocher called Certificate Revocation Trees. This approach uses of Merkle Trees (the heart of the Bitcoin ledger) to provide a very space efficient solution to this problem. Unfortunately, we were unable to popularize this at the time.

Ben Laurie began work on a variation on this approach that leveraged sparse Merkle Trees that he called Revocation Transparency. I personally like the idea of this approach because it leverages the work done to make Certificate Transparency scalable. For example, Trillian, the foundational server for Google’s next generation log server is designed to scale to Trillions of certificates.

That said, there are a number of similar approaches that could be equally scalable.

While I do think that an approach similar to the above could be made to work today, I also think it is more of a long-term solution in that even with the significantly increased rate of technological adoption it would take close to ten years given the state of things for such a solution to be fully deployed if we started right now.

As such I would start with the problem definition, which would need to involve a more formal analysis of the role of revocation checking today so that the right solution was built.

In parallel I would want to see the industry adopt a more strategic plan to address the more practical and immediately solvable problems, including:

  • Measuring and improving the revocation infrastructure operated by CAs,
  • Establishing global performance and reliability metrics and reporting that all CAs must meet,
  • Funding improvements to Nginx and Apache’s OCSP Stapling implementations,
  • Working with browsers to adopt the performance optimization firefox has implemented for revocation checking,
  • Working with TLS stacks, User Agents, Servers and Service Providers to adopt OCSP Must-Staple,
  • Defining an OCSP transport based on DNS that would reduce dependency on CA infrastructure reliability,
  • Evangalizing the adoption of OCSP stapling with administrators.

Ryan

P.S.

The author also has also added in someone else who has asked some questions or more correctly seems to question my version of the historical narrative. To provide some context, my narrative comes from my practical experience working with Microsoft, eBay, Amazon, and other large companies in the mid to late 90s and through the mid-2000s.

I too have worked with the BBN Safekeeper, I have a fun story how we hired some people to extract the keys from one of these boxes I would be happy to share over a beer sometime.

However, a cool device, the first one I remember working with was the KOV-8 in the 1993/4 timeframe.

Anyway, it is true that SSL started its life in 1994/5 at which point only software implementations of crypto were used (they were all BSAFE) but it is also true that mass deployment of SSL (relatively speaking) did not start until the late 90s and early 00s and that is the time my narrative was based on.

He also has also questioned the narrative of what Windows supported in the context of key protection. Since the author knows me personally he must have simply forgotten that I was the PM for these technologies and was at Microsoft working in this area for about 15 years.

Again I think there is some confusion here, the author states:

The software based Cryptographic Service Provider for RSA allowed keys to be marked ‘not for export’ from a very early release if not the first.

and:

The CAPI features used to protect private keys were expanded and exposed as a separate API in Windows 2000 as the Data Protection API.

As someone who worked at Microsoft on these technologies for a long time I can say with absolute confidence they were not built to provide key isolation, do not provide key isolation properties and were actually not used by the SSL implementation (SCHANNEL) for the server keys. If you’re interested in learning more about the capabilities of Windows in this area check out this post I did recently.

He has also questioned the role of ValiCert in the definition of the RFC, thankfully the IETF PKIX archives are still there and if you care to look you can see Mike was basically checked out, Warwick was not publicly paticipating and the work to finalize the protocol was largely done by Ambarish Malpani the founder of Valicert.

It is my turn, or short-lived certificates part 2

5 Replies

My response to a recent post suggesting short-lived certificates were intended to remove the need to do revocation seems to have spurred a response from its author.

The thesis of the latest response is that:

Revocation can easily handle “key compromise” situation and do so by offering more security than short-lived certs.

The largest problem with this thesis is that it is based on an incorrect understanding. That being the author believes short-lived certificates do not get checked for revocation, or they believe it is a choice between short-lived certificates and revocation.

Both statements are not true. User Agents do not say “oh, this certification is short-lived so we won’t do revocation checking”. If User Agents do revocation checking of end-entity certificates, and not all do, the short-lived certificate will get checked just like the long-lived certificate.

Now there is a reasonable argument to be made that if CAs are allowed to produce revocation lists and OCSP responses every 7 days, and User agents will trust them for that time, that it wouldn’t be an unreasonable performance optimization. At least if the behavior was limited to certificates with validity periods shorter than 7 days. With that said, User Agents don’t do this today and I didn’t suggest they should in my post.

[Update 10:43PM May 4th 2017 It seems a changed happened while I was asleep at the wheel. Firefox has implemented the below optimization as of Firefox 41. A corrected statement follows in the next  two paragraphs]

Both statements are not true, well except for one performance optimization implemented by Firefox I will explain shortly. Basically, if User Agents do revocation checking of end-entity certificates, and not all do, the short-lived certificate will get checked just like the long-lived certificate as long as it is younger than the corresponding CRL or OCSP response would be (Firefox only).

This performance optimization Firefox implemented is based on the fact that  CAs are allowed to rely on 7-day old OCSP responses and CRLS. As a result, 7 days becomes the precision of revocation checking, It is not clear what value Firefox chose but it is a subset of that figure, not 90 days. But either way, no major CA that I am aware of issues such short-lived certs today due to time skew issues.

The second problem with this thesis is it presumes the user who does know of a compromise wants to announce it to the world and can. As I mentioned in an earlier post on revocation reasons subscribers are not keen to announce to the world that there was “a compromise”. I mention “can” because in some cases the one who knows about the comromise do not even have sufficient permissions to the associated CA consoles to request the revocation.

And finally, the last issue in this thesis is that it presumes an effectiveness of revocation checking. Today over 9% of OCSP responses fail due to issues with the CA’s revocation infrastructure (the connections time out).

That’s right 9 out of 100 revocation checks fail because CAs fail to operate capable enough infrastructure to meet the needs of the clients that rely on them. It is actually worse than that though, the largest websites use a technique called Domain Sharding to make their sites load faster, this means that the failure rate you would experience as a user if hard-fail was implemented could be 2-4x higher than that.

This is before we consider the fact that due to the poor performance and failures in CA revocation infrastructure revocation checking has been largely turned off in Chrome and other browsers.

I say this because for “revocation checking” to work for a key compromise case you need two things:

  • The CA to know the compromise occurred,
  • Revocation checking to actually work.

Now I want to be clear, I think revocation checking is a good thing and I would like to see the situation improved, as a proof of that statement here is an example of some of my work in this area:

  • While at ValiCert (the lead creators of OCSP) I worked on the standardization of RFC 2560 (OCSP) including running the interoperability testing that led to its ultimate standardization,
  • I also am the author of RFC 5019 which is the profile of OCSP in use by CAs and clients,
  • I have led the development of numerous PKI SDKs and servers which have implemented these standards (including leading the team that added support for OCSP to Windows),
  • I led the implementation of the first, and most reliable implementation of OCSP stapling (in SCHANNEL),
  • I led the CASC project to get OCSP stapling added Nginx,
  • I have helped create, and/or recreate numerous CAs and along with their revocation infrastructure,
  • And I led the first wide scale measurement and efforts of the CA revocation infrastructure via my X509LABS Revocation Report project which led to all CAs adopting the same design philosophies to begin to address abhorrent response times and uptime.

Basically, I see the value in revocation checking and think the investments need to be made to make it work and be relevant for the WebPKI of today. That said, this topic has zero to do with short-lived certificates.

Ryan

How to keep a secret in Windows

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Protecting cryptographic keys is always a balancing act. For the keys to be useful they need to be readily accessible, recoverable and their use needs to be sufficiently performant so their use does not slow your application down. On the other hand, the more accessible and recoverable they are the less secure the keys are.

In Windows, we tried to build a number of systems to help with these problems, the most basic was the Window Data Protection API (DPAPI). It was our answer to the question: “What secret do I use to encrypt a secret”. It can be thought of as a policy or DRM system since as a practical matter it is largely a speed bump for privileged users who want access to the data it protects.

Over the years there have been many tools that leverage the user’s permissions to decrypt DPAPI protected data, one of the most recent was DPAPIPick. Even though I have framed this problem in the context of Windows, Here is a neat paper on this broad problem called “Playing hide and seek with stored keys”.

The next level of key protection offered by Windows is a policy mechanism called “non-exportable keys” this is primarily a consumer of DPAPI. Basically, when you generate the key you ask Windows to deny its export, as a result the key gets a flag set on it that can not, via the API, be changed. The key and this flag are then protected with DPAPI. Even though this is just a policy enforced with a DRM-like system it does serve its purpose, reducing the casual copying of keys.

Again over the years, there have been numerous tools that have leveraged the user’s permissions to access these keys, one of the more recent I remember was called Jailbreak (https://github.com/iSECPartners/jailbreak-Windows). There have also been a lot of wonderful walkthroughs of how these systems work, for example, this nice NCC Group presentation.

The problem with all of the above mechanisms is that they are largely designed to protect keys from their rightful user. In other words, even when these systems work they key usually ends up being loaded into memory in the clear where it is accessible to the user and their applications. This is important to understand since the large majority of applications that use cryptography in Windows do so in the context of the user.

A better solution to protecting keys from the user is putting them behind protocol specific APIs that “remote” the operation to a process in another user space. We would call this process isolation and the best example of this in Windows is SCHANNEL.

SCHANNEL is the TLS implementation in Windows, prior to Windows 2003 the keys used by SCHANNEL were loaded into the memory of the application calling it. In 2003 we moved the cryptographic operations into Local Security Authority Subsystem Service (LSAS) which is essentially RING 0 in Windows.

By moving the keys to this process we help protect, but don’t prevent, them from user mode processes but still enable applications to do TLS sessions. This comes at an expense, you now need to marshal data to and from user mode and LSAS which hurts performance.

[Nasko, a former SCHANNEL developer tells me he believes it was the syncronous nature of SSPI that hurt the perf the most, this is likely, but the net result is the same]

In fact, this change was cited as one of the major reasons IIS was so much slower than Apache for “real workloads” in Windows Server 2003. It is worth noting those of us involved in the decision to make this change surely felt vindicated when Heartbleed occurred.

This solution is not perfect either, again if you are in Ring 0 you can still access the cryptographic keys.

When you want to address this risk you would then remote the cryptographic operation to a dedicated system managed by a set of users that do not include the user. This could be TCP/IP remoted cryptographic service (like Microsoft KeyVault, or Google Cloud Key Manager) or maybe a Hardware Security Module (HSM) or smart card. This has all of the performance problems of basic process isolation but worse because the transition from the user mode to the protected service is even “further” or “bandwidth” constrained (smart cards often run at 115k BPS or slower).

In Windows, for TLS, this is accomplished through providers to CNG, CryptoAPI CSPs, and Smartcard minidrivers. These solutions are usually closed source and some of the security issues I have seen in them over the years are appalling but despite their failings, there is a lot of value in getting keys out of the user space and this is the most effective way of doing that.

These devices also provide, to varying degrees, protection from physical, timing and other more advanced attacks.

Well, that is my summary of the core key protection schemes available in Windows. Most operating systems have similar mechanisms, Windows just has superior documentation and has answers to each of these problems in “one logical place” vs from many different libraries from different authors.

Ryan

Why bother with short-lived certificates and keys in TLS?

7 Replies

There seems to be a lot of confusion and misinformation about the idea of short-lived certificates and keys so I thought I would pen some thoughts about the topic in the hope of providing some clarification.

I have seen some argue the rationale behind short-lived certificates is to address the shortcoming in the CA and browser revocation infrastructure, I would argue this is not the case at all.

In my mind, the main reason for them is to address the issue of key compromise. Long-lived keys have a long period in which they are exposed to theft (see Heartbleed) and therefor are a higher value to an attacker since a stolen key enables the attacker to impersonate the associated website for that period.

The most important thing to keep in mind is that the nature of key compromise is such that you almost never know it has happened until it is too late (consider Diginotar as an extreme example).

The importance of protecting the SSL private key is why in the 90s when SSL deployment started in earnest, large companies tried to deploy SSL in their environment using SSL “accelerators” and “security modules” that protected the keys. This was the “right technical” thing to do not not the right “practical thing” because it significantly reduced the scalability of SSL protected services and at the same time massively increased the cost to deploy SSL.

By the mid 00’s we recognized this was not workable and the use of accelerators basically stopped outside a few edge cases and software keys were used instead. Some implementations, like the Microsoft SCHANNEL implementation, tried to protect the keys by moving them to a separate process mitigating the risk of theft to some degree. Others simply loaded the keys into the web servers process and as a result exposed them to Heartbleed like attacks.

What’s important here is that when this shift happened no one pushed meaningful changes to the maximum validity period of certificates and by association their private keys. This meant that we had software keys exposed to theft for five years (max validity period at the time) with no reliable way to detect they were stolen and in the unlikely event we did find out of the theft we would rely on the unreliable CA revocation infrastructure to communicate that issue to User Agents — clearly this was not an ideal plan.

In short, by significantly reducing the validity of the certificate and keys, we also significantly reduce the value to the attacker.

Another issue that short-lived certificates help mitigate is the evolution of the WebPKI, with a long-lived certificate you get virtually no security benefits of policy and technology improvements until the old certificates and keys are expunged from the ecosystem. Today this is rollout of new policy largely accomplished via “natural expiration” which means you have to wait until the last certificate that was issued under the old policies expires before absolute enforcement is possible.

So what is the ideal certificate validity period then? I don’t think there is a one-size-fits-all answer to that question. The best I can offer is:

As short as possible but no shorter.

With some systems it is not possible to deploy automation and until it is one needs to pick a validity period that is short enough to mitigate the key compromise and policy risks I discussed while long enough to make management practical.

It is probably easier to answer the question what is the shortest validity period we can reliably use. The answer to that question is buried in the clock skew of the relying parties. Chrome recently released a new clock synchronization feature that significantly reduces errors related certificate validity periods. But until that is fully deployed and other UAs adopt similar solutions you are probably best to keep certificate validity periods at 30 days to accommodate skew and potential renewal failures.

In-short, long-lived keys for SSL exist because we never re-visited the threat model of key compromise when we stopped using hardware-protected keys in our SSL deployments and short-lived keys help deal with the modern reality of SSL deployments.

Ryan

– I have added a few pictures for fun and made some minor text changes for clarity.