Certificate Transparency is really not a replacement for key pinning

I’ve noticed that there’s a common misconception that Certificate Transparency is a replacement for HTTP public key pinning. If those words make no sense to you: HTTP public key pinning was a now-mostly-defunct mechanism whereby websites could “pin” themselves to a set of public keys, so that browsers would not accept a certificate for that website’s hostname unless one of those pinned public keys appeared in its certificate chain. Certificate Transparency is a system whereby certificates must be logged in public append-only logs, so that domain owners can observe them as they are issued and detect malicious certificates (see my previous overview here).

Public key pinning was mostly removed from the web due its brittleness; it was very easy for websites to accidentally DoS themselves with PKP, for reasons that deserve their own lengthy post. CT is often seen as the modern replacement for PKP. It’s true that the deployment of CT partly motivated the deprecation of PKP, and it’s also true that both CT and PKP were motivated by the need to protect against malicious or mistakenly issued certificates, but CT and PKP actually provide quite different security properties. I believe that we could theoretically build upon CT to provide similar security properties to PKP, but the resulting system would be very hacky and/or quite baroque (a castle of duct tape and toothpicks, really) and would ultimately be an abuse of CT, which was not designed to provide these security properties.

Security properties

Let’s compare and contrast the security models of PKP versus CT.

Typically, when a website enabled public key pinning, it would bless a set of root or intermediate certificates that could be used to issue certificates for that website. This reduced the attack surface of the web PKI: once a browser was aware of a website’s PKP configuration, until that configuration expired, it would no longer be the case that any trusted CA could issue a certificate for that website. The many-single-points-of-failure property is a deep weakness of the web PKI, and public key pinning mitigated it. A pinned website would be vulnerable to compromise of only the single-digit number of CAs that it pinned to, not the several hundred CAs that browsers/OSes typically trust.

PKP also defined a reporting functionality. The website owner could configure PKP such that the browser would send a report of any pinning violation it observed to a reporting endpoint of the website owner’s choosing. However, this was not a security feature, as an attacker using a malicious certificate would typically be able to simply block reports of the violation as they were sent. PKP reporting was designed to help site owners detect and debug pinning misconfigurations, not attacks.

In contrast, CT gives a much fuzzier and qualified set of security properties. As designed, CT ensures that if a browser accepts a certificate, it will appear in at least one public log within 24 hours of submission to said log; submission typically (but not always) happens around certificate issuance time. In CT as designed, ideally, this guarantee holds even in the event of a compromised or malicious log, but the basic CT design doesn’t flesh out details fully enough to say exactly how malicious log behavior will be detected (specifically for the attack of a split view, where the log presents one view to e.g. a client and another to e.g. a domain owner monitoring for malicious certificates).

This ideal security guarantee is already pretty fuzzy, but it gets even fuzzier when you consider that CT is in different states of deployment and maturity across different clients, and no client currently implements CT fully as designed. For example, in Chrome’s current deployment, the actual security guarantee of CT is something like this: if a browser accepts a certificate, it will appear in at least one public log within 24 hours of submission to said log, as long as the log(s) are not compromised or malicious. In the event of a compromised or malicious log, a failure to incorporate a certificate into the log as presented to Google will be detected probabilistically (in expectation, if a malicious certificate is used on 10,000 connections, log misbehavior will be detected, though this probability is tweakable in exchange for some privacy and performance tradeoffs). Building on that, Google can and does “gossip” with other parties to detect when they may be seeing different views of the same log, mitigating split view attacks – though there isn’t yet universally implemented. (Thanks to Andrew Ayer for a correction in this paragraph, pointing out that Google does in fact gossip with his Cert Spotter monitoring tool and that gossip is not a theoretically hard problem.)

CT also has an Achilles’ heel: what is a domain owner supposed to actually do if they find a malicious certificate for their domain in a CT log? Your first thought might be that they should revoke it, which they should, but revocation in the web PKI is fraught, with some browsers implementing minimal support for revocation or only implementing revocation infrastructure for hand-picked high-value certificates. This is a complex topic and my best summary is that revocation, or other reactive action, is an unsolved part of the CT design.

As you can probably surmise just from the number of sentences and amount of handwaving used to describe their respective security guarantees, public key pinning offered a much cleaner and mostly stronger security model than CT. It is nearly impossible to even cleanly state CT’s security model, thus it cannot be seen as a drop-in replacement for PKP, which offers fairly clean and strong security guarantees. PKP directly protects users against attackers with malicious certificates issued from non-authorized CAs. CT makes it more likely that domain owners will notice malicious certificates issued for their domain.

While PKP generally looks like the stronger security model, there are two ways in which one could argue that CT offers stronger security than PKP:

  • First, PKP will not protect against, or even permit detection of, the compromise of the CAs to which a website is pinned. If a website pins to some CA, and the attacker compromises that CA, the attacker wins. In contrast, CT will (ideally, but in reality subject to all the caveats above) allow a website owner to detect a malicious certificate regardless of which CA issued it.
  • Second, CT has second-order protective effects in the ecosystem. Publishing ~all certificates has enabled the detection of various CA incidents and bad practices, helping improve CA security before larger compromises occur. All websites are safer when CAs adopt better security practices.

Therefore, there is not a strict ordering between the security properties of CT and PKP; neither fully subsumes the other.

What would it take for CT to subsume PKP?

If CT does not currently match PKP’s security guarantees, it invites the question: could it be made to? As a warning, this section is purely a thought exercise, and definitely not a design proposal. The short answer is that I believe we could, in theory, build upon CT to get a lot closer to PKP’s security model, but it would be laughably complicated and would likely end up just as brittle as pinning was. The point of this thought exercise is to underline that CT is really not anywhere close to a replacement for PKP. The degree of hackery needed to make CT into a replacement for PKP illustrates that it wasn’t designed for this security model.

I’ll introduce two avenues for evolving CT into something that gets closer to PKP. Both are bad ideas. Also note that this section assumes some more in-depth knowledge of how CT works; you might want to skip this section if you only have a high-level understanding of CT, or read my previously-linked overview post first.

Enforce pinning in the logs

The first bad idea for building upon CT to get something more like HPKP is simple: CT logs could enforce pins at certificate submission time, instead of the client enforcing pins at connection time. A website owner that wishes to pin would configure pins with CT log operators, instead of configuring pins to be consumed directly by clients.

If we assume that CT logs are honest, this straightforwardly achieves the security goals of public key pinning, without nearly as much brittleness. There are two sources of brittleness with pinning that are mitigated by pinning in the logs instead of in clients:

  • Traditional HTTP public key pinning is brittle because if a pinning configuration needs to be updated, approximately every browser instance in the world needs to learn about that updated configuration. With pinning in the logs, however, a website owner only needs to reach the ~10 CT log operators to update their pinning configuration, instead of every browser instance.
  • Another source of brittleness in traditional HTTP public key pinning is the difficulty of predicting which certificate chains clients might build. If a website owner pins their site to a particular root or intermediate CA, they must be sure that affected clients are going to verify their certificate through a path that includes that CA – which is not easy to predict, due to differences in root CA trust stores across clients and cross-signed certificates. However, if logs enforce the pins instead of clients, website owners only need to be able to predict the paths that CT logs build, not the paths that all clients will build. This is simpler to reason about, reducing the likelihood of bad pinning configurations; plus, as mentioned above, bad pinning configurations are easier to deal with when there are fewer points of enforcement.

Despite these appealing properties, enforcing pins at log submission time is a terrible idea. Here’s why:

  • First of all, it doesn’t really solve the security problem. By design, CT logs are not considered trustworthy, thus they should not be trusted to enforce pins.
  • Secondly, it’s already difficult and expensive to run a CT log, so CT log operators shouldn’t be burdened with additional responsibility, especially responsibility that runs counter to a CT log’s goal of providing comprehensive transparency. Except as needed to mitigate spam and abuse, CT logs should seek to accept and publish as much as possible, not to reject certificates. Specifically, CT logs should be the way that a domain owner or other interested party can find out about a malicious or misissued certificate; CT logs shouldn’t be responsible for rejecting such a certificate and preventing it from being used.

Despite the terribleness of this idea, I think it’s still an interesting thought exercise because it reveals some properties that we might want to preserve in a new system designed to replace HTTP public key pinning. For example, an off-the-cuff idea for a pinning replacement might be that if a website owner wishes to pin, they register their pins with each browser vendor, and the website must present a countersignature from the browser vendor alongside its certificate. The browser vendor enforces the pins before issuing this countersignature. This proposal (which surely has plenty of other downsides) preserves the idea of having fewer, more predictable pinning enforcement points, but doesn’t abuse CT logs to achieve that property.

“Pinning” to transparency

The second terrible idea is to do away with public key pinning as the basis for security, and instead provide stronger transparency guarantees. The goal would be to strengthen CT’s guarantees on an opt-in basis for website owners who are willing to accept some inconvenience in exchange.

Recall that CT’s guarantee is (approximately, and depending on deployment model) that if a browser accepts a certificate as valid, it will appear in at least one public log within 24 hours of issuance. What if instead, for an opted-in website, the browser enforces that the certificate was submitted to more than one log at least 48 hours ago and that the certificate comes with a stapled OCSP response? This would entail some inconvenience for the site owner (legitimate certificates would not be usable for 48 hours after issuance, and the website would need to implement OCSP stapling), but it would improve security by ensuring that the website owner has a chance to see malicious certificates in CT logs and revoke them before they become usable for attacking users.

This idea of “pinning” to increased transparency requirements is not the same mechanism as PKP, but it achieves similar security: malicious certificates would not be accepted by clients, as long as the website owner actively monitors CT logs and revokes unexpected certificates within the >=24 hour window from when they appear until when they start being accepted by browsers. In a way this is even an improvement over PKP’s security guarantees, because it allows website owners to act on any malicious certificate, even if it is issued by a CA they consider trustworthy.

However, there are two big asterisks on the security model of this proposal.

The first is that the attacker could obtain a valid OCSP response for a malicious certificate and continue to use that for several days after the website owner notices and revokes the certificate, until the OCSP response expires. Therefore an attacker may be able to use a malicious certificate for some period of time, unlike PKP where the browser will always reject the malicious certificate.

The second asterisk is that we’re assuming that each certificate is logged in at least one trustworthy log. As mentioned previously, this violates a design goal of CT: logs are not meant to be trustworthy.

The browser could potentially reduce trust in logs by requiring not just an SCT (the signed statement that a log intends to publish a certificate) but a stapled inclusion proof (a proof that the log actually incorporated the certificate). This alone isn’t sufficient, though; a log might have incorporated the certificate into the view that the web server and browser are seeing, but not into the view that is being presented to the website owner as they are monitoring for malicious certificates. We could go even further and have the browser vendor distribute STHs to clients so that the browser can verify that the view being presented to the web server and browser instance is consistent with the view presented to the browser vendor. To take this all the way to completion, website owners would be obligated to interact with the browser vendor to monitor for malicious certificates, to be sure that the website owner is seeing the same view of the log as the browser vendor and the browser instance; for example, the browser vendor could operate a CT monitoring service that the website owner uses to detect malicious certificates. This would ensure that the browser, website owner, and browser vendor are all seeing the same view of the log; if a malicious certificate is accepted by the browser, even in the face of a malicious log, the certificate must have been visible to the website owner for at least 24 hours, allowing them to notice and revoke it.

Altogether, this is now a very complex proposal, and still with many details to be ironed out (such as what happens if a browser instance can’t obtain up-to-date STHs from the browser vendor). It would surely involve multiple person-years of work to design and build, and I’m not at all confident that the resulting system would be any less brittle than public key pinning. As mentioned above, my point in presenting it is to illustrate that, while it may theoretically be possible to build upon CT to achieve something similar to PKP, CT is quite far off from being a PKP replacement.

A whole separate misconception: CAA

There is a whole separate misconception that CAA (Certificate Authority Authorization) is a replacement for PKP. CAA is a DNS-based policy mechanism that evolved around the same timeframe as CT. CAA is a DNS record that indicates the CAs that are authorized to issue certificates for a domain. CAs must check this record before issuing certificates. However, CAA is a sign, not a cop. PKP was designed to protect against malicious or compromised CAs, and CAA does not do this; a malicious or compromised CA would simply ignore CAA and issue a malicious certificate. There is essentially no technical security value to CAA, only protection against CAs’ non-adversarial mistakes. (CAs make a lot of non-adversarial mistakes, so there is still a lot of value! Just not direct technical security value in the way that PKP has security value.)

Both CT and CAA are very valuable, and one could argue that together these two mechanisms add up to significantly diminish PKP’s marginal value – but neither is a direct replacement for PKP in terms of security guarantees.

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