SKS Keyserver Network Under Attack

Posted by & filed under Security Alerts.

Terminological Note

“OpenPGP” refers to the OpenPGP protocol, in much the same way that HTML refers to the protocol that specifies how to write a web page. “GnuPG”, “SequoiaPGP”, “OpenPGP.js”, and others are implementations of the OpenPGP protocol in the same way that Mozilla Firefox, Google Chromium, and Microsoft Edge refer to software packages that process HTML data.

Executive Summary

In the last week of June 2019 unknown actors deployed a certificate spamming attack against two high-profile contributors in the OpenPGP community (Robert J. Hansen and Daniel Kahn Gillmor, better known in the community as “rjh” and “dkg”). This attack exploited a defect in the OpenPGP protocol itself in order to “poison” rjh and dkg’s OpenPGP certificates. Anyone who attempts to import a poisoned certificate into a vulnerable OpenPGP installation will very likely break their installation in hard-to-debug ways. Poisoned certificates are already on the SKS keyserver network. There is no reason to believe the attacker will stop at just poisoning two certificates. Further, given the ease of the attack and the highly publicized success of the attack, it is prudent to believe other certificates will soon be poisoned.

This attack cannot be mitigated by the SKS keyserver network in any reasonable time period. It is unlikely to be mitigated by the OpenPGP Working Group in any reasonable time period. Future releases of OpenPGP software will likely have some sort of mitigation, but there is no time frame. The best mitigation that can be applied at present is simple: stop retrieving data from the SKS keyserver network.

How Keyservers Work

When Phil Zimmermann first developed PGP (“Pretty Good Privacy”) in the early 1990s there was a clear chicken and egg problem. Public key cryptography could revolutionize communications but required individuals possess each other’s public keys. Over time terminology has shifted: now public key cryptography is mostly called “asymmetric cryptography” and public keys are more often called “public certificates”, but the chicken-and-egg problem remains. To communicate privately, each party must have a small piece of public data with which to bootstrap a private communication channel.

Special software was written to facilitate the discovery and distribution of public certificates. Called “keyserver software”, it can be thought of as analogous to a telephone directory. Users can search the keyserver by a variety of different criteria to discover public certificates which claim to belong to the desired user. The keyserver network does not attest to the accuracy of the information, however: that’s left for each user to ascertain according to their own criteria.

Once a user has verified a certificate really and truly belongs to the person in question, they can affix an affidavit to the certificate attesting that they have reason to believe the certificate really belongs to the user in question.

For instance: John Hawley ( and I ( are good friends in real life. We have sat down face-to-face and confirmed certificates. I know with complete certainty a specific public certificate belongs to him; he knows with complete certainty a different one belongs to me. John also knows H. Peter Anvin ( and has done the same with him. If I need to communicate privately with Peter, I can look him up in the keyserver. Whichever certificate bears an attestation by John, I can trust really belongs to Peter.

Keyserver Design Goals

In the early 1990s we were concerned repressive regimes would attempt to force keyserver operators to replace certificates with different ones of the government’s choosing. (I speak from firsthand experience. I’ve been involved in the PGP community since 1992. I was there for these discussions.) We made a quick decision that keyservers would never, ever, ever, delete information. Keyservers could add information to existing certificates but could never, ever, ever, delete either a certificate or information about a certificate.

To meet this goal, we started running an international network of keyservers. Keyservers around the world would regularly communicate with each other to compare directories. If a government forced a keyserver operator to delete or modify a certificate, that would be discovered in the comparison step. The maimed keyserver would update itself with the content in the good keyserver’s directory. This was a simple and effective solution to the problem of government censorship.

In the early 1990s this design seemed sound. It is not sound in 2019. We’ve known it has problems for well over a decade.

Why Hasn’t It Been Fixed?

There are powerful technical and social factors inhibiting further keyserver development.

  • The software is Byzantine. The standard keyserver software is called SKS, for “Synchronizing Key Server”. A bright fellow named Yaron Minsky devised a brilliant algorithm that could do reconciliations very quickly. It became the keystone of his Ph.D thesis, and he wrote SKS originally as a proof of concept of his idea. It’s written in an unusual programming language called OCaml, and in a fairly idiosyncratic dialect of it at that. This is of course no problem for a proof of concept meant to support a Ph.D thesis, but for software that’s deployed in the field it makes maintenance quite difficult. Not only do we need to be bright enough to understand an algorithm that’s literally someone’s Ph.D thesis, but we need expertise in obscure programming languages and strange programming customs.
  • The software is unmaintained. Due to the above, there is literally no one in the keyserver community who feels qualified to do a serious overhaul on the codebase.
  • Changing a design goal is not the same as fixing a bug. The design goal of the keyserver network is “baked into” essentially every part of the infrastructure. This isn’t a case where there’s a bug that’s inhibiting the keyserver network from functioning correctly. Bugs are generally speaking fairly easy to fix once you know where the problem is. Changing design goals often requires an overhaul of such magnitude it may be better to just start over with a fresh sheet of paper.
  • There is no centralized authority in the keyserver network. The lack of centralized authority was a feature, not a bug. If there is no keyserver that controls the others, there is no single point of failure for a government to go after. On the other hand it also means that even after the software is overhauled and/or rewritten, each keyserver operator has to commit to making the upgrade and stomping out the difficulties that inevitably arise when new software is fielded. The confederated nature of the keyserver network makes changing the design goals even harder than it would normally be—and rest assured, it would normally be very hard!

The Vulnerabilities

The keyserver network is susceptible to a variety of attacks as a consequence of its write-only design. The keyserver network can be thought of as an extremely large, extremely reliable, extremely censorship-resistant distributed filesystem which anyone can write to.

Imagine if Dropbox allowed any Tom, Dick, or Harry to not only put information in your public Dropbox folder, but made it impossible for you to delete it. How would everyone from spammers to child pornographers abuse this?

Many of the same attacks are possible on the keyserver network. We have known about these vulnerabilities for well over a decade. Fixing the keyserver network is, however, problematic for the reasons listed above.

In order to limit the scope of this document a detailed breakdown of only one such vulnerability will be presented (see below).

The Certificate Spamming Attack

Consider public certificates. In order to make them easier to use, they have a list of attestations: statements from other people, represented by their own public certificates, that this certificate really belongs to the individual in question. In my example from before, John Hawley attested to H. Peter Anvin’s certificate. When I looked for H. Peter Anvin’s certificate I checked all the certificates which claimed to belong to him and selected the one John attested as being really his.

These attestations — what we call certificate signatures — can be made by anyone for any purpose. And once made, they never go away. Ever. Even when a certificate signature gets revoked the original remains on the certificate: all that happens is a second signature is affixed saying “don’t trust the previous one I made”.

The OpenPGP specification puts no limitation on how many signatures can be attached to a certificate. The keyserver network handles certificates with up to about 150,000 signatures.

GnuPG, on the other hand … doesn’t. Any time GnuPG has to deal with such a spammed certificate, GnuPG grinds to a halt. It doesn’t stop, per se, but it gets wedged for so long it is for all intents and purposes completely unusable.

My public certificate as found on the keyserver network now has just short of 150,000 signatures on it.

Further, pay attention to that phrase any time GnuPG has to deal with such a spammed certificate. If John were to ask GnuPG to verify my signature on H. Peter Anvin’s certificate, GnuPG would attempt to comply and in the course of business would have to deal with my now-spammed certificate.

The Consequences

We’ve known for a decade this attack is possible. It’s now here and it’s devastating. There are a few major takeaways and all of them are bad.

  • If you fetch a poisoned certificate from the keyserver network, you will break your GnuPG installation.
  • Poisoned certificates cannot be deleted from the keyserver network.
  • The number of deliberately poisoned certificates, currently at only a few, will only rise over time.
  • We do not know whether the attackers are intent on poisoning other certificates.
  • We do not even know the scope of the damage.

That last one requires some explanation. Any certificate may be poisoned at any time, and is unlikely to be discovered until it breaks an OpenPGP installation.

The number one use of OpenPGP today is to verify downloaded packages for Linux-based operating systems, usually using a software tool called GnuPG. If someone were to poison a vendor’s public certificate and upload it to the keyserver network, the next time a system administrator refreshed their keyring from the keyserver network the vendor’s now-poisoned certificate would be downloaded. At that point upgrades become impossible because the authenticity of downloaded packages cannot be verified. Even downloading the vendor’s certificate and re-importing it would be of no use, because GnuPG would choke trying to import the new certificate. It is not hard to imagine how motivated adversaries could employ this against a Linux-based computer network.


At present I (speaking only for myself) do not believe the global keyserver network is salvageable. High-risk users should stop using the keyserver network immediately.

Users who are confident editing their GnuPG configuration files should follow the following process:

  1. Open gpg.conf in a text editor. Ensure there is no line starting with keyserver. If there is, remove it.
  2. Open dirmngr.conf in a text editor. Add the line keyserver hkps:// to the end of it. is a new experimental keyserver which is not part of the keyserver network and has some features which make it resistant to this sort of attack. It is not a drop-in replacement: it has some limitations (for instance, its search functionality is sharply constrained). However, once you make this change you will be able to run gpg --refresh-keys with confidence.


If you know which certificate is likely poisoned, try deleting it: this normally goes pretty quickly. If your OpenPGP installation becomes usable again, congratulations. Acquire a new unpoisoned copy of the certificate and import that.

If you don’t know which certificate is poisoned, your best bet is to get a list of all your certificate IDs, delete your keyrings completely, and rebuild from scratch using known-good copies of the public certificates.


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