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January 18, 2023

The SSL Certificate Issuer Field is a Lie

A surprisingly hard, and widely misunderstood, problem with SSL certificates is figuring out what organization (called a certificate authority, or CA) issued a certificate. This information is useful for several reasons:

  • You've discovered an unauthorized certificate for your domain via Certificate Transparency logs and need to contact the certificate authority to get the certificate revoked.
  • You've discovered a certificate via Certificate Transparency and want to know if it was issued by one of your authorized certificate providers.
  • You're a researcher studying the certificate ecosystem and want to count how many certificates each certificate authority has issued.

On the surface, this looks easy: every certificate contains an issuer field with human-readable attributes, including an organization name. Problem solved, right?

Not so fast: a certificate's issuer field is frequently a lie that tells you nothing about the organization that really issued the certificate. Just look at the certificate chain currently served by

C = IE O = Baltimore OU = CyberTrust CN = Baltimore CyberTrust Root 3ABBE63DAF756C5016B6B85F52015FD8 E8ACBE277C5087B127A60563A841ED8A C = US O = Cloudflare, Inc. CN = Cloudflare Inc ECC CA-3 6E5C90EB2E592F95FABF68AFAF7D05C5 3CBD536EEE7EE2057FDE63704F3E1CA1

According to this, DoorDash's certificate was issued by an intermediate certificate belonging to "Cloudflare, Inc.", which was issued by a root certificate belonging to "Baltimore". Except Cloudflare is not a certificate authority, and Baltimore is a city.

In reality, both DoorDash's certificate and the intermediate certificate were issued by DigiCert, a name which is mentioned nowhere in the above chain. What's going on?

First, Cloudflare has paid DigiCert to create and operate an intermediate certificate with Cloudflare's name in it. DigiCert, not Cloudflare, controls the private key and performs the security-critical validation steps prior to issuance. All Cloudflare does is make an API call to DigiCert. Certificates issued from the "Cloudflare" intermediate are functionally no different from certificates issued from any of DigiCert's other intermediates. This type of white-labeling is common in the certificate industry, since it lets companies appear to be CAs without the expense of operating a CA.

Sidebar: that time everyone freaked out about Blue Coat

In 2016 Symantec created an intermediate certificate with "Blue Coat" in the organization name. This alarmed many non-experts who thought Blue Coat, a notorious maker of TLS interception devices, was now operating a certificate authority. In reality, it was just a white-label Symantec intermediate certificate, operated by Symantec under their normal audits with their normal validation procedures, and it posed no more risk to the Internet than any of the other intermediate certificates operated by Symantec.

What about "Baltimore"? That's short for Baltimore Technologies, a now-defunct infosec company, who acquired GTE's certificate authority subsidiary (named CyberTrust) in 2000, which they then sold to a company named Betrusted in 2003, which merged with TruSecure in 2004, who rebranded back to CyberTrust, which was then acquired by Verizon in 2007, who then sold the private keys for their root certificates to DigiCert in 2015. So "Baltimore" hasn't been accurate since 2003, and the true owner has changed four times since then.

Mergers and acquisitions are common in the certificate industry, and since the issuer name is baked into certificates, the old name can persist long after a different organization takes over. Even once old certificates expire, the acquiring CA might keep using the old name for branding purposes. Consider Thawte, which despite not existing since 1999, could still be found in new certificates as recently as 2017. (Thawte was sold to Verisign, then Symantec, and then DigiCert, who finally stopped putting "Thawte" in the issuer organization name.)

Consequentially, the certificate issuer field is completely useless for human consumption, and causes constant confusion. People wonder why they get Certificate Transparency alerts for certificates issued by "Cloudflare" when their CAA record has only in it. Worse, people have trouble revoking certificates: consider this incident where someone tried to report a compromised private key to the certificate reseller named in the certificate issuer field, who failed to revoke the certificate and then ghosted the reporter. If the compromised key had been reported to the true certificate authority, the CA would have been required to revoke and respond within 24 hours.

I think certificate tools should do a better job helping people understand who issued certificates, so a few years ago I started maintaining a database which maps certificate issuers to their actual organization names. When Cert Spotter sends an alert about an unknown certificate found in Certificate Transparency logs, it shows the name from this database - not the name from the certificate issuer field. It also includes correct contact information for requesting revocation.

As of this month, the same information is available through SSLMate's Certificate Transparency Search API, letting you integrate useful certificate issuer information into your own applications. Here's what the API looks like for the certificate (some fields have been truncated for clarity):

{ "id":"3779499808", "tbs_sha256":"eb3782390d9fb3f3219129212b244cc34958774ba289453a0a584e089d0f2b86", "cert_sha256":"6e5c90eb2e592f95fabf68afaf7d05c53cbd536eee7ee2057fde63704f3e1ca1", "dns_names":["*","",""], "pubkey_sha256":"456d8df5c5b1097c775a778d92f50d49b25720f672fcb0b8a75020fc85110bea", "issuer":{ "friendly_name":"DigiCert", "website":"", "caa_domains":["","","","", ...], "operator":{"name":"DigiCert","website":""}, "pubkey_sha256":"144cd5394a78745de02346553d126115b48955747eb9098c1fae7186cd60947e", "name":"C=US, O=\"Cloudflare, Inc.\", CN=Cloudflare Inc ECC CA-3" }, "not_before":"2022-05-29T00:00:00Z", "not_after":"2023-05-29T23:59:59Z", "revoked":false, "problem_reporting":"Send email to or visit" }

(Here's the API query)

Note the following fields:

  • The friendly_name field contains "DigiCert", not "Cloudflare". This field is useful for displaying to humans.
  • The caa_domains field contains the CAA domains used by the CA. You can compare this array against your domain's CAA record set to determine if the certificate is authorized - at least one of the domains in the array should also be in your CAA record set.
  • The operator field contains details about the company which operates the CA. In this example, the operator name is the same as the friendly name, but later in this post I'll describe an edge case where they are different.
  • The problem_reporting field contains instructions on how to contact the CA to request the certificate be revoked.

The data comes from a few places:

  • The Common CA Database (CCADB)'s AllCertificateRecords report, which is a CSV file listing every intermediate certificate trusted by Apple, Microsoft, or Mozilla. To find out who operates an intermediate certificate, you can look up the fingerprint in the "SHA-256 Fingerprint" column, and then consult the "Subordinate CA Owner" column, or if that's empty, the "CA Owner" column.

  • The CCADB's CAInformationReport, which lists the CAA domains and problem reporting instructions for a subset of CAs.

  • For CAs not listed in CAInformationReport, the information comes from the CA's Certificate Policy (CP) and Certificate Practice Statement (CPS), a pair of documents which describe how the CA is operated. The URL of the applicable CP and CPS can be found in AllCertificateRecords. Section 1.5.2 of the CPS contains problem reporting instructions, and Section 4.2 of either the CP or CPS lists the CAA domains.

In a few cases I've manually curated the data to be more helpful. The most notable example is Amazon Certificate Manager. When you get a certificate through ACM, it's issued by DigiCert from a white-label intermediate certificate with "Amazon" in its name, similar to Cloudflare. However, Amazon has gone several steps further than Cloudflare in white-labeling:

  • To authorize ACM certificates, you put in your CAA record, not

  • Amazon operates their own root certificates which have signed the white-label intermediates operated by DigiCert. This is highly unusual. Recall that the DigiCert-operated Cloudflare intermediate is signed by a DigiCert-operated root, as is typical for white-label intermediates. (Why does Amazon operate roots whose sole purpose is to cross-sign intermediates operated by another CA? I assume it was to get to market more quickly. I have no clue why they are still doing things this way after 8 years.)

If you look up one of Amazon's intermediates in AllCertificateRecords, it will say that it is operated by DigiCert. But due to the extreme level of white-labeling, I think telling users that ACM certificates were issued by "DigiCert" would cause more confusion than saying they were issued by "Amazon". So here's what SSLMate's CT Search API returns for an ACM certificate:

{ "id":"3837618459", "tbs_sha256":"9c312eef7eb0c9dccc6b310dcd9cf6be767b4c5efeaf7cb0ffb66b774db9ca52", "cert_sha256":"7e5142891ca365a79aff31c756cc1ac7e5b3a743244d815423da93befb192a2e", "dns_names":["","","", ...], "pubkey_sha256":"8c296c2d2421a34cf2a200a7b2134d9dde3449be5a8644224e9325181e9218bd", "issuer":{ "friendly_name":"Amazon", "website":"", "caa_domains":["","","","",""], "operator":{"name":"DigiCert","website":""}, "pubkey_sha256":"252333a8e3abb72393d6499abbacca8604faefa84681ccc3e5531d44cc896450", "name":"C=US, O=Amazon, OU=Server CA 1B, CN=Amazon" }, "not_before":"2022-06-13T00:00:00Z", "not_after":"2023-06-11T23:59:59Z", "revoked":false, "problem_reporting":"Send email to or visit" }

(API query)

As you can see, friendly_name and website refer to Amazon. However, the problem_reporting field tells you to contact DigiCert, and the operator field makes clear that the issuer is really operated by DigiCert.

I've overridden a few other cases as well. Whenever a certificate issuer uses a distinct set of CAA domains, I override the friendly name to match the domains. My reasoning is that CAA and Certificate Transparency are often used in conjunction - a site operator might first publish CAA records, and then monitor Certificate Transparency to detect violations of their CAA records. Or, they might first use Certificate Transparency to figure out who their certificate authorities are, and then publish matching CAA records. Thus, ensuring consistency between CAA and CT provides the best experience. In fact, the certificate authority names that you see on SSLMate's CAA Record Helper are the exact same values you can see in the friendly_name field.

If you're looking for a certificate monitoring solution, consider Cert Spotter, which notifies you when certificates are issued for your domains, or SSLMate's Certificate Transparency Search API, which lets you search Certificate Transparency logs by domain name.


January 10, 2023

whoarethey: Determine Who Can Log In to an SSH Server

Filippo Valsorda has a neat SSH server that reports the GitHub username of the connecting client. Just SSH to, and if you're a GitHub user, there's a good chance it will identify you. This works because of two behaviors: First, GitHub publishes your authorized public keys at Second, your SSH client sends the server the public key of every one of your key pairs.

Let's say you have three key pairs, FOO, BAR, and BAZ. The SSH public key authentication protocol works like this:

Client: Can I log in with public key FOO?
Server looks for FOO in ~/.ssh/authorized_keys; finds no match
Server: No
Client: Can I log in with public key BAR?
Server looks for BAR in ~/.ssh/authorized_keys; finds no match
Server: No
Client: Can I log in with public key BAZ?
Server looks for BAZ in ~/.ssh/authorized_keys; finds an entry
Server: Yes
Client: OK, here's a signature from private key BAZ to prove I own it works by taking each public key sent by the client and looking it up in a map from public key to GitHub username, which Filippo populated by crawling the GitHub API. If it finds a match, it tells the client the GitHub username:

Client: Can I log in with public key FOO?
Server looks up FOO, finds no match
Server: No
Client: Can I log in with public key BAR?
Server looks up BAR, finds no match
Server: No
Client: Can I log in with public key BAZ?
Server looks up BAZ, finds a match to user AGWA
Server: Aha, you're AGWA!

This works the other way as well: if you know that AGWA's public keys are FOO, BAR, and BAZ, you can send each of them to the server to see if the server accepts any of them, even if you don't know the private keys:

Client: Can I log in with public key FOO?
Server: No
Client: Can I log in with public key BAR?
Server: No
Client: Can I log in with public key BAZ?
Server: Yes
Client: Aha, AGWA has an account on this server!

This behavior has several implications:

  1. If you've found a server that you suspect belongs to a particular GitHub user, you can confirm it by downloading their public keys and asking if the server accepts any of them.

  2. If you want to find servers belonging to a particular GitHub user, you could scan the entire IPv4 address space asking each SSH server if it accepts any of the user's keys. This wouldn't work with IPv6, but scanning every IPv4 host is definitely practical, as shown by masscan and zmap.

  3. If you've found a server and want to find out who controls it, you can try asking the server about every GitHub user's keys until it accepts one of them. I'm not sure how practical this would be; testing every GitHub user's keys would require sending an enormous amount of traffic to the server.

As a proof of concept, I've created whoarethey, a small Go program that takes the hostname:port of an SSH server, an SSH username, and a list of GitHub usernames, and prints out the GitHub username which is authorized to connect to the server. For example, you can try it on a test server of mine:

$ whoarethey root github:AGWA github:FiloSottile github:AGWA

whoarethey reports that I, but not Filippo, can log into root@

You can also use whoarethey with public key files stored locally, in which case it prints the name of the public key file which is accepted:

$ whoarethey root

Note that just because a server accepts a key (or claims to accept a key), it doesn't mean that the holder of the private key authorized the server to accept it. I could take Filippo's public key and put it in my authorized_keys file, making it look like Filippo controls my server. Therefore, this information leak doesn't provide incontrovertible proof of server control.

Nevertheless, I think it's a useful way to deanonymize a server, and it concerns me much more than I only SSH to servers which already know who I am, and I'm not very worried about being tricked into connecting to a malicious server - it's not like the Web where it's trivial to make someone visit a URL. However, I do have accounts on a few servers which are not otherwise linkable to me, and it came as an unpleasant surprise that anyone would be able to learn that I have an account just by asking the SSH server.

The simplest way to thwart whoarethey would be for SSH servers to refuse to answer if a particular public key would be accepted, and instead make clients pick a private key and send the signature to the server. Although I don't know of any SSH servers that can be configured to do this, it could be done within the bounds of the current SSH protocol. The user experience would be the same for people who use a single key per client, which I assume is the most common configuration. Users with multiple keys would need to tell their client which key they want to use for each server, or the client would have to try every key, which might require the user to enter a passphrase or press a physical button for each attempt. (Note that to prevent a timing leak, the server should verify the signature against the public key provided by the client before checking if the public key is authorized. Otherwise, whoarethey could determine if a public key is authorized by sending an invalid signature and measuring how long it takes the server to reject it.)

There's a more complicated solution (requiring protocol changes and fancier cryptography) that leverages private set intersection to thwart both whoarethey and However, it treats SSH keys as encryption keys instead of signing keys, so it wouldn't work with hardware-backed keys like the YubiKey. And it requires the client to access the private key for every key pair, not just the one accepted by the server, so the user experience for multi-key users would be just as bad as with the simple solution.

Until one of the above solutions is implemented, be careful if you administer any servers which you don't want linked to you. You could use unique key pairs for such servers, or keep SSH firewalled off from the Internet and connect over a VPN. If you do use a unique key pair, make sure your SSH client never tries to send it to other servers - a less benign version of could save the public keys that it sees, and then feed them to whoarethey to find your servers.


December 12, 2022

No, Google Did Not Hike the Price of a .dev Domain from $12 to $850

It was perfect outrage fodder, quickly gaining hundreds of upvotes on Hacker News:

As you know, domain extensions like .dev and .app are owned by Google. Last year, I bought the domain for one of our projects. When I tried to renew it this year, I was faced with a renewal price of $850 instead of the normal price of $12.

It's true that most .dev domains are just $12/year. But this person never paid $12 for According to his own screenshots, he paid 4,360 Turkish Lira for the initial registration on December 6, 2021, which was $317 at the time. So yes, the price did go up, but not nearly as much as the above comment implied.

According to a Google worker, this person should have paid the same, higher price in 2021, since is a "premium" domain, but got an extremely favorable exchange rate so he ended up paying less. That's unsurprising for a currency which is experiencing rampant inflation.

Nevertheless, domain pricing has become quite confusing in recent years, and when reading the ensuing Hacker News discussion, I learned that a lot of people have some major misconceptions about how domains work. Multiple people said untrue or nonsensical things along the lines of "Google has a monopoly on the .dev domain. GoDaddy doesn't have a monopoly on .com, .biz, .net, etc." So I decided to write this blog post to explain some basic concepts and demystify domain pricing.

Registries vs Registrars

If you want to have an informed opinion about domains, you have to understand the difference between registries and registrars.

Every top-level domain (.com, .biz, .dev, etc.) is controlled by exactly one registry, who is responsible for the administration of the TLD and operation of the TLD's DNS servers. The registry effectively owns the TLD. Some registries are:


Registries do not sell domains directly to the public. Instead, registrars broker the transaction between a domain registrant and the appropriate registry. Registrars include Gandi, GoDaddy, Google, Namecheap, and Companies can be both registries and registrars: e.g. GoDaddy and Google are registrars for many TLDs, but registries for only some TLDs.

When you buy or renew a domain, the bulk of your registration fee goes to the registry, with the registrar adding some markup. Additionally, 18 cents goes to ICANN (Internet Corporation for Assigned Names and Numbers), who is in charge of the entire domain system.

For example, Google's current .com price of $12 is broken down as follows:

$0.18ICANN fee
$8.97Verisign's registry fee
$2.85Google's registrar markup

Registrars typically carry domains from many different TLDs, and TLDs are typically available through multiple registrars. If you don't like your registrar, you can transfer your domain to a different one. This keeps registrar markup low. However, you'll always be stuck with the same registry. If you don't like their pricing, your only recourse is to get a whole new domain with a different TLD, which is not meaningful competition.

At the registry level, it's not true that there is no monopoly on .com - Verisign has just as much of a monopoly on .com as Google has on .dev.

At the registrar level, Google holds no monopoly over .dev - you can buy .dev domains through registrars besides Google, so you can take your business elsewhere if you don't like the Google registrar. Of course, the bulk of your fee will still go to Google, since they're the registry.

ICANN Price Controls

So if .com is just as monopoly-controlled as .dev, why are all .com domains the same low price? Why are there no "premium" domains like with .dev?

It's not because Verisign is scared by the competition, since there is none. It's because Verisign's contract with ICANN is different from Google's contract with ICANN.

The .com registry agreement between Verisign and ICANN capped the price of .com domains at $7.85 in 2020, with at most a 7% increase allowed every year. Verisign has since imposed two 7% price hikes, putting the current price at $8.97.

In contrast, .dev is governed by ICANN's standard registry agreement, which has no price caps. It does, however, forbid "discriminatory" renewal pricing:

In addition, Registry Operator must have uniform pricing for renewals of domain name registrations ("Renewal Pricing"). For the purposes of determining Renewal Pricing, the price for each domain registration renewal must be identical to the price of all other domain name registration renewals in place at the time of such renewal, and such price must take into account universal application of any refunds, rebates, discounts, product tying or other programs in place at the time of renewal. The foregoing requirements of this Section 2.10(c) shall not apply for (i) purposes of determining Renewal Pricing if the registrar has provided Registry Operator with documentation that demonstrates that the applicable registrant expressly agreed in its registration agreement with registrar to higher Renewal Pricing at the time of the initial registration of the domain name following clear and conspicuous disclosure of such Renewal Pricing to such registrant, and (ii) discounted Renewal Pricing pursuant to a Qualified Marketing Program (as defined below). The parties acknowledge that the purpose of this Section 2.10(c) is to prohibit abusive and/or discriminatory Renewal Pricing practices imposed by Registry Operator without the written consent of the applicable registrant at the time of the initial registration of the domain and this Section 2.10(c) will be interpreted broadly to prohibit such practices.

This means that Google is only allowed to increase a domain's renewal price if it also increases the renewal price of all other domains. If Google wants to charge more to renew a "premium" domain, the higher price must be clearly and conspicuously disclosed to the registrant at time of initial registration. This prevents Google from holding domains hostage: they can't set a low price and later increase it after your domain becomes popular.

(By displaying prices in lira instead of USD for, did Google violate the "clear and conspicuous" disclosure requirement? I'm not sure, but if I were a registrar I would display prices in the currency charged by the registry to avoid misunderstandings like this.)

I wouldn't assume that the .com price caps will remain forever. .org used to have price caps too, before switching to the standard registry agreement in 2019. But even if .com switched to the standard agreement, we probably wouldn't see "premium" .com domains: at this point, every .com domain which would be considered "premium" has already been registered. And Verisign wouldn't be allowed to increase the renewal price of already-registered domains due to the need for disclosure at the time of initial registration.

There ain't no rules for ccTLDs (.io, .tv, .au, etc.)

It's important to note that registries for country-code TLDs (which is every 2-letter TLD) do not have enforceable registry agreements with ICANN. Instead, they are governed by their respective countries (or similar political entities), which can do as they please. They can sucker you in with a low price and then hold your domain hostage when it gets popular. If you register your domain in a banana republic because you think the TLD looks cool, and el presidente wants your domain to host his cat pictures, tough luck.

This is only scratching the surface of what's wrong with ccTLDs, but that's a topic for another blog post. Suffice to say, I do not recommend using ccTLDs unless all of the following are true:

  • You live in the country which owns the ccTLD and don't plan on moving.
  • You don't expect the region where you live to secede from the political entity which owns the ccTLD. (Just ask the British citizens who had .eu domains.)
  • You trust the operator of the ccTLD to be fair and competent.

Further Complications

To make matters more confusing, sometimes when you buy a domain from a registrar, you're not getting it from the registry, but from an existing owner who is squatting the domain. In this case, you pay a large upfront cost to get the squatter to transfer the domain to you, after which the domain renews at the lower, registry-set price. It used to be fairly obvious when this was happening, as you'd transact directly with the squatter, but now several registrars will broker the transaction for you. The Google registrar calls these "aftermarket" domains, which I think is a good name, but other registrars call them "premium" domains, which is confusing because such domains may or may not be considered "premium" by the registry and subject to higher renewal prices.

Yet another confounding factor is that registrars sometimes steeply discount the initial registration fee, taking a loss in the hope of making it up with renewals and other services.

To sum up, there are multiple scenarios you may face when buying a domain:

ScenarioInitial FeeRenewal Fee
Non-premium domain, no discount$$$$
Non-premium domain, first year discount$$$
Premium domain, no discount$$$$$$
Premium domain, first year discount$$$$$
Aftermarket non-premium domain$$$$$$
Aftermarket premium domain$$$$$$$
ccTLD domainVariesSky's the limit!

I was curious how different registrars distinguish between these cases, so I tried searching for the following domains at Gandi, GoDaddy, Google, Namecheap, and

  • and - decidedly non-premium domains
  • - premium domain
  • - aftermarket domain


Non-premium domain, no discount:

Screenshot of Gandi showing price of $17.75/year

Non-premium domain, first year discount:

Screenshot of Gandi showing 1 year price of $4.50 then $30.46/year

Premium domain:

Screenshot of Gandi showing price of $779.05/year

Gandi does not seem to sell aftermarket domains.


Non-premium domain, first year discount:

Screenshot of GoDaddy showing $19.99 crossed out, followed by $0.01 for the first year with a 2 year registration Screenshot of GoDaddy showing $49.99 crossed out, followed by $1.99 for the first year

Premium domain:

Screenshot of GoDaddy showing $929.99 in large text, then $929.99/yr when you renew

Aftermarket domain:

Screenshot of GoDaddy showing $4,888 + $19.99/yr


Non-premium domain, no discount:

Screenshot of Google showing $12/year

Premium domain:

Screenshot of Google showing $720/year

Aftermarket domain:

Screenshot of Google showing $4,900 + $12/year


Non-premium domain, first year discount:

Screenshot of Namecheap showing $7.98/yr in black text, and then in gray text Retail $13.98/yr Screenshot of Namecheap showing $1.88/yr in black text, and then in gray text Retail $32.98/yr

Premium domain:

Screenshot of Namecheap showing $843.70

Aftermarket domain:

Screenshot of Namecheap showing $4,888.00 in black text, then in gray text Renews at $14.58/yr

Non-premium domain, first year discount:

Screenshot of showing $34.99 in gray text and crossed out, followed by $1.99

Premium domain:

Screenshot of showing $811.25 in black text, then in gray text RENEWAL: $811.25

Aftermarket domain:

Screenshot of showing $5,621.20 in black text, then in gray text RENEWAL: $15.99


I think Gandi and Google do the best job conveying the first year and renewal prices using clear and consistent UI. Namecheap is the worst, only showing a clear renewal price when it's less than the initial price, but obscuring it when it's the same or higher (note the use of the term "Retail" instead of "Renews at", and the lack of a "/yr" suffix for the price). also obscures the renewal price for (I very much doubt it's $1.99). GoDaddy also fails to show a clear renewal price for the non-premium domains, but at least says the quoted price is "for the first year."

My advice is to pay very close attention to the renewal price when buying a domain, because it may be the same, lower, or higher than the first year's fee. And be very wary of 2-letter TLDs (ccTLDs).


December 1, 2022

Checking if a Certificate is Revoked: How Hard Can It Be?

This wasn't my first rodeo so I knew it would be hard. And I was right! The only question was what flavor of dysfunction I'd be encountering.

SSLMate's Certificate Transparency Search API now returns two new fields that tell you if, why, and when the certificate was revoked:

"revoked":true, "revocation":{"time":"2021-10-27T21:38:48Z","reason":0,"checked_at":"2022-10-18T14:49:56Z"},

(See the complete API response)

This simple-sounding feature was obnoxious to implement, and required dealing with some amazingly creative screwups by certificate authorities, and a clunky system called the Common CA Database that's built on Salesforce. Just how dysfunctional is the WebPKI? Buckle up and find out!

Background on Certificate Revocation

There are two ways for a CA to publish that a certificate is revoked: the online certificate status protocol (OCSP), and certificate revocation lists (CRLs).

With OCSP, you send an HTTP request to the CA's OCSP server asking, "hey is the certificate with this serial number revoked?" and the CA is supposed to respond "yeah" or "nah", but often responds with "I dunno" or doesn't respond at all. CAs are required to support OCSP, and it's easy to find a CA's OCSP server (the URL is included in the certificate itself) but I didn't want to use it for the CT Search API: each API responses can contain up to 100 certificates, so I'd have to make up to 100 OCSP requests just to build a single response. Given the slowness and unreliability of OCSP, that was a no go.

With CRLs, the CA publishes one or more lists of all revoked serial numbers. This would be much easier to deal with: I could write a cron job to download every CRL, insert all the entries into my trusty PostgreSQL database, and then building a CT Search API response would be as simple as JOINing with the crl_entry table!

Historically, CRLs weren't an option because not all CAs published CRLs, but on October 1, 2022, both Mozilla and Apple began requiring all CAs in their root programs to publish CRLs. Even better, they required CAs to disclose the URLs of their CRLs in the Common CA Database (CCADB), which is available to the public in the form of a chonky CSV file. Specifically, two new columns were added to the CSV: "Full CRL Issued By This CA", which is populated with a URL if the CA publishes a single CRL, and "JSON Array of Partitioned CRLs", which is populated with a JSON array of URLs if the CA splits its list of revoked certificates across multiple CRLs.

So I got to work writing a cron job in Go that would 1) download and parse the CCADB CSV file to determine the URL of every CRL 2) download, parse, and verify every CRL and 3) insert the CRL entries into PostgreSQL.

How hard could this be?

This wasn't my first rodeo so I knew it would be hard. And I was right! The only question was what flavor of dysfunction I'd be encountering.


The CCADB is a database run by Mozilla that contains information about publicly-trusted certificate authorities. The four major browser makers (Mozilla, Apple, Chrome, and Microsoft) use the CCADB to keep track of the CAs which are trusted by their products.

CCADB could be a fairly simple CRUD app, but instead it's built on Salesforce, which means it's actual crud. CAs use a clunky enterprise-grade UI to update their information, such as to disclose their CRLs. Good news: there's an API. Bad news: here's how to get API credentials:

Salesforce will redirect to the callback url (specified in 'redirect_uri'). Quickly freeze the loading of the page and look in the browser address bar to extract the 'authorization code', save the code for the next steps.

To make matters worse, CCADB's data model is wrong (it's oriented around certificates rather than subject+key) which means the same information about a CA needs to be entered in multiple places. There is very little validation of anything a CA inputs. Consequentially, the information in the CCADB is often missing, inconsistent, or just flat out wrong.

In the "Full CRL Issued By This CA" column, I saw:

  • URLs without a protocol
  • Multiple URLs
  • The strings "expired" and "revoked"

Meanwhile, the data for "JSON Array of Partitioned CRLs" could be divided into three categories:

  • The empty array ([]).
  • A comma-separated list of URLs, with no square brackets or quotes.
  • A comma-separated list of URLs, with square brackets but without quotes.

In other words, the only well-formed JSON in sight was the empty array.

Initially, I assumed that CAs didn't know how to write non-trivial JSON, because that seems like a skill they would struggle with. Turned out that Salesforce was stripping quotes from the CSV export. OK, CAs, it's not your fault this time. (Well, except for the one who left out square brackets.) But don't get too smug, CAs - we haven't tried to download your CRLs yet.

(The CSV was eventually fixed, but to unblock my progress I had to parse this column with a mash of strings.Trim and strings.Split. Even Mozilla had to resort to such hacks to parse their own CSV file.)

CAs Suck

Once I got the CCADB CSV parsed successfully, it was time to download some CRLs! Surely, this would be easy - even though CRLs weren't mandatory before October 1, the vast majority of CAs had been publishing CRLs for years, and plenty of clients were already consuming them. Surely, any problems would have been discovered and fixed by now, right?

Ah hah hah hah hah.

I immediately ran into some fairly basic issues, like Amazon's CRLs returning a 404 error, D-TRUST encoding CRLs as PEM instead of DER, or Sectigo disclosing a CRL with a non-existent hostname because they forgot to publish a DNS record, as well as some more... interesting issues:


Since root certificate keys are kept offline, CRLs for root certificates have to be generated manually during a signing ceremony. Signing ceremonies are extremely serious affairs that involve donning ceremonial robes, entering a locked cage, pulling a laptop out of a safe, and manually running openssl commands based on a script - and not the shell variety, but the reams of dead tree variety. Using the openssl command is hell in the best of circumstances - now imagine doing it from inside a cage. The smarter CAs write dedicated ceremony tooling instead of using openssl. The rest bungle ceremonies on the regular, as GoDaddy did here when they generated CRLs with an obsolete version number and missing a required extension, which consequentially couldn't be parsed by Go. To GoDaddy's credit, they are now planning to switch to dedicated ceremony tooling. Sometimes things do get better!


Instead of setting the CRL signature algorithm based on the algorithm of the issuing CA's key, GlobalSign was setting it based on the algorithm of the issuing CA's signature. So when an elliptic curve intermediate CA was signed by an RSA root CA, the intermediate CA would produce CRLs that claimed to have RSA signatures even though they were really elliptic curve signatures.

After receiving my report, GlobalSign fixed their logic and added a test case.

Google Trust Services

Here is the list of CRL revocation reason codes defined by RFC 5280:

CRLReason ::= ENUMERATED { unspecified (0), keyCompromise (1), cACompromise (2), affiliationChanged (3), superseded (4), cessationOfOperation (5), certificateHold (6), -- value 7 is not used removeFromCRL (8), privilegeWithdrawn (9), aACompromise (10) }

And here is the protobuf enum that Google uses internally for revocation reasons:


As you can see, the reason code for unspecified is 0, and the protobuf enum value for unspecified is 1. The reason code for keyCompromise is 1 and the protobuf enum value for keyCompromise is 2. Therefore, by induction, all reason codes are exactly one less than the protobuf enum value. QED.

That was the logic of Google's code, which generated CRL reason codes by subtracting one from the protobuf enum value, instead of using a lookup table or switch statement. Of course, when it came time to revoke a certificate for the reason "privilegeWithdrawn", this resulted in a reason code of 7, which is not a valid reason code. Whoops.

At least this bug only materialized a few months ago, unlike most of the other CAs mentioned here, who had been publishing busted CRLs for years.

After receiving my report, Google fixed the CRL and added a test case, and will contribute to CRL linting efforts.


There are still some problems that I haven't investigated yet, but at this point, SSLMate knows the revocation status of the vast majority of publicly-trusted SSL certificates, and you can access it with just a simple HTTP query.

If you need to programmatically enumerate all the SSL certificates for a domain, such as to inventory your company's SSL certificates, then check out SSLMate's Certificate Transparency Search API. I don't know of any other service that pulls together information from over 40 Certificate Transparency logs and 3,500+ CRLs into one JSON API that's queryable by domain name. Best of all, I stand between you and all the WebPKI's dysfunction, so you can work on stuff you actually like, instead of wrangling CSVs and debugging CRL parsing errors.


May 18, 2022

Parsing a TLS Client Hello with Go's cryptobyte Package

In my original post about SNI proxying, I showed how you can parse a TLS Client Hello message (the first message that the client sends to the server in a TLS connection) in Go using an amazing hack that involves calling tls.Server with a read-only net.Conn wrapper and a GetConfigForClient callback that saves the tls.ClientHelloInfo argument. I'm using this hack in snid, and if you accessed this blog post over IPv4, it was used to route your connection.

However, it's pretty gross, and only gives me access to the parts of the Client Hello message that are exposed in the tls.ClientHelloInfo struct. So I've decided to parse the Client Hello properly, using the package, which is a great library that makes it easy parse length-prefixed binary messages, such as those found in TLS.

cryptobyte was added to Go's quasi-standard x/crypto library in 2017. Since then, more and more parts of Go's TLS and X.509 libraries have been updated to use cryptobyte for parsing, often leading to significant performance gains.

In this post, I will show you how to use cryptobyte to parse a TLS Client Hello message, and introduce, an HTTP server that returns a JSON representation of the Client Hello message sent by your client.

Using cryptobyte

The cryptobyte parser is centered around the cryptobyte.String type, which is just a slice of bytes that points to the message that you are parsing:

type String []byte

cryptobyte.String contains methods that read a part of the message and advance the slice to point to the next part.

For example, let's say you have a message consisting of a variable-length string prefixed by a 16-bit big-endian length, followed by a 32-bit big-endian integer:

00 06 'A' 'n' 'd' 'r' 'e' 'w' 00 5A F6 0C
len(name) name id

First, you create a cryptobyte.String variable, message, which points to the above bytes.

Then, to read the name, you use ReadUint16LengthPrefixed:

var name cryptobyte.String message.ReadUint16LengthPrefixed(&name)

ReadUint16LengthPrefixed reads two things. First, it reads the 16-bit length. Second, it reads the number of bytes specified by the length. So, after the above function call, name points to the 6 byte string "Andrew", and message is mutated to point to the remaining 4 bytes containing the ID.

To read the ID, you use ReadUint32:

var id uint32 message.ReadUint32(&id)

After this call, id contains 5961228 (0x5AF60C) and message is empty.

Note that cryptobyte.String's methods return a bool indicating if the read was successful. In real code, you'd want to check the return value and return an error if necessary.

It's also a good idea to call Empty to make sure that the string is really empty at the end, so you can detect and reject trailing garbage.

cryptobyte.String's methods are generally zero-copy. In the above example, name will point to the same memory region which message originally pointed to. This makes cryptobyte very efficient.

Parsing the TLS Client Hello

Let's write a function that takes the bytes of a TLS Client Hello handshake message as input, and returns a struct with info about the TLS handshake:

func UnmarshalClientHello(handshakeBytes []byte) *ClientHelloInfo

We start by constructing a cryptobyte.String from handshakeBytes:

handshakeMessage := cryptobyte.String(handshakeBytes)

For guidance, we turn to Section 4 of RFC 8446, which describes TLS 1.3's handshake protocol.

Here's the definition of a handshake message: struct { HandshakeType msg_type; /* handshake type */ uint24 length; /* remaining bytes in message */ select (Handshake.msg_type) { case client_hello: ClientHello; case server_hello: ServerHello; case end_of_early_data: EndOfEarlyData; case encrypted_extensions: EncryptedExtensions; case certificate_request: CertificateRequest; case certificate: Certificate; case certificate_verify: CertificateVerify; case finished: Finished; case new_session_ticket: NewSessionTicket; case key_update: KeyUpdate; }; } Handshake;

The first field in the message is a HandshakeType, which is an enum defined as:

enum { client_hello(1), server_hello(2), new_session_ticket(4), end_of_early_data(5), encrypted_extensions(8), certificate(11), certificate_request(13), certificate_verify(15), finished(20), key_update(24), message_hash(254), (255) } HandshakeType;

According to the above definition, a Client Hello message has a value of 1. The last entry of the enum specifies the largest possible value of the enum. In TLS, enums are transmitted as a big-endian integer using the smallest number of bytes needed to represent the largest possible enum value. That's 255, so HandshakeType is transmitted as an 8-bit integer. Let's read this integer and verify that it's 1:

var messageType uint8 if !handshakeMessage.ReadUint8(&messageType) || messageType != 1 { return nil }

The second field, length, is a 24-bit integer specifying the number of bytes remaining in the message.

The third and last field depends on the type of handshake message. Since it's a Client Hello message, it has type ClientHello.

Let's read these two fields using ReadUint24LengthPrefixed and then make sure there are no bytes remaining in handshakeMessage:

var clientHello cryptobyte.String if !handshakeMessage.ReadUint24LengthPrefixed(&clientHello) || !handshakeMessage.Empty() { return nil }

clientHello now points to the bytes of the ClientHello structure, which is defined in Section 4.1.2 as follows:

struct { ProtocolVersion legacy_version; Random random; opaque legacy_session_id<0..32>; CipherSuite cipher_suites<2..2^16-2>; opaque legacy_compression_methods<1..2^8-1>; Extension extensions<8..2^16-1>; } ClientHello;

The first field is legacy_version, whose type is defined as a 16-bit integer:

uint16 ProtocolVersion;

To read it, we do:

var legacyVersion uint16 if !clientHello.ReadUint16(&legacyVersion) { return nil }

Next, random, whose type is defined as:

opaque Random[32];

That means it's an opaque sequence of exactly 32 bytes. To read it, we do:

var random []byte if !clientHello.ReadBytes(&random, 32) { return nil }

Next, legacy_session_id. Like random, it is an opaque sequence of bytes, but this time the RFC specifies the length as a range, <0..32>. This syntax means it's a variable-length sequence that's between 0 and 32 bytes long, inclusive. In TLS, the length is transmitted just before the byte sequence as a big-endian integer using the smallest number of bytes necessary to represent the largest possible length. In this case, that's one byte, so we can read legacy_session_id using ReadUint8LengthPrefixed:

var legacySessionID []byte if !clientHello.ReadUint8LengthPrefixed((*cryptobyte.String)(&legacySessionID)) { return nil }

Now we're on to cipher_suites, which is where things start to get interesting. As with legacy_session_id, it's a variable-length sequence, but rather than being a sequence of bytes, it's a sequence of CipherSuites, which is defined as a pair of 8-bit integers:

uint8 CipherSuite[2];

In TLS, the length of the sequence is specified in bytes, rather than number of items. For cipher_suites, the largest possible length is just shy of 2^16, which means a 16-bit integer is used, so we'll use ReadUint16LengthPrefixed to read the cipher_suites field:

var ciphersuitesBytes cryptobyte.String if !clientHello.ReadUint16LengthPrefixed(&ciphersuitesBytes) { return nil }

Now we can iterate to read each item:

for !ciphersuitesBytes.Empty() { var ciphersuite uint16 if !ciphersuitesBytes.ReadUint16(&ciphersuite) { return nil } // do something with ciphersuite, like append to a slice }

Next, legacy_compression_methods, which is similar to legacy_session_id:

var legacyCompressionMethods []uint8 if !clientHello.ReadUint8LengthPrefixed((*cryptobyte.String)(&legacyCompressionMethods)) { return nil }

Finally, we reach the extensions field, which is another variable-length sequence, this time containing the Extension struct, defined as:

struct { ExtensionType extension_type; opaque extension_data<0..2^16-1>; } Extension;

ExtensionType is an enum with maximum value 65535 (i.e. a 16-bit integer).

As with cipher_suites, we read all the bytes in the field into a cryptobyte.String:

var extensionsBytes cryptobyte.String if !clientHello.ReadUint16LengthPrefixed(&extensionsBytes) { return nil }

Since this is the last field, we want to make sure clientHello is now empty:

if !clientHello.Empty() { return nil }

Now we can iterate to read each Extension item:

for !extensionsBytes.Empty() { var extType uint16 if !extensionsBytes.ReadUint16(&extType) { return nil } var extData cryptobyte.String if !extensionsBytes.ReadUint16LengthPrefixed(&extData) { return nil } // Parse extData according to extType }

And that's it! You can see working code, including parsing of several common extensions, in my tlshacks package.

To test this out, I wrote an HTTP server that returns a JSON representation of the Client Hello. This is rather handy for checking what ciphers and extensions a client supports. You can check out what your client's Client Hello looks like at

Making the Client Hello message available to an HTTP handler required some gymnastics, including writing a net.Conn wrapper struct that peeks at the first TLS handshake message and saves it in the struct, and then a ConnContext callback that grabs the saved message out of the wrapper struct and makes it available in the request's context. You can read the code if you're curious.

I'm happy to say that deploying this HTTP server was super easy thanks to snid. This service cannot run behind an HTTP reverse proxy - it has to terminate the TLS connection itself. Without snid, I would have needed to use a dedicated IPv4 address.


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