Put your CAA in DNS!

There are hundreds of public, trusted* certificate authorities (CAs) in the world. These CAs have had their root CA Certificate published into the Trust Store of many solutions that the world uses. These Trust Stores include widely used web browsers (like the one you’re using now), to the various programming language run times, and individuals operating systems.

A trust store is literally a store of certificates which are deemed trusted. While users can edit their trust store, or make their own, they come with a set that have been selected by your software vendor. Sometimes these are manipulated in the corporate environment to include a company Certificate Authority, or remove specific distrusted authorities.

Over time, some CAs fall into disrepute, and eventually software distributors will issue updates that remove a rouge CA. Of course, issuing an update for systems that the public never apply doesn’t change much in the short term (tip: patch your environments, including the trust store).

Like all x509 certificates the CA root certificates have an expiry, typically over a very long 20+year period, and before expiry, much effort is put into creating a new root Certificate and having it issued distributed and updated in deployed applications.

Legitimate public certificate authorities are required to undertake some mandatory checks when they issue their certificates to their customers. These checks are called the Baseline Requirements, and are governed by the Browser/CA Forum industry body. CAs that are found to be flouting the Baseline Requirements are expelled from the Browser/CA Forum, and subsequently, most software distributions then remove them from their products (sometimes retrospectively via patches as mentioned above).

Being a Certificate Authority has been a lucrative business over the years. In the early days, it was enough to make Mark Shuttleworth a tidy packet with Thawte – enough for him to become a very early Space Tourist, and then start Canonical. With a trusted CA Root certificate widely adopted, a CA can then issue certificates for whatever they wish to charge.

What’s important to note though, is that any certificate in use has no bearing on the strength of encryption or negotiation protocol being used when a client connects to an HTTPS service. The only thing a CA-issued certificate gives you is a reasonably strong validation that the controller of the DNS name you’re connecting to has validate themselves to the CA vetting process.

It doesn’t tell you that the other end of your connection is someone you can TRUST, but you can reasonably TRUST that a given Certificate Authority thinks the entity at the other end of your connection may be the controller of their DNS (in Domain Validated (DV) certificates). Why reasonably? Well what if the controll erof the web site you’re trying to talk to accidentally published their PRIVATE key somewhere; a scammer could then set up a site that may look legitimate, poison some DNS or control a network segment your traffic routes over….

When a CA issues a certificate, it adds a digital signature (typically RSA based) around the originating certificate request. With in the certificate data are the various fields about the subject of the certificate, as well as information about who the issuer is, including a fingerprint (hash) of the issuer’s public certificate.

Previously CAs would issue certificates with an MD5 of their certificate. MD5 was replaced with SHA1, and around 2014, SHA1 was replaced with SHA2-256.

This signature algorithm is effectively the strength of the trust between the issuing CA, and the subjects certificate that you see on a web site. RSA gets very slow as key sizes get larger; today’s services typically use RSA at 2048 bits, which is currently strong enough to be deemed secure, and fast enough not to be a major performance overhead; make that 4096 bits and its another story.

Not only is the RSA algorithm being replaced, but eventually the SHA2-256 will be as well. The replacement for RSA is likely to be Eliptic Curve based, and SHA2-256 will either grow longer (SHA2-384), or to a new algorithm (SHA3-256), or a completely new method.

But back to the hundreds of CAs: you probably only use a small number in your organisation. LetsEncrypt, Amacon, Google, Verisign, GlobalTrust, etc. However, all CAs are seen as equally trusted when presented with a valid signed certificate. What can you do to prevent other CAs from issuing certificates in your (DNS) name?

The answer is simple: the DNS CAA record: Certificate Authority Authorisation. Its a list that says which CA(s) are allowed to issue certificates for your domain. It’s a record in DNS that is looked up by CAs just before they’re about to issue a certificate: if their indicator flag is not found, they don’t issue.

As it is so rarely issued, you can set this DNS record up with an extremely low TTL (say, 60 seconds). If you get the record wrong, or you forget to whitelist a new CA you’re moving to, update the record.

DNS isn’t perfect, but this slight incremental step may help keep public CAs to only issue from the CA’s you’ve made a decision to trust, and for your customers to trust as well.

DNS CAA was defined in 2010, and an IETF RFC in 2014. I worked with AWS Route53 team to have the record type supported in 2015. You can inspect CAA records using the dig command:

dig caa advara.com
; <<>> DiG 9.10.6 <<>> caa advara.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5546
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;advara.com. IN CAA
;; ANSWER SECTION:
advara.com. 60 IN CAA 0 issue "amazon.com"

Here you can see that advara.com has permitted AWS’s Certificate Manager, with its well known flag of “amazon.com” (and its a 60 second TTL).

You’ll also see that various online services will let you inspect this, including SSLLabs.com, Hardenize.com, and more.

Putting a CAA record in DNS typically costs nothing; its rarely looked up and can easily be changed. It protects you from someone tricking another CA into issuing certificates they think are legitimate; and this has been seen several times (think how valuable a google.com certificate would be ot intercept (MITM) mobile phones, searches, gmail, etc) – and while mis-issuance like this MAy lead to Browser/CA forum expulsion, and eventual client updates to distrust this CA, its far easier to prevent issuance with this simple record.

Of course, DNS Sec would be nice too…

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