DANE does not only challenge the certificate authority (CA) system, but also has the potential to revolutionize email security; namely making encrypted email delivery the norm. Halon Security’s mission is to bring DANE to the world of email and to help maintain data privacy and security on a global scale.
Even today (2016), email is being transmitted unencrypted, with very few exceptions. The fact that many servers use TLS doesn’t do much more than create a false sense of security; while it prevents passive wiretapping, the opportunistic TLS mode that is being used today doesn’t protect your email transmissions from active attacks. DANE is currently the most promising piece of technology that might change all that. Despite being (primarily) a DNSSEC-based trust scheme for X.509 certificates, it also provides the means to know if a domain supports encrypted email transfer. That last part is very important; it’s what has been missing all these years.
Public-key cryptography is a fundamental component that makes the modern Internet work the way it does. One of the most widely known encryption protocols is TLS (formerly SSL), used for web browsing, instant messaging, and much more.
Email however, despite being “the go-to form of communication in the Business world” with a constantly growing user base, hasn’t changed much over the years, and is generally not encrypted.
Before we start talking about DANE, let’s first look at the differences between transport and end-to-end encryption;
While I would certainly love to see widely deployed end-to-end email encryption in the future, having transport security in place would be a very good start. When visiting your bank’s website, it’s surely encrypted. Still, when sending them an email, the transport from your email server to theirs, likely isn’t. Why is that? Since email transfer (SMTP) supports TLS, why not use a PKI such as the CA system, like we do for the web? Unfortunately, there’s no standard defining a way for the sending server to know whether to use encryption or not. For the web, you type https://, and there’s simply no equivalent for email. Until DANE.
DANE is proposed in RFC 6698 by Hoffman and Schlyter as an alternative to CAs. The CA system has been subject to criticism during the last years, following compromise and mishaps of several major CAs. To make things worse, the revocation systems doesn’t work very well in practice. DANE builds on DNSSEC which provides a hierarchal scheme (the root, TLDs, domains and so on), as opposed to the flat CA system where any trusted CA can issue a certificate in the name of anyone.
Is the general consensus that DANE should replace CAs all together? As always there are arguments for and against. While a hierarchal system is preferable, the transfer of control and responsibility to the DNS system’s owners (governments, DNS admins) is a topic much discussion. There’s also work to improve the CA system with supplementary techniques such as key pinning and added perspective. Nevertheless, DANE stands strong to improve the security in many areas, such as email.
Postfix was probably the first email server to support RFC 7672 (DANE for SMTP), and Exim support is underway. The fact that those two are the most widely used SMTP servers, and the somewhat strong DNSSEC adoption, is a solid foundation for DANE. Halon’s MTA supports DANE since 3.4-rocky-r2.
Most of our customers can start sending email with DANE right away, by simply changing TLS mode to dane. We recommend that you use the built-in DNS resolver Unbound with DNSSEC enabled, instead of relying on an external DNS server’s AD-bit. In the logs you can identify DANE-verified connections by
Connecting to [2001:1900:2254:206a::19:1]:25 (DNSSEC)
X.509: /OU=Domain Control Validated/OU=Gandi Standard SSL/CN=mx1.freebsd.org issued by...
DANE: validated successfully
Connection is now using TLS
If you or your customers’ domains are DNSSEC signed, you should look into receiving email with DANE as well. In theory, it’s no more difficult than publishing your SMTP servers’ certificate signatures as TLSA records in your DNS using the same name as the MX points at, but prefixed with _25._tcp, for example:
mx1.freebsd.org. IN A 184.108.40.206
_25._tcp.mx1.freebsd.org. IN TLSA 3 0 1 2B73BB905F...
We have been pioneering email security in our scriptable SMTP server for a long time; being early adopters of DMARC, DNSSEC and much more. If you’re curious, go ahead and download the software from our website.