Tag: encryption

Dude, where’s my email?

Ensuring high deliverability in email is no walk in the park. As a high-volume sender of email, there are many things to take in consideration, especially with cybercriminals keeping a fast pace in innovation.

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Why you need to demand high deliverability and security of email

Trends come and go but fashion is always in style. 20 years of emails shows that it is a consistent way of communication, despite a relatively standstill in development. While the world sings their praise for new and sexy messaging services like Slack, gets rid of stationary phones, and transfer customer communication to Facebook chat bots, the B2B world still rely steadily on email. After three days of WebSummit in Lisbon, Portugal, I am even more convinced than before.

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What Gmail’s new TLS icon really means

Google’s recent announcement that they’ll be adding encryption (TLS) and authentication (DKIM/DMARC) status icons to Gmail is a great initiative.

What they do

In addition to being displayed for received email, the encryption indicator works when composing email as well. It’s however not updated in realtime, and seems to be cached per domain. For example, a warning is displayed “”, but not for “”, despite pointing to the same MX. We haven’t been able to verify how long it takes for the warning to display; after setting up a new (non-TLS) domain one day ago and sending numerous email to it from different Gmail accounts, the warning is nowhere to be seen. We presume that the sessions of sent email are being used is material, since we didn’t record any additional probes/callouts to our test server.

I fully appreciate Google’s rationale behind displaying a red broken lock icon when TLS is absent, rather than a green lock when it is used; I just wished they addressed the shortcomings of non-authenticated TLS and took a stance on how we could move on to proper email encryption.

The problem

As I discussed in a recent blog, TLS is widely deployed for email, but virtually no-one is doing verification (checking authenticity). In the case of a web browser, TLS without verification triggers a warning (such as a red lock). It might be confusing for users that a similar symbol in Gmail indicates something different. In my opinion, there should have been at least an explaination or comment that non-authenticated (opportunistic) TLS doesn’t protect against active attacks (eavesdropping, downgrade, etc).

The main issue, of course, is that there’s currently no widely deployed mechanism for SMTP TLS name verification. This stems from the fact that email is security-agnostic (unlike the web, where you type “https://”) and that the DNS MX structure makes normal CA-based name verification ambiguous.

What they could do

The most promising technology right now for proper, verified encryption (TLS) is probably DANE, which builds on DNSSEC. It’s already implemented in Postfix (the second most popular email server) and (of course) Halon. Furthermore, many European providers and operators have either deployed, or is planning to deploy, DANE. It even serves as a cornerstone for Trusted Email Services initiative.

While its dependency on DNSSEC is likely hampering adoption, I’m encouraged by its strong community support. It feels like widespread email encryption is finally within reach!

What is DANE?

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.

The DNS-based Authentication of Named Entities (DANE) is a proposed standard that binds X.509 certificates to DNS names using Domain Name System Security Extensions (DNSSEC).

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.

The state of email encryption

Before we start talking about DANE, let’s first look at the differences between transport and end-to-end encryption;

  • Transport encryption works on domain level (an organization) and protects the email transfer. It is similar to HTTPS, used when browsing secure websites.
  • End-to-end encryption works on the individual level (a person) and protects the content of the email for its entire lifetime. The two most popular standards are S/MIME and PGP, but none of them have achieved widespread adoption.

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.

Introducing 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.

DANE for 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.

Start using DANE

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/ 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: IN A IN TLSA 3 0 1 2B73BB905F…

Enter Halon

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.