Web giants publish new e-mail security standard
Engineers from some of the world’s largest e-mail service providers have banded together to improve the security of email traffic traversing the Internet.
Devised by engineers from Google, Microsoft, Yahoo, Comcast, LinkedIn and 1&1 Mail & Media Development & Technology, the SMTP Strict Transport Security is a new mechanism that allows e-mail providers to define policies and rules for establishing encrypted e-mail communications.
The new mechanism is defined in a draft that was published late last week for consideration as an Internet Engineering Task Force (IETF) standard.
The Simple Mail Transfer Protocol (SMTP), which is used to transfer e-mail messages between e-mail clients and servers, as well as from one provider to another, dates back to 1982 and was not built with any encryption option.
For this reason, in 2002, an extension called STARTTLS was added to the protocol as a way to include Transport Layer Security (TLS) with SMTP connections. Unfortunately, over the following decade, the extension was not widely adopted, and e-mail traffic exchanged between servers remained largely unencrypted.
That changed after 2013, when former US National Security Agency contractor Edward Snowden leaked secret documents that revealed widespread surveillance of Internet communications by intelligence agencies from the US, UK, and other countries.
In May 2014, Facebook, which sends billions of notification e-mails to users every day, ran a test and found that 58% of those e-mails passed through a connection encrypted with STARTTLS. By August that same year, the rate jumped to 95%.
There is one problem, however, unlike HTTP Secure (HTTPS), STARTTLS allows for what is known as opportunistic encryption. It does not validate the digital certificates presented by e-mail servers, under the assumption that even if a server’s identity cannot be verified, encrypting the traffic is still better than nothing.
This means that STARTTLS connections are vulnerable to man-in-the-middle attacks, where a hacker in a position to intercept the traffic could present the e-mail sender with any certificate, even a self-signed one, and it will be accepted, allowing for the traffic to be decrypted. Furthermore, STARTTLS connections are vulnerable to so-called encryption downgrade attacks, where the encryption is simply removed.
The newly proposed SMTP Strict Transport Security (SMTP STS) addresses both of those issues. It gives email providers the means to inform connecting clients that TLS is available and should be used. It also tells them how the presented certificate should be validated and what should happen if a TLS connection cannot be safely negotiated.
These SMTP STS policies are defined through special DNS records added to the e-mail server’s domain name. The protocol provides mechanisms for clients to automatically validate these policies and to report back on any failures.
The proposed protocol is similar to the HTTP Strict Transport Security (HSTS), which is meant to prevent HTTPS downgrade attacks by caching a domain’s HTTPS policy locally in the browser. It does, however, assume that the first connection from a particular client to the server was performed without being intercepted; otherwise, a fraudulent policy might have been cached.
According to Google’s latest data, 83% of e-mail messages sent by Gmail users to other e-mail providers from around the world are encrypted, but only 69% of incoming emails from other providers are received over an encrypted channel.
There are also large discrepancies in email encryption between regions of the world, with email providers in Asia and Africa faring much worse than providers in Europe and the US.
IDG News Service