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Get cracking on STARTTLS says Facebook

E-mail security protocol has 'critical mass'

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Facebook has debunked the idea that SMTP STARTTLS encryption still isn't taking hold, after an analysis of the billions of messages it sends to millions of servers each day.

In this blog post, The Social NetworkTM says the numbers are clear: “STARTTLS has achieved critical mass and there is immediate value in deploying it”, and the more people that use e-mail encryption, the more value there is in it.

The analysis was prepared by Facebook mail integrity engineer Michael Adkins.

The short version of Facebook's numbers: out of billions of messages to millions of domains, 76 per cent of unique MX hostnames it sends to use STARTTLS, and 58 per cent of notification e-mails are successfully encrypted.

“Additionally, certificate validation passes for about half of the encrypted email, and the other half is opportunistically encrypted. 74% of hosts that support STARTTLS also provide Perfect Forward Secrecy”, the post continues.

The company is at pains to assure its customers that the log file analysis underneath the numbers didn't involve delving into sensitive customer data: it only looked at data reported from the recipient server including STARTTLS results (whether encryption could be negotiated and which cipher suite was used), the recipient's domain, the MX hostname and the receiving server's IP address.

Interestingly, while there's lots of STARTTLS out there, Facebook notes that certificate problems are also widespread. As a result, only about 30 per cent of all e-mails in the analysis pass strict validation, meaning the rest have to rely on the protocol's opportunistic encryption capabilities.

Most often, that's because of a mismatch between the certificate and the hostname (99.35 per cent of all cases).

While DHE-RSA-AES128-SHA is the most common cipher suite used, the next-most-common is AES128-SHA, “which is concerning because it does not provide Perfect Forward Secrecy”. In spite of this, across all e-mails PFS is supported in more than 98 per cent of connections. ®

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