TLS and e-mail security

Question

Our client has asked us to send sensitive data via TLS and to install a certificate on our mail servers so that they can verify the authenticity of our e-mails.

If I understand correctly, TLS encrypts the 'tunnel' that the message is communicated through, while not encrypting the actual content of the e-mail or any of its attachments. Is this correct?

If this is the case, am I right in saying that the content of the e-mail is potentially vulnerable to being intercepted at any server that is involved in SMTP relay between our outbound mail server and the recipients mail server?

Answer 1

• TLS is just for connection authentication or as you called it, the "tunnel". It's really just for mail flow connections between servers and/or clients to make sure that unauthenticated clients/servers aren't permitted to send mail through said host/server.
• If they want you to install a certificate on your server, that's for the purpose of verifying that there isn't a rogue or spoofing mail server trying to send them mail using your domain name but not your server. It's one of many ways to verify the sending server's authenticity to send mail for the respective domain.
• Encryption of the email data itself is a completely different technology. There are various third-party vendors that offer this service, along with open source options such as PGP

Answer 2

If I understand correctly, TLS encrypts the 'tunnel' that the message is communicated through, while not encrypting the actual content of the e-mail or any of its attachments. Is this correct?

The email content passes encrypted through the tunnel, so it's won't be in plain through the wire (). It's like downloading a zip file through https. The zip itself won't have a password, but it was downloaded securely.

(Of course, you could use PGP in the email as well)

if TLS is specified, all relaying servers in between will use TLS…

That's wrong. TLS in emails, is hop-by-hop. You may transmit encrypted to the next relay, but if encryption is unavailable to the next, it will be transmitted in the plain. You could specify the XSECURITY ESMTP extension to request otherwise, but I think only courier mailservers support it.

If this is the case, am I right in saying that the content of the e-mail is potentially vulnerable to being intercepted at any server that is involved in SMTP relay between our outbound mail server and the recipients mail server?

Yes. While protected in transit, the content is fully available in the intermediate hosts so a malicious email relay could steal the contents. Thus, their output email host is probably connecting to your web server directly, not through any untrusted relay. Then, by using a (trusted) certificate, their mailserver can ensure that their server is connecting to yours, with no intermediate relay.

There are a number of caveats if you want true security, though:

• First of all, they would somehow ensure they are not receiving a malicious MX dns reply, since you are probably not using dnssec.
• The sending email server would need to verify that the certificate is trusted (it is pinned, or signed by a trusted CA…). By default, MTA will accept any certificate, as there too many self-signed ones so far.
• Equally, when you are replying, your server should need to be equally hardened. What's good in having a perfect email server, if your replies are then quoting your secret contents, and sent with a MTA that is not taking advantage of the secure tunnel you offer?

Still, just will protect against passive eavesdroppers, such as a government agency snooping on the submarine cables.

Answer 3

Why would other servers be involved in the SMTP relay? Your server makes a secure connection to the destination MX server and sends the email. The only way for it to be intercepted is if the destination server is compromised. If the domain only uses secure MX servers then the message cannot be intercepted.

edit:

There can be multiple servers involved in an email exchange, however these servers are all either smtp relays configured by the originating server, or secondary MX servers configured by the destination domain. If a domain uses secure MX servers, and these servers have been properly configured for secure internal transfers, then you have the whole connection controlled.

It's not like 3rd party servers can inject themselves into the delivery pipeline.

Answer 4

Yes, this is correct. It sounds like they are hoping to connect directly to your mailserver.

These intermediate mailservers will be connecting to you over TLS and then making their own TLS connection to the next server. This in between is not encrypted. Essentially they are a man-in-the-middle.