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Posts Tagged ‘starttls’

SMTP TLS: All About Secure Email Delivery over TLS

Monday, October 2nd, 2017

TLS stands for “Transport Layer Security” and is the successor of “SSL” (Secure Socket Layer). TLS is one of the standard ways that computers on the Internet transmit information over an encrypted channel. In general, when one computer connects to another computer and uses TLS, the following happens:

  1. Computer A connects to Computer B (no security)
  2. Computer B says “Hello” (no security)
  3. Computer A says “Lets talk securely over TLS” (no security)
  4. Computer A and B agree on how to do this (secure)
  5. The rest of the conversation is encrypted (secure)

In particular:

  • The meat of the conversation is encrypted
  • Computer A can verify the identity of Computer B (by examining its SSL certificate, which is required for this dialog)
  • The conversation cannot be eavesdropped upon (without Computer A knowing)
  • The conversation cannot be modified by a third party
  • Other information cannot be injected into the conversation by third parties.

Basic email security starts with SMTP TLS

TLS (and SSL) is used for many different reasons on the Internet and helps make the Internet a more secure place, when used. One of the popular uses of TLS is with SMTP for transmitting email messages between servers in a secure manner.  See also:

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Does TLS Corruption Spell the end of SMTP TLS?

Tuesday, November 3rd, 2015

We have seen discussions recently about how attackers can interfere with SMTP TLS, influencing connections, and causing them to be downgraded to insecure — SMTP without TLS.  E.g. Ars Technica’s – “Don’t Count on STARTTLS to Automatically Encrypt your Sensitive Emails“.

What is being discussed here is a very real attack on Opportunistic TLS. I.e. the kind of automated establishment of encryption that can happen when two email servers being their dialog and discover that “hey, great, we both support TLS so lets use it!”  In such cases, servers take the “opportunity” to use TLS to encrypt the delivery of an email message from one server to another.  Opportunistic TLS is great as it is enabling automatic encryption of more and more email over time (see: Who supports TLS?).

The problem is that the initial negotiation of the SMTP email connection, before TLS is established, occurs over an insecure channel.  A man-in-the-middle attacker can interfere with this connection so that it appears that TLS (i.e. the STARTTLS command) is not supported by the server (when it really is).  As a result, the sending server will never try to use TLS and the connection will remain insecure — transmitting the email message “in the clear” and ripe for eavesdropping.

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How to Tell Who Supports SMTP TLS for Email Transmission

Tuesday, January 27th, 2015

SMTP TLS (Transport Layer Security) is the mechanism by which two email servers, when communicating, can automatically negotiate an encrypted channel between them so that the emails transmitted are secured from eavesdroppers.

It is becoming ever more important to use a company that supports TLS for email transmission as more and more banks, health care, and other organizations who have any kind of security policy are requiring their vendors and clients to use this type of encryption for emailed communications with them. Additionally, if your email provider supports TLS for email transmission, and you are communicating with people whose providers do also, then you can be reasonably sure that all of the email traffic between you and them will be encrypted.

How do you find out if someone to whom you are sending email uses a provider who’s servers support TLS-encrypted communications? We will take you through the whole process step-by-step, but first let us note some 6 important truths about TLS connection encryption.

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