There’s often quite a confusion about the different terms: SSL, TLS, STARTTLS and STLS.
SSL and TLS
SSL and TLS are cryptographic protocols, both provide a way to encrypt communication channel between two machines over the Internet (e.g. client computer and a server). SSL stands for Secure Sockets Layer and current version is 3.0. TLS stands for Transport Layer Security and the current version is 1.2. TLS is the successor to SSL. The terms SSL and TLS can be used interchangeably, unless you’re referring to a specific protocol version.
Version numbering is inconsistent between SSL and TLSs. When TLS took over SSL as the preferred protocol name, it began with a new version number. The ordering of protocols in terms of oldest to newest is: SSLv2, SSLv3, TLSv1.0, TLSv1.1, TLSv1.2.
STARTTLS and STLS
STARTTLS is a protocol command, that is issued by an email client. It indicates, that the client wants to upgrade existing, insecure connection to a secure connection using SSL/TLS cryptographic protocol. STARTTLS command name is used by SMTP and IMAP protocols, whereas POP3 protocol uses STLS as the command name.
Despite having TLS in the name, STARTTLS doesn’t mean TLS will be used. Both SSL and TLS are acceptable protocols for securing the communication.
Clear text/Plain text
No security protocol is used at all. All commands, responses and data are transferred in plain text.
Implict SSL mode
Implict SSL mode means, that you connect to SSL/TLS encrypted port.
Explicit SSL mode
Explicit SSL mode means, that you connect to plaint text port and secure the connection by issuing STARTTLS (or STLS) command afterwards (you explicitly secure the connection).
Securing the connection
Regardless of whether you use implict (connecting to an SSL/TLS encrypted port) or explicit (using STARTTLS to upgrade an existing connection) mode, both sides will negotiate which protocol and which version to use. This negotiation is based on how client and server have been configured and what each side supports.
Support for SSL/TLS is virtually universal, however which versions are supported is variable. Pretty much everything supports SSLv3. Most machines support TLSv1.0.
TLS vs STARTTLS naming problem
One significant complicating factor is that some email software incorrectly uses the term TLS when they should have used “STARTTLS” or “explicit SSL/TLS”. Older versions of Thunderbird used “TLS” to mean “enforce use of STARTTLS to upgrade the connection, and fail if STARTTLS is not supported” and “TLS, if available” to mean “use STARTTLS to upgrade the connection, if the server advertises support for it, otherwise just use an insecure connection” (very problematic, as we’ll see below).
To add security to some existing protocols (IMAP, POP3, SMTP), it was decided to just add SSL/TLS encryption as a layer underneath the existing protocol. However to distinguish that software should talk the SSL/TLS encrypted version of the protocol rather than the plaintext one, a different port number was used for each protocol:
||587 or 25
Too many ports? Solution: Plain text + STARTTLS
At some point, it was decided that having 2 ports for every protocol was wasteful, and instead it’s better to have 1 port, that starts off as plain text, but clients can upgrade the connection to an SSL/TLS encrypted one using STARTTLS (or STLS for POP3 protocol) command.
There were a few problems with this. There exists lots of software, that used the alternate port numbers with pure SSL/TLS connections. Client software can be very long lived, so you can’t just disable the encrypted ports until all software has been upgraded.
Each protocol received mechanisms to tell clients that the server supported upgrading to SSL/TLS (e.g. STARTTLS in IMAP’s CAPABILITY response), and that they should not attempt to login without doing the STARTTLS upgrade (LOGINDISABLED in IMAP’s CAPABILITY response). This created two unfortunate situations:
Some software just ignored the “login disabled until upgraded” announcement (LOGINDISABLED, STARTTLS) and just tried to log in anyway, sending the user login name and password over clear text channel. The server rejected the login and password, but the details had already been sent over the Internet in plain text.
Other software saw the “login disabled until upgraded” announcement, but then wouldn’t upgrade the connection automatically, and thus reported login errors back to the user, which caused confusion about what was wrong.
Both of these problems resulted in significant compatibility issues with existing clients, and so most system administrators continued to just use plain text connections on one port, and encrypted connections on a separate port number.
Disable plain text for IMAP and POP3
Many companies (e.g. Gmail, Outlook.com) disabled plain IMAP (port 143) and plain POP3 (port 110), so people must use a SSL/TLS encrypted connection – this removes the need for having STARTTLS command completely.
SMTP STARTTLS stays
The one real exception to the above is SMTP. Most email software used SMTP on port 25 to submit messages to the email server for onward transmission to the destination. However SMTP was originally designed for transfer, not submission. So yet another port (587) was defined for message submission.
Port 587 doesn’t mandate requiring STARTTLS, however the use of port 587 became popular around the same time as the realization that SSL/TLS encryption of communications between clients and servers was an important issue. The result is that most systems, that offer message submission over port 587 require clients to use STARTLS to upgrade the connection. Login and password to authenticate is also required.
There has been an additional benefit to this approach as well. By moving users away from using port 25 for email submission, ISPs can block outgoing port 25 connections from users’ computers, which were a significant source of spam, due to user computers infected with spam sending viruses.
Using SSL/TLS with IMAP
Using SSL/TLS with SMTP
Using SSL/TLS with POP3