STARTTLS

Opportunistic TLS refers to extensions in plain text communication protocols, which offer a way to upgrade a plain text connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication. Several protocols use a command named "STARTTLS" for this purpose. It is primarily intended as a countermeasure to passive monitoring.

TLS is application-neutral; in the words of RFC 5246:

The style used to specify how to use TLS matches the same layer distinction that is also conveniently supported by several library implementations of TLS. E.g., the RFC 3207 SMTP extension illustrates with the following dialog how a client and server can start a secure session:

The last EHLO command above is issued over a secure channel. Note that authentication is optional in SMTP, and the omitted server reply may now safely advertise an AUTH PLAIN SMTP extension, which is not present in the plain-text reply.

Before opportunistic TLS was well established, a number of TCP ports were defined for SSL-secured versions of well-known protocols. These establish secure communications and then present a communication stream identical to the old un-encrypted protocol. These are no longer recommended, since opportunistic TLS makes more efficient use of scarce port numbers and allows simpler device configuration. On the other hand, SSL ports have the advantage of fewer round-trips; also less meta-data is transmitted in unencrypted form. Some examples include:

Opportunistic TLS is an opportunistic encryption mechanism. Because the initial handshake takes place in plain text, an attacker in control of the network can modify the server messages via a man-in-the-middle attack to make it appear that TLS is unavailable (called a STRIPTLS attack). Most SMTP clients will then send the email and possibly passwords in plain text, often with no notification to the user. In particular, many SMTP connections occur between mail servers, where user notification is not practical.

In September 2014, two ISPs in Thailand were found to be doing this to their own customers. In October 2014, Aio Wireless, then a subsidiary of Cricket Wireless, was found to be doing this to their customers.

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