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Opportunistic TLS (Transport Layer Security) 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" or "Explicit TLS" for this purpose. It is a form of opportunistic encryption and is primarily intended as a countermeasure to passive monitoring. The STARTTLS command for IMAP and
POP3 In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. Today, POP version 3 (POP3) is the most commonly used version. Together with IMAP, i ...
is defined in , for
SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typi ...
in , for
XMPP Extensible Messaging and Presence Protocol (abbreviation XMPP, originally named Jabber) is an Open standard, open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML (Ext ...
in and for NNTP in . For
IRC IRC (Internet Relay Chat) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called '' channels'', but also allows one-on-one communication via private messages as well as chat ...
, the IRCv3 Working Group defined a STARTTLS extension, though it was later deprecated.
FTP The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and dat ...
uses the command "AUTH TLS" defined in and
LDAP The Lightweight Directory Access Protocol (LDAP ) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed Directory service, directory information services over an Internet Protocol (IP) networ ...
defines a protocol extension OID in .
HTTP HTTP (Hypertext Transfer Protocol) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, wher ...
uses an upgrade header.


Layering

TLS is application-neutral; in the words of : :One advantage of TLS is that it is application protocol independent. Higher-level protocols can layer on top of the TLS protocol transparently. The TLS standard, however, does not specify how protocols add security with TLS; the decisions on how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS. 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 SMTP extension illustrates with the following dialog how a client and server can start a secure session: S: <waits for connection on TCP port 25> C: <opens connection> S: 220 mail.example.org ESMTP service ready C: EHLO client.example.org S: 250-mail.example.org offers a warm hug of welcome S: 250 STARTTLS C: STARTTLS S: 220 Go ahead C: <starts TLS negotiation> C & S: <negotiate a TLS session> C & S: <check result of negotiation> C: EHLO client.example.org . . . 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.


SSL ports

Besides the use of opportunistic TLS, 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. Separate SSL ports have the advantage of fewer round-trips; also less meta-data is transmitted in unencrypted form. Some examples include: At least for the email related protocols, favors ''Implicit TLS'' (using separate SSL ports) instead of STARTTLS.


Weaknesses and mitigations

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 In cryptography and computer security, a man-in-the-middle (MITM) attack, or on-path attack, is a cyberattack where the attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communi ...
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 Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...
were found to be doing this to their own customers. In October 2014, Cricket Wireless, a subsidiary of
AT&T AT&T Inc., an abbreviation for its predecessor's former name, the American Telephone and Telegraph Company, is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the w ...
, was revealed to be doing this to their customers. This behavior started as early as September 2013 by Aio Wireless, who later merged with Cricket where the practice continued. STRIPTLS attacks can be blocked by configuring SMTP clients to require TLS for outgoing connections (for example, the Exim Message transfer agent can require TLS via the directive "hosts_require_tls"). However, since not every mail server supports TLS, it is not practical to simply require TLS for all connections. An example of a STRIPTLS attack of the type used in Thai
mass surveillance Mass surveillance is the intricate surveillance of an entire or a substantial fraction of a population in order to monitor that group of citizens. The surveillance is often carried out by Local government, local and federal governments or intell ...
technology: 220 smtp.gmail.com ESMTP mail.redacted.com - gsmtp ehlo a 250-smtp.gmail.com at your service, EDACTED SERVICE 250-SIZE 35882577 250-8BITMIME # The STARTTLS command is stripped here 250-ENHANCEDSTATUSCODES 250-PIPELINING 250 SMTPUTF8 220 smtp.gmail.com ESMTP - gsmtp ehlo a 250-smtp.gmail.com at your service 250-SIZE 35882577 250-8BITMIME 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-PIPELINING 250 SMTPUTF8 Supposing the client side supports it (name resolution of the client and upstream DNS server of the client), this problem can be addressed by DNS-based Authentication of Named Entities (DANE), a part of DNSSEC, and in particular by for SMTP. DANE allows to advertise support for secure SMTP via a TLSA record. This tells connecting clients they should require TLS, thus preventing STRIPTLS attacks. The STARTTLS Everywhere project from the
Electronic Frontier Foundation The Electronic Frontier Foundation (EFF) is an American international non-profit digital rights group based in San Francisco, California. It was founded in 1990 to promote Internet civil liberties. It provides funds for legal defense in court, ...
works in a similar way. However, DNSSEC, due to deployment complexities and peculiar criticism, faced a low adoption rate and a new protocol called SMTP MTA Strict Transport Security or MTA-STS has been drafted by a group of major email service providers including Microsoft, Google and Yahoo. MTA-STS does not require the use of DNSSEC to authenticate DANE TLSA records but relies on the
certificate authority In cryptography, a certificate authority or certification authority (CA) is an entity that stores, signs, and issues digital certificates. A digital certificate certifies the ownership of a public key by the named subject of the certificate. Thi ...
(CA) system and a trust-on-first-use (TOFU) approach to avoid interceptions. The TOFU model reduces complexity but without the guarantees on first use offered by DNSSEC. In addition, MTA-STS introduces a mechanism for failure reporting and a report-only mode, enabling progressive roll-out and auditing for compliance.


Popularity

Following the revelations made by
Edward Snowden Edward Joseph Snowden (born June 21, 1983) is a former National Security Agency (NSA) intelligence contractor and whistleblower who leaked classified documents revealing the existence of global surveillance programs. Born in 1983 in Elizabeth ...
in light of the global mass surveillance scandal, popular email providers have bettered their email security by enabling STARTTLS.
Facebook Facebook is a social media and social networking service owned by the American technology conglomerate Meta Platforms, Meta. Created in 2004 by Mark Zuckerberg with four other Harvard College students and roommates, Eduardo Saverin, Andre ...
reported that after enabling STARTTLS and encouraging other providers to do the same, until Facebook discontinued its email service in February 2014, 95% of outbound email was encrypted with both perfect forward secrecy and strict certificate validation.


References


External links


Secure Email Tests and Tools
verify STARTTLS in real-time dialog like example above
Verify if a receiving domain has STARTTLS enabled for email and with which security level
* A mechanism enabling mail service providers to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections. {{DEFAULTSORT:Starttls Internet mail protocols Transport Layer Security fr:StartTLS