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
TLS may refer to:
Computing
* Transport Layer Security, a cryptographic protocol for secure computer network communication
* Thread level speculation, an optimisation on multiprocessor CPUs
* Thread-local storage, a mechanism for allocating vari ...
or
SSL SSL may refer to:
Entertainment
* RoboCup Small Size League, robotics football competition
* ''Sesame Street Live'', a touring version of the children's television show
* StarCraft II StarLeague, a Korean league in the video game
Natural language ...
) connection instead of using a separate port for encrypted communication. Several protocols use a command named "STARTTLS" for this purpose. It is a form of
opportunistic encryption and is primarily intended as a countermeasure to
passive monitoring
Passive may refer to:
* Passive voice, a grammatical voice common in many languages, see also Pseudopassive
* Passive language, a language from which an interpreter works
* Passivity (behavior), the condition of submitting to the influence of on ...
.
The STARTTLS command for
IMAP and
POP3 is defined in , for
SMTP in , for
XMPP
Extensible Messaging and Presence Protocol (XMPP, originally named Jabber) is an open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML (Extensible Markup Language), i ...
in and for
NNTP in . For
IRC, the IRCv3 Working Group has defined the STARTTLS extension.
FTP uses the command "AUTH TLS" defined in and
LDAP defines a protocol extension
OID in .
HTTP
The Hypertext Transfer Protocol (HTTP) 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 We ...
uses
upgrade header
The Upgrade header field is an List of HTTP header fields, HTTP header field introduced in HTTP/1.1. In the exchange, the client begins by making a cleartext request, which is later upgraded to a newer HTTP protocol version or switched to a differ ...
.
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 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 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 ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...
were found to be doing this to their own customers.
In October 2014,
Cricket Wireless
Cricket Wireless is an American prepaid wireless service provider, owned by AT&T. It provides wireless services to ten million subscribers in the United States. Cricket Wireless was founded in March 1999 by Leap Wireless International. AT&T ac ...
, a subsidiary of
AT&T
AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile ...
, 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 and federal governments or governmental organizati ...
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
This problem is addressed by
DNS-based Authentication of Named Entities
DNS-based Authentication of Named Entities (DANE) is an Internet security protocol to allow X.509 digital certificates, commonly used for Transport Layer Security (TLS), to be bound to domain names using Domain Name System Security Extensions (DNS ...
(DANE), a part of
DNSSEC
The Domain Name System Security Extensions (DNSSEC) are a suite of extension specifications by the Internet Engineering Task Force (IETF) for securing data exchanged in the Domain Name System (DNS) in Internet Protocol (IP) networks. The protocol ...
, 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 international non-profit digital rights group based in San Francisco, California. The foundation was formed on 10 July 1990 by John Gilmore, John Perry Barlow and Mitch Kapor to promote Internet ...
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 (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 in light of the
global mass surveillance scandal, popular email providers have bettered their email security by enabling STARTTLS.
Facebook
Facebook is an online social media and social networking service owned by American company Meta Platforms. Founded in 2004 by Mark Zuckerberg with fellow Harvard College students and roommates Eduardo Saverin, Andrew McCollum, Dust ...
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
In cryptography, forward secrecy (FS), also known as perfect forward secrecy (PFS), is a feature of specific key agreement protocols that gives assurances that session keys will not be compromised even if long-term secrets used in the session key ...
and strict certificate validation.
References
External links
Secure Email Tests and Toolsverify 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