Opportunistic encryption (OE) refers to any

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

that, when connecting to another system, attempts to

encrypt In cryptography, encryption (more specifically, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the information, known as plai ...

communications channels, otherwise falling back to unencrypted communications. This method requires no pre-arrangement between the two systems. Opportunistic encryption can be used to combat passive wiretapping. (an ''active'' wiretapper, on the other hand, can disrupt encryption negotiation to either force an unencrypted channel or perform 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 ...

on the encrypted link.) It does not provide a strong level of

security Security is protection from, or resilience against, potential harm (or other unwanted coercion). Beneficiaries (technically referents) of security may be persons and social groups, objects and institutions, ecosystems, or any other entity or ...

as authentication may be difficult to establish and secure communications are not mandatory. However, it does make the encryption of most Internet traffic easy to implement, which removes a significant impediment to the mass adoption of Internet traffic security. Opportunistic encryption on the Internet is described in "Opportunistic Encryption using the Internet Key Exchange (IKE)", "Opportunistic Security: Some Protection Most of the Time", and in "Opportunistic Security for HTTP/2".

Routers

The FreeS/WAN project was one of the early proponents of OE. The effort is continued by the former freeswan developers now working on Libreswan. Libreswan aims to support different authentication hooks for opportunistic encryption with IPsec. Version 3.16, which was released in December 2015, had support for Opportunistic IPsec using AUTH-NULL which is based o
RFC 7619
The Libreswan Project is currently working on (forward)

Domain Name System Security Extensions The Domain Name System Security Extensions (DNSSEC) is a suite of Extension Mechanisms for DNS, extension specifications by the Internet Engineering Task Force (IETF) for securing data exchanged in the Domain Name System (DNS hijacking, DNS) in In ...

(DNSSEC) and Kerberos support for Opportunistic IPsec. Openswan has also been ported to the

OpenWrt OpenWrt (from ''open wireless router'') is an open-source project for embedded operating systems based on Linux kernel, Linux, primarily used on Embedded system, embedded devices to Router (computing), route network traffic. The main components ...

project. Openswan used reverse DNS records to facilitate the key exchange between the systems. It is possible to use OpenVPN and networking protocols to set up dynamic VPN links which act similar to OE for specific domains.

Linux and Unix-like systems

The FreeS/WAN and forks such as Openswan and strongSwan offer VPNs that can also operate in OE mode using IPsec-based technology. Obfuscated TCP is another method of implementing OE.

Windows OS

Microsoft Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

platforms have an implementation of OE installed by default. This method uses IPsec to secure the traffic and is a simple procedure to turn on. It is accessed via the MMC and "IP Security Policies on Local Computer" and then editing the properties to assign the "(Request Security)" policy. This will turn on optional IPsec in a Kerberos environment. Many systems also have problems when either side is behind a NAT. This problem is addressed by

NAT traversal Network address translation traversal is a computer networking technique of establishing and maintaining Internet Protocol connections across Gateway (telecommunications), gateways that implement network address translation (NAT). NAT traversal te ...

(NAT-T) and is accomplished by editing a registry item. Using the filtering options provided in MMC, it is possible to tailor the networking to require, request or permit traffic to various domains and protocols to use encryption.

Email

Opportunistic encryption can also be used for specific traffic like

e-mail Electronic mail (usually shortened to email; alternatively hyphenated e-mail) is a method of transmitting and receiving Digital media, digital messages using electronics, electronic devices over a computer network. It was conceived in the ...

using the

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 ...

STARTTLS 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 ...

extension for relaying messages across the Internet, or the Internet Message Access Protocol (IMAP) STARTTLS extension for reading e-mail. With this implementation, it is not necessary to obtain a certificate from a

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 ...

, as a self-signed certificate can be used. * Using TLS with IMAP, POP3 and ACAP * SMTP Service Extension for Secure SMTP over TLS
STARTTLS and postfixSTARTTLS and Exchange
Many systems employ a variant with third-party add-ons to traditional email packages by first attempting to obtain an encryption key and if unsuccessful, then sending the email in the clear. PGP, p≡p, Hushmail, and Ciphire, among others can all be set up to work in this mode. In practice, STARTTLS in SMTP is often deployed with self-signed certificates, which represents a minimal one-time task for a system administrator, and results in most email traffic being opportunistically encrypted.

VoIP

Some

Voice over IP Voice over Internet Protocol (VoIP), also known as IP telephony, is a set of technologies used primarily for voice communication sessions over Internet Protocol (IP) networks, such as the Internet. VoIP enables voice calls to be transmitted as ...

(VoIP) solutions provide for painless encryption of voice traffic when possible. Some versions of the Sipura Technology and Linksys lines of analog telephony adapters (ATA) include a hardware implementation of SRTP with the installation of a certificate from Voxilla, a VoIP information site. When the call is placed an attempt is made to use SRTP, if successful a series of tones are played into the handset, if not the call proceeds without using encryption.

Skype Skype () was a proprietary telecommunications application operated by Skype Technologies, a division of Microsoft, best known for IP-based videotelephony, videoconferencing and voice calls. It also had instant messaging, file transfer, ...

and Amicima use only secure connections and Gizmo5 attempts a secure connection between its clients. Phil Zimmermann, Alan Johnston, and Jon Callas have proposed a new VoIP encryption protocol called

ZRTP ZRTP (composed of Z and Real-time Transport Protocol) is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over IP (VoIP) phone telephony call based on the Real-time Transport Protocol ...

. They have an implementation of it called Zfone whose source and compiled binaries are available.

Websites

For encrypting WWW/

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 ...

connections,

HTTPS Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It uses encryption for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protoc ...

is typically used, which requires strict encryption and has significant administrative costs, both in terms of initial setup and continued maintenance costs for the

website A website (also written as a web site) is any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, educatio ...

operator. Most browsers verify the webserver's identity to make sure that an SSL certificate is signed by a trusted

and has not expired, usually requiring the website operator to manually change the certificate every one or two years. The easiest way to enable some sort of opportunistic website encryption is by using self-signed certificates, but this causes browsers to display a warning each time the website is visited unless the user manually marks the website's certificate as trusted. Because unencrypted websites do not currently display any such warnings, the use of self-signed certificates is not well received. In 2015,

Mozilla Mozilla is a free software community founded in 1998 by members of Netscape. The Mozilla community uses, develops, publishes and supports Mozilla products, thereby promoting free software and open standards. The community is supported institution ...

started to roll out opportunistic encryption in

Firefox Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements curr ...

version 37. This was quickly rolled back (in update 37.0.1) due to a serious

vulnerability Vulnerability refers to "the quality or state of being exposed to the possibility of being attacked or harmed, either physically or emotionally." The understanding of social and environmental vulnerability, as a methodological approach, involves ...

that could bypass SSL certificate verification. Browser extensions like HTTPS Everywhere and HTTPSfinder find and automatically switch the connection to HTTPS when possible. Several proposals were available for true, seamless opportunistic encryption of

HTTP/2 HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google. HTTP/2 was developed by the HTTP Working ...

protocol. These proposals were later rejected. Poul-Henning Kamp, lead developer of

Varnish Varnish is a clear Transparency (optics), transparent hard protective coating or film. It is not to be confused with wood stain. It usually has a yellowish shade due to the manufacturing process and materials used, but it may also be pigmente ...

and a senior

FreeBSD FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...

kernel developer, has criticized the

IETF The Internet Engineering Task Force (IETF) is a standards organization for the Internet standard, Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster ...

for following a particular

political agenda In politics, a political agenda is a list of subjects or problems (issues) to which government officials as well as individuals outside the government are paying serious attention to at any given time. The political agenda is most often shaped by ...

with HTTP/2 for not implementing opportunistic encryption in the standard.

Weaknesses

implementations often used with

are vulnerable to STRIPTLS attacks when subject to active wiretapping.

References

External links

Enabling Email Confidentiality through the use of Opportunistic Encryption
by Simson Garfinkel of the MIT Laboratory for Computer Science, May 2003
Windows KB article on NAT-T and DH2048
* – Opportunistic Encryption using the Internet Key Exchange (IKE) * – Pervasive Monitoring Is an Attack {{SSL/TLS Cryptographic software Internet Protocol based network software Internet privacy