Secure communication is when two entities are communicating and do not want a third party to listen in. For that they need to communicate in a way not susceptible to eavesdropping or interception. Secure communication includes means by which people can share information with varying degrees of certainty that third parties cannot intercept what was said. Other than spoken face-to-face communication with no possible eavesdropper, it is probably safe to say that no communication is guaranteed secure in this sense, although practical obstacles such as legislation, resources, technical issues (interception and encryption), and the sheer volume of communication serve to limit surveillance.
With many communications taking place over long distance and mediated by technology, and increasing awareness of the importance of interception issues, technology and its compromise are at the heart of this debate. For this reason, this article focusses on communications mediated or intercepted by technology.
Also see Trusted Computing, an approach under present development that achieves security in general at the potential cost of compelling obligatory trust in corporate and government bodies.
One of the most famous systems of secure communication was the Green Hornet. During WWII, Winston Churchill had to discuss vital matters with Franklin D. Roosevelt. At first, the calls were made using a voice scrambler as this was thought to be secure. When this was found to be untrue the engineers started work on a whole new system, the Green Hornet or SIGSALY. Anyone listening in would just hear white noise but the conversation was clear to the parties. As secrecy was paramount, the location of the Green Hornet was only known by the people who built it and Winston Churchill, and if anyone did see him entering the room it was kept in, all they would see was the Prime Minister entering a closet labeled 'Broom Cupboard.' It is said that because the Green Hornet works by a one-time pad it cannot be beaten.
Nature and limits of security 
Types of security 
Security can be broadly categorised under the following headings, with examples:
- Hiding the content or nature of a communication
- Code – a rule to convert a piece of information (for example, a letter, word, phrase, or gesture) into another form or representation (one sign into another sign), not necessarily of the same type. In communications and information processing, encoding is the process by which information from a source is converted into symbols to be communicated. Decoding is the reverse process, converting these code symbols back into information understandable by a receiver. One reason for coding is to enable communication in places where ordinary spoken or written language is difficult or impossible. For example, semaphore, where the configuration of flags held by a signaler or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.
- Identity Based
- Hiding the parties to a communication – preventing identification, promoting anonymity
- Hiding the fact that a communication takes place
- "Security by obscurity" – similar to needle in a haystack
- Random traffic – creating random data flow to make the presence of genuine communication harder to detect and traffic analysis less reliable
Each of the three is important, and depending on the circumstances any of these may be critical. For example, if a communication is not readily identifiable, then it is unlikely to attract attention for identification of parties, and the mere fact a communication has taken place (regardless of content) is often enough by itself to establish an evidential link in legal prosecutions. It is also important with computers, to be sure where the security is applied, and what is covered.
Borderline cases 
A further category, which touches upon secure communication, is software intended to take advantage of security openings at the end-points. This software category includes trojan horses, keyloggers and other spyware.
These types of activity are usually addressed with everyday mainstream security methods, such as antivirus software, firewalls, programs that identify or neutralize adware and spyware, and web filtering programs such as Proxomitron and Privoxy which check all web pages being read and identify and remove common nuisances contained. As a rule they fall under computer security rather than secure communications.
Tools used to obtain security 
Encryption is where data is rendered hard to read by an unauthorised party. Since encryption can be made extremely hard to break, many communication methods either use deliberately weaker encryption than possible, or have backdoors inserted to permit rapid decryption. In some cases government authorities have required backdoors be installed in secret. Many methods of encryption are also subject to "man in the middle" attack whereby a third party who can 'see' the establishment of the secure communication is made privy to the encryption method, this would apply for example to interception of computer use at an ISP. Provided it is correctly programmed, sufficiently powerful, and the keys not intercepted, encryption would usually be considered secure. The article on key size examines the key requirements for certain degrees of encryption security.
The encryption can be implemented in a way to require the use of encryption, i.e. if encrypted communication is impossible then no traffic is sent, or opportunistically. Opportunistic encryption is a lower security method to generally increase the percentage of generic traffic which is encrypted. This is analogous to beginning every conversation with "Do you speak Navajo?" If the response is affirmative, then the conversation proceeds in Navajo, otherwise it uses the common language of the two speakers. This method does not generally provide authentication or anonymity but it does protect the content of the conversation from eavesdropping.
Steganography ("hidden writing") is the means by which data can be hidden within other more innocuous data. Thus a watermark proving ownership embedded in the data of a picture, in such a way it is hard to find or remove unless you know how to find it. or, for communication, the hiding of important data (such as a telephone number) in apparently innocuous data (an MP3 music file). An advantage of steganography is plausible deniability, that is, unless one can prove the data is there (which is usually not easy), it is deniable that the file contains any. (Main article: Steganography)
Identity based networks 
Unwanted or malicious behavior is possible on the web since it is inherently anonymous. True identity based networks replace the ability to remain anonymous and are inherently more trustworthy since the identity of the sender and recipient are known. (The telephone system is an example of an identity based network.)
Anonymized networks 
Recently, anonymous networking has been used to secure communications. In principle, a large number of users running the same system, can have communications routed between them in such a way that it is very hard to detect what any complete message is, which user sent it, and where it is ultimately going from or to. Examples are Crowds, Tor, I2P, Mixminion, various anonymous P2P networks, and others.
Anonymous communication devices 
In theory, an unknown device would not be noticed, since so many other devices are in use. This is not altogether the case in reality, due to the presence of systems such as Carnivore and Echelon which can monitor communications over entire networks, and the fact that the far end may be monitored as before. Examples include payphones, Internet cafe, etc.
Methods used to "break" security 
The placing covertly of monitoring and/or transmission devices either within the communication device, or in the premises concerned.
Computers (general) 
Any security obtained from a computer is limited by the many ways it can be compromised – by hacking, keystroke logging, backdoors, or even in extreme cases by monitoring the tiny electrical signals given off by keyboard or monitors to reconstruct what is typed or seen (TEMPEST, which is quite complex).
Laser audio surveillance 
Sounds, including speech, inside rooms can be sensed by bouncing a laser beam off a window of the room where a conversation is held, and detecting and decoding the vibrations in the glass caused by the sound waves.
Systems offering partial security 
Anonymous cellphones 
Cellphones can easily be obtained, but are also easily traced and "tapped". There is no (or only limited) encryption, the phones are traceable – often even when switched off – since the phone and SIM card broadcast their International Mobile Subscriber Identity (IMSI). It is possible for a cellphone company to turn on some cellphones when the user is unaware and use the microphone to listen in on you, and according to James Atkinson, a counter-surveillance specialist cited in the same source, "Security-conscious corporate executives routinely remove the batteries from their cell phones" since many phones' software can be used "as-is", or modified, to enable transmission without user awareness  and the user can be located within a small distance using signal triangulation and now using built in GPS features for newer models.
Analogue landlines are not encrypted, and it is very easy to tap them. Such tapping requires physical access to the line, easily obtained from a number of places, e.g. the phone location, distribution points, cabinets and the exchange itself. Tapping a landline in this way can enable an attacker to make calls which appear to originate from the tapped line.
Anonymous Internet 
Using a third party system of any kind (payphone, Internet cafe) is often quite secure, however if that system is used to access known locations (a known email account or 3rd party) then it may be tapped at the far end, or noted, and this will remove any security benefit obtained. Some countries also impose mandatory registration of Internet cafe users.
Anonymous proxies are another common type of protection, which allow one to access the net via a third party (often in a different country) and make tracing difficult. Note that there is seldom any guarantee that the plaintext is not tappable, nor that the proxy does not keep its own records of users or entire dialogs. As a result anonymous proxies are a generally useful tool but may not be as secure as other systems whose security can be better assured. Their most common use is to prevent a record of the originating IP, or address, being left on the target site's own records. Typical anonymous proxies are found at both regular websites such as Anonymizer.com and spynot.com, and on proxy sites which maintain up to date lists of large numbers of temporary proxies in operation.
A recent development on this theme arises when wireless Internet connections ("Wi-Fi") are left in their unsecured state. The effect of this is that any person in range of the base unit can piggyback the connection – that is, use it without the owner being aware. Since many connections are left open in this manner, situations where piggybacking might arise (willful or unaware) have successfully led to a defense in some cases, since it makes it difficult to prove the owner of the connection was the downloader, or had knowledge of the use to which unknown others might be putting their connection. An example of this was the Tammie Marson case, where neighbours and anyone else might have been the culprit in the sharing of copyright files. Conversely, in other cases, people deliberately seek out businesses and households with unsecured connections, for illicit and anonymous Internet usage, or simply to obtain free bandwidth.
Programs offering more security 
- Skype – secure voice over Internet, secure chat. Uses 128-bit AES (256-bit is the standard) and 1024-bit asymmetrical protocols to exchange initial keys (which is considered relatively weak by NIST). Proprietary. No information on backdoors. An article in 2004 suggested that Skype has relatively weak encryption, but more recent analyses, one by invitation and one by reverse engineering presented at DEF CON 2005, both conclude that Skype uses encryption effectively. Criticism focuses upon its proprietary "black box" design, its relatively short (1536 bit) keys, excessive bandwidth use of user supernodes, and excessive trust of other computers able to "speak Skype". (See Skype#Security)
- Zfone is an open source secure voice over Internet program implementing the ZRTP encryption protocol, by Phil Zimmermann, the creator of PGP.
- Jitsi is another open source voice and chat client implementing ZRTP and STRP encryption for calls, as well as OTR security for chat.
- I2P-Messenger is a simple secure (end-to-end encrypted), anonymous, and serverless instant messenger with file transfer support.
- pbxnsip is a SIP-based PBX that uses TLS and SRTP to encrypt the voice traffic. In contrast to other proprietary protocols, the protocol is open so that devices from independent vendors can be used. The encryption includes the relay of instant messaging, presence information, and the management interface.
- Secure IRC and web chat – Some IRC clients and systems use security such as SSL. This is not standardised. Likewise some web chat clients such as Yahoo Messenger use secure communications on their web based program. Again the security of these is unverified, and it is likely the communication is not secured other than to and from the client.
- Trillian – offers secure IM facility, however appears to have weaknesses in key exchange which would enable a "man in the middle" attack with ease. Proprietary, no information on backdoors.
- Off-the-Record Messaging (OTR) is a plugin which adds end-to-end encryption, authentication and perfect forward secrecy (PFS) to instant messaging. It is not a separate protocol but runs under most every instant messaging (IM) protocol.
- WASTE – open source secure IM, high strength "end to end" encryption, within an anonymised network.
- Secure email – some email networks such as "hushmail" or Opolis Secure Mail, are designed to provide encrypted and/or anonymous communication. They authenticate and encrypt on the users own computer, to prevent transmission of plain text, and mask the sender and recipient. Mixminion and I2P-Bote provide a higher level of anonymity by using a network of anonymizing intermediaries, (similar to how Tor and crowds work above, but at a higher latency).
- Cryptocat is a simple, open source, online encrypted chat service and software project supporting encrypted file transfers and up to 10 users in a room. The service can be used any modern browser, including those on mobile devices. A local app for Chrome exists, with more platforms in development.
- AESpad.com – open source online encrypted secure chat. Uses 256-bit AES symmetrical encryption. Relies on a pre-shared key between chat participants.
- ChatCrypt.com – another online encrypted secure chat. It uses the 256-bit AES symmetrical encryption in CTR mode.
See also 
General background 
Software selections and comparisons 
- Hepting vs. AT&T, a 2006 lawsuit in which the Electronic Frontier Foundation alleges AT&T Inc. allowed the NSA to tap all of its clients' Internet and Voice over IP communications
- NSA warrantless surveillance controversy
- Kish cypher
- Secret cell phone
- D. P. Agrawal and Q-A. Zeng, Introduction to Wireless and Mobile Systems (2nd Edition, published by Thomson, April 2005) ISBN 978-0-534-49303-5
- J.K. and K. Ross, Computer Networking (2nd Ed, Addison Wesley, 2003) ISBN 978-0-321-17644-8
- The schematics are illustrated in U.S. Patent 613,809 and describes "rotating coherers".
- "High-tech bugging techniques, and a costly fix". Popular Science. Google. August 1987.
- Wall Street Journal: How concerned are you that the iPhone tracks and stores your location?
- Open Wi-Fi proves no defence in child porn case, The Register
- 'Extortionist' turns Wi-Fi thief to cover tracks, The Register
- NIST recommendations from 2005 state that 1024-bit asymmetric ciphers are the lowest standard it considers when evaluating ciphers, and that this is expected to be categorized as technically weak or breakable from around 2006–2010.  (click on NIST and scroll to the last table, or see article: Key size).