An anonymizer or an anonymous proxy is a tool that attempts to make activity on the Internet untraceable. It is a proxy server computer that acts as an intermediary and privacy shield between a client computer and the rest of the Internet. It accesses the Internet on the user's behalf, protecting personal information by hiding the client computer's identifying information.
There are many reasons for using anonymizers. Anonymizers help minimize risk. They can be used to prevent identity theft, or to protect search histories from public disclosure.
Some countries apply heavy censorship on the internet. Anonymizers can help in allowing free access to all of the internet content, but cannot help against persecution for accessing the Anonymizer website itself. Furthermore, as information itself about Anonymizer websites are banned in these countries, users are wary that they may be falling into a government-set trap.
Anonymizers are also used by people who wish to receive objective information with the growing target marketing on the internet and targeted information. For example, large news outlets such as CNN target the viewers according to region and give different information to different populations. Websites such as YouTube obtain information about the last videos viewed on a computer, and propose "recommended" videos accordingly, and most of the online targeted marketing is done by showing advertisements according to that region. Anonymizers are used for avoiding this kind of targeting and getting a more objective view of information.
Use of anonymizers 
Protocol specific anonymizers 
Sometimes anonymizers are implemented to work only with one particular protocol. The advantage is that no extra software is needed. The operation occurs in this manner: A connection is made by the user to the anonymizer. Commands to the anonymizer are included inside a typical message. The anonymizer then makes a connection to the resource specified by the in-bound command and relays the message with the command stripped out.
Protocol independent anonymizers 
In this case either the desired application must support the tunneling protocol, or a piece of software must be installed to force all connections through the tunnel. Web browsers, FTP and IRC clients often support SOCKS for example, unlike telnet.
Use of multiple relays 
Proxies can be daisy chained. Chaining anonymous proxies can make traffic analysis far more complex and costly by requiring the eavesdropper to be able to monitor different parts of the Internet. An anonymizing remailer can use this concept by relaying a message to another remailer, and eventually to its destination.
Even stronger anonymity can be gained by using Tor. Tor is not merely a proxy chain, but an onion router, which means that routing information (as well as message content) is encrypted in such a way as to prevent linking the origin and destination. Like all anonymity networks, Tor cannot end-to-end encrypt messages destined for the public Internet; that must be arranged between the sender and recipient. Tor's hidden service protocol does, however, provide end-to-end encryption, along with the ability to anonymize servers to make them more censorship-resistant.
Another anonymity network is I2P. Unlike Tor, I2P is a fully internal network. The philosophy behind I2P is that each node routes traffic for others and blends its own traffic in, whereas one's own traffic will be relayed by other peers through so-called tunnels made up of various other peers. As you never know if a given mix logs all connections or not, the only way to be really sure there is no logging is to run your own anonymizing mix node and blend your traffic with those of other users, who in turn need not trust you, as they blend their traffic with yours and other users' traffic in their own mix nodes. The network is highly dynamic and totally decentralized. It also takes care of other nodes learning about your node existing, for without peers using your node, there would be no traffic to blend yours with. As all traffic always stay within the I2P network, a routing user's I2P can remain end-to-end encrypted and will never show on public websites' logs.
See also 
- Anonymous web browsing
- CGIProxy, web based proxy Perl script often used as anonymizer solution,
- Java Anon Proxy - a proxy system designed to allow browsing the Web with revocable pseudonymity.
- "How Anonymizers Work". The Living Internet. Retrieved 2007-08-03.
- (Reference from Censorship in Singapore#Internet) Mixing welfare and elitism in Singapore", Alex Au, Asia Times, November 23, 2006.
- (Reference from Censorship in Saudi Arabia#Internet) http://www.tgdaily.com/business-and-law-features/53403-saudi-arabia-bans-blogging-without-a-licence
- (Reference from Censorship in North Korea - where internet access itself is illegal) "List of the 13 Internet enemies". Reporters Without Borders. Archived from the original on 2 January 2008. Retrieved January 9, 2008.
- (Reference from Internet censorship in Iran) OpenNet Initiative. (2006.) "Internet Filtering in Iran in 2004-2005: A Country Study". In these countries most Anonymizer websites are banned
- See references above i.e. Iran banned Tor
- Anonymizer to provide censor-free internet to China (Anonymizer.com website)
- US FBI uses 'Anonymizer trap' to catch online pedofiles (Security Focus website)
- About targeted advertising on an anonymizer (FastCompany Anonymizer website) - an article describing the typical targeted advertising avoidance by most anonymizer services, while this one enables continued online targeted marketing
- The hack of the year
- Privacy-friendly law enforcement 2009