The ICQ system was originally created for communication using personal computers. But as the availability of mobile phones increased with access to the Internet, programmers began to come up with ways to use this system from them. At first, all applications for this purpose were unofficial, but then official ones appeared.
Instructions
Step 1
During the period when ICQ was owned by AOL, the agreement between the company and the user prohibited the use of alternative clients, and the official ones were only for computers running Mac OS and Windows. But in practice, no one was punished for using alternative programs. Those were created both for Mac OS and Windows, and for OS where there were no official clients, for example, Linux. And there was no software solution for mobile phones yet. WAP browsers were already built into many of them, but the ability to run Java applications was not yet available in all. Therefore, a website called TJAT was created. It worked like this: an alternative client was launched on the server, and the user from a phone with a WAP browser went to the web interface, entered the number and password, and then got the opportunity to send and receive messages. The server acted as a kind of bridge, interacting with the ICQ server in the "understandable" language, and with the WAP browser - in the "understandable" to it. Once this server was hacked, and attackers gained access to a number of passwords. But by that time it was no longer relevant for most users.
Step 2
This was due to the fact that phones with the ability to run Java applications soon became widespread. This made it possible to create ICQ clients directly on mobile phones, without requiring a "translator" server. The programmers created several alternative clients, the most famous of which was JIMM. From the point of view of the interaction protocol, he imitated the official client, so the ICQ server willingly interacted with him. AOL then declared a quiet war on unofficial customers, including JIMM. Changes were made to the protocol, which were reflected in the official client, but the authors of the unofficial ones did not immediately have time to decipher and reflect these changes in their developments. After several such attempts, AOL gave up, realizing that sooner or later the developers would "pull up" their programs to change the protocol. For Linux users then there was already an official client in the form of a Flash application, which, however, worked much worse than third-party developments. For mobile phones, there were only unofficial applications.
Step 3
Jabber users, where unofficial clients were never banned, and therefore there were many such programs for mobile phones, could access ICQ through gateways. These are also programs running on servers. Like TJAT, they "talked" to the ICQ server in a "understandable" language, but when interacting with a mobile phone they had to exchange information not with a WAP browser, but with a Jabber client. During the "quiet war" with alternative clients, they often refused to function as well. There have been cases of hacking of such gateways, but they were rare.
Step 4
The situation changed for the better after ICQ was bought from AOL by the Mail. Ru group. The new owner allowed the creation of alternative clients and gave programmers access to the protocol description. But on the other hand, the need for unofficial applications has almost disappeared. Initially, ICQ support was added to Mail. Ru Agent, for which there was already an official client then. Then the official ICQ mobile client was released with support also for Mail. Ru Agent. In fact, these were two almost identical programs, differing in the main design. Both of them interacted with the server directly, and also soon introduced Jabber support into both programs. The result is multi-protocol clients that differ little from third-party developments.
Step 5
Today official ICQ clients, interacting with the server directly, are available for most common mobile platforms. There is also an official client for Linux desktops, as well as an official web client that works the same way as TJAT. It does not require Flash, but you can use it through a regular browser from both a computer and a mobile phone.
