Psi (pronounced [ saɪ ]) is an instant messaging - program for the XMPP (XMPP).
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The aim of the Psi project is a powerful XMPP offer client. So attempts at programming work, to adhere strictly to the XMPP templates and XEPs. This means that the Psi team no features implemented, as long as they do not from the XMPP community as standard were accepted. Thus, to ensure that Psicompatible remains and stable.
Psi is a powerful XMPP client who strictly adhere to the standards of the XMPP Standards Foundation maintains.
With preference profiles can (also on a computer, on a [operating system] account) for multiple users separate settings are saved. Support for ID prioritization easier from the simultaneous use of an account from multiple computers.
Psi provides extensive presence notification that also determines, for example, and transmits, if the user just typed in or not (english "typing notification").
Psi is also full-duplex audio Connected ( IP telephony ) and between XMPP users (unencrypted) files transferred.
Psi allows end-to-end encryption of text objects to the OpenPGP standard and point-to-point encryption with TLS .
Psi provides a graphical user interface, which also window transparent to serve. Several open chat sessions can a tabs are displayed system under their own tabs in the same window. The surface was in many languages translated (language packs, for example, German , Chinese , French , Italian and Spanish ).Many in the popular Jisp format (used by AIM , iChat , Trillian , ...) available emoticon packages ( smileys ) can be used.
It is available as free software in source code under the terms of version 2 or higher of the GNU General Public License available (GPL), with the additional permission for binding to the Qt (also the commercial version) and OpenSSL libraries without their having to enclose source.
Psi runs under Linux , Windows , Mac OS X and eComStation . Installation packages ( RPM , deb ) are available for many Linux distributions . Available ports are also available for FreeBSD - andSolaris - Operating Systems . It is in most popular Linux distributions installed directly from the standard repositories.   
Psi is in the programming language C ++ written and has one on the Qt -based graphical user interface library. The program in the library Iris  encapsulated XMPP functionality is developed in a separate sub-project of a common parent project (the "Delta XMPP Project"). The telephony functionality uses the protocol extension jingle and transmits the voice signal by means of the speech codecSpeex .
For the point-to-point encryption for communication servers TLS implementation of is OpenSSL -Project used for the OpenPGP end-to-end encryption of text messages can GnuPG be involved. In addition, for Psi exists a plugin for off-the-record messaging (OTR)  . Also in Psi +  OTR can be used (Windows and Linux)  .
The project was initiated in 2001 by Justin Karneges.  The programming of the base code could be paid in the meantime Karneges - then Psi developed very well. Nevertheless, the release of the proceeds Publications relatively slow. Justin Karneges left the project in late 2004. The maintenance of the project was since then in the hands of Kevin Smith. In early 2009, Smith returned to the scepter Karneges which again revives the somewhat lately creeping developing new wind. The core team also consists of Mikhail Pishchagin, Hal Rottenberg and Remko Tronçon. From mid-October 2007 published development version 0.11 was the Jabbin project cleaved , the support for IP telephony introduced on XMPP with free software.  Psi took this functionality in the end of June 2009 released version 0.13 worked at the end of 2005 was,  after the property occupied by the functional protocol extension Jingle was standardized. The latest stable version (0.15) comes from 3 October 2012. Since then, found, for example in a spin-off called "Psi +" still active development work instead. 
"Psi" is the abbreviation of Psynergy , a word created from psychic (supernatural) and energy ( energy ). Psi is also a Greek letter ( Ψ ), this is the logo of the Messengers.