Although many people criticize the implementation of the Siri system in iOS and consider that Apple's assistant is extremely useless, just as many people are very interested in installing it on old iDevices. Until now, we have been shown that Siri can run on old iDevices, but the big problem was that ordinary users could not reproduce this functionality on their own terminals, until the hackers "took heart in their teeth" and started to release tweaks that violate Apple's copyright.
Several tweaks were released in Cydia that brought an almost functional Siri for iDevices and the most recent of them is called H1 Siri. This tweak allows iPhone 4 and iPod Touch 4G owners to install Siri on their terminals, but by installing the tweak, the possibility to use the video recording function is blocked. Moreover, by installing H1Siri you will connect your terminal to the server of the hackers who developed the tweak and with each query made through Siri you will send them information about: contact book, Calendar, bookmarks, Reminders, locations and email hate.
Unfortunately, now there is no 100% good and functional method to install Siri on old iDevices without risk. It has been said repeatedly that this will not happen until the iPhone 4S is jailbroken and for now the statement is true. Any tweaks that promise to install Siri could leave you without the Settings app, without the ability to record video, without the ability to use iOS, and worse could send confidential information to hackers' servers.
I recommend that you avoid, at least for now, any tweaks that promise to install Siri.
