iPhone jailbreak developer chpwn, along with the help of planetbeing and Ryan Petrich, have released Spire — a legal way to port Siri to currently unsupported iOS devices like iPhone 4 and iPad 2. Spire uses a new method to obtain the files necessary for Siri, so it doesn’t have the copyright issues encountered by previous attempts. Similarly, rather than directing all traffic through a specific proxy server (and the associated privacy issues), Spire allows you to specify your own proxy server.
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Legally port Siri to unsupported iPhones, iPad with Spire [Jailbreak]