Meaning: A kind of cracking software on iPhone, iPod Touch, ipad and other devices, similar to the client of the software platform of Apple's online software store iTunes Store, was loaded into the system during jailbreak, and most of them were third-party software and patches of iPhone, iPod touch and iPad, mainly to make up for the shortcomings of the system.

Led by Jay Freeman(Saurik), it was jointly developed by Okori Group and UCSB University.

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Cydia will download the software package directly and install it in the same /Applications directory as the built-in program of iOS. At the same time, this will not affect the original purchase and download of software in the App Store.

In September 2009, Cydia provided a new function besides software, that is, saving the digital signature required for iOS recovery, also known as SHSH (Certificate for Verifying the Legitimacy of iTunes Firmware Recovery Operation), which was used by Apple to verify the process of iOS Firmware Recovery. This allows users to restore the iOS firmware to an earlier version on the new bootrom iPhone 3Gs and higher devices, and can easily use the vulnerability of the earlier version of iOS to escape from prison in the form of a replay attack.