With Apple no longer signing the iOS 10.2 firmware, users who upgraded to iOS 10.2.1 find themselves in a situation that makes jailbreaking impossible, since iOS 10.2 is the latest version that can be jailbroken. It is actually possible with Prometheus, but before you get too excited, you need to know what are the requirements to downgrade from iOS 10.2.1 or higher to iOS 10.2.
Prometheus is a set of tools that enables downgrading to older iOS versions, regardless of them being signed by Apple or not. It uses nonce collisions for the downgrading process from iOS 10.3 beta 1 to iOS 10.2, consequently it works on devices that produce these collisions. It must be either an iPhone 5s or possibly but not confirmed on iOS 10.3 beta 1, an iPad Air.
The method is to first download and install iOS 10.3 beta 1 and then downgrade to iOS 10.2 with futurerestore. This was confirmed by futurerestore’s creator Thimstar after this successful report of an iPhone 5s user downgrading from iOS 10.3 beta 1 to iOS 10.2 was tweeted.
Here are all the requirements you have to meet before proceeding to downgrade your iPhone 5 or possibly iPad Air to iOS 10.2:
1. Your device is running iOS 10.2.1 or iOS 10.3 beta.
2. You have a device which produces nonce collisions (iPhone 5s or iPad Air).
3. You saved iOS 10.2 .SHSH2 blobs before Apple stopped signing the firmware.
If all these apply to you, you are ready to jailbreak your device. Start by moving from iOS 10.2.1 to iOS 10.3 beta 1. Afterwards, it is recommended to run noncestatistics so you can see which nonces collide the most for your device on iOS 10.3 beta 1 and; then you should check your saved iOS 10.2 blobs to see if there are blobs saved with those nonces.
This step is optional, whether you choose to do it or not, you can proceed to running futurestore with all your saved nonces and see if any collides to match your blob.
It is very likely for the two nonce which collide on iOS 10.3 beta 1 to be the same with the ones which collided for iPhone 5s on iOS 10.2.
If you are lucky enough, having the requisite blobs and technical skills to use this tool should give you the chance to successfully downgrade to iOS 10.2 and enjoy jailbreaking your device.