IMPORTANT ADDITION! UNIVERSAL FIX, in-depth guide. Noob friendly.
After trying out pretty much everything and getting nothing to work, I managed to fix ALL possible problems: a ton of important things were not mentioned that are CRUCIAL for the sideloading to work.
I separated the guide into various sections, go through them top to bottom. (Start here)
Here are some errors I personally had and managed to fix:
1.) File is not a zip file
2.) Can't listdir a file
3.) Installation failed: 0 APIInternalError
4.) And other various errors. (Like 4 more I can't remember)
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Prerequisites:
1.) Uninstall iTunes and iCloud (if you don't have it installed, go to step 2)
2.) Install iTunes and iCloud WEB VERSION
3.) LOG IN TO BOTH APPLICATIONS!
4.) Install "7-Zip"
Do NOT JUST install it, LOG IN with your Apple ID.
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Download:
1.) Download the .ipa file from the iOS gods thread
2.) In some browsers, an error or warning may pop up that the file was detected as malicious and could not be downloaded. If this happens to you, use the default installation of Edge (no such error is generated, safe option).
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CRUCIAL STEPS:
Once you have downloaded everything you need, you need to disable some security options on your Windows PC to make sure everything works without interference. I am not 100% sure which ones are essential, so I recommend turning off everything I'm going to list! (Don't forget to turn it on again after the installation is finished.)
1.) Settings -> Update & Security -> Windows Security -> Virus & threat protection -> Virus & threat protection settings. Inside "Virus & threat protection settings": turn off "Real-time protection", "Cloud-delivered protection", and "Tamper Protection".
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The following is PROBABLY not needed, but I did it anyway:
Firewall & network -> disable "Domain Network" and "Private Network".
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Modify/Edit the IPA: NO WinZip needed
1.) Right-click on the IPA file and extract it to a random folder.
2.) Inside of the folder "CSR 2 - Realistic Drag Racing Hack" etc. will be a folder named "Payload", go into that folder and then go into the folder that comes up next "customstreetracer2.app".
3.) Inside of the folder "customstreetracer2.app" there will be TWO (2) important folders you HAVE to delete. Find and delete "PlugIns" AND "Extensions", that's ALL you need to do.
4.) Now go back until you see the folder "Payload" again. NOW you right click onto the "Payload" folder NOT "CSR 2 - Realistic Drag Racing Hack" folder, hover over "7-Zip", and you should now see an option to save as "Payload.7z" and "Payload.zip". USE THE "PAYLOAD.ZIP" option.
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Sideloadly configuration and install:
1.) Make sure your phone is connected and the PC is authorized to access it. If you're unsure, unplug your phone and plug it in again. When you plug your phone in, you should get a pop-up on your phone asking for access; press yes/confirm.
1.1. UNINSTALL the ORIGINAL CSR 2 app if you have it on your phone!
2.) ADMINISTRATOR PERMISSIONS: Find your Sideloadly icon/program and right-click to select "Run as administrator".
3.) Inside Sideloadly: Click the icon on the top left and select your "Payload.ipa" file. You may or may not be asked to enter your Apple credentials. Simply enter your Apple ID and password.
4.) Wait for the installation to finish. This CAN take a LOOOOOONG time (60 minutes or more), so do not panic.
5.) Once done, switch back to your iPhone/iPad and enable the Developer app you should see in your profiles (where you authorized the iOSGods app as well).
Important things to add:
You must have Developer Mode turned on on your iPhone/iPad
TURN ON THE SECURITY FEATURES YOU DISABLED ON YOUR PC ONCE THE INSTALLATION IS FINISHED!
Advanced Data protection can also mess up the installation. Disable it on your iPhone/iPad (Settings> my Apple ID > iCloud > Advanced Data Protection.)