The process is very simple all you have to do is to add Saurik’s Server address to your system hosts file. This trick make iTunes thinks it is talking to Apple, it is talking to Cydia instead while the restoring process. Doing this will allow iTunes to access signatures already stored by Cydia “on file” feature. In simple words this duplicates the functionality exposed by Apple’s signature server, except using “on file” results rather than live request.
Note: 1. This process is only for those who saved their SHSH to Cydia Server already.
2. Read the following process two-three times before starting the process.
How to Edit and Save Hosts file in Windows
Open Notepad with Administrative privileges by clicking Start, All Programs, Accessories, and then right-click Notepad and select to open as Administrator.
Now that Notepad is open with Administrative privileges browse to (C:\Windows\System32\drivers\etc\) in Notepad and open the hosts file, and add the following entry to the bottom of the file.
# 184.108.40.206 gs.apple.com
Note: If you do not see the hosts file, please be sure to select all files as seen in the picture below. If that doesn’t work then you can also access this file by typing this into run (C:\Windows\System32\drivers\etc\hosts)
Now you navigate to files menu and save the hosts file. By doing this you changed the communication server, and then we will ensure that iTunes checks the firmware on the server rather than opens Saurik toApple! Because your ECID is on that server, the verification will fail and restore the previous version will be done without too many problems!
How to Edit and Save Hosts file in Mac OS X
Navigate to /etc and look for hosts file, right click on hosts file and open with Text Edit, and add the following entry to the bottom of the file.
Now save the file and click on Save As. Because Text Editor can’t save your system files, here comes the tricky part, in save as windows use the following settings. Or take a close look at the following image.
A host file will be appear on your Desktop, just drag and drop to /etc and follow the onscreen instructions.
Press the Replace and enter your Password
By doing this you changed the communication server, and then we will ensure that iTunes checks the firmware on the server rather than opens Saurik to Apple! Because your ECID is on that server, the verification will fail and restore the previous version will be done without too many problems!
Now you can do your Downgrading from iOS 4 to 3.1.3 or below (with DFU Mode) as usual.
To Downgrade iPhone 3GS from iOS 4 to 3.1.3 or below:
1. Connect your iPhone to your computer
2. Turn iPhone off.
3. Hold power and home together for 10 seconds (exactly).
4. Release power but keep holding home until the computers beeps (observed on a PC) as a USB device is recognized.
5. A few seconds later iTunes should detect your iPhone.
Note: If the Restore Logo is present on the screen, you are in Recovery Mode, not DFU.
Once your iPhone is in DFU mode following message will be appeared in iTunes. Click on “OK”, Now hold shift key while clicking Restore in iTunes and browse the OS you wish to restore to your iPhone 3GS. Now the restore process will take 10 – 15 minutes. That’s it your done, you may now enjoy your downgraded OS.
Note: If you encountered error message like 1601, 1602, 1600 or anything else then try putting the iPhone into DFU mode and try again or check to make sure your hosts file is correct and that you did have SHSH’s on file on Cydia for the OS your attempting to restore to.