iPhone Understandable explanation of IOS downgrades?

[hakr100]

I'm just wondering why it is not possible to downgrade the IOS on an iPhone from 6.14 to 6.12 or whatever.

I know I can wipe a hard drive clean and install an older OS on a computer, assuming the OS supports the hardware on the computer, and usually it does, within limits or by downloading drivers.

So, I guess the question is, why can't you erase what is on the iPhone and reinstall the OS (within reason) you want?

Plain English if you can...I only have an M.A. in English lit.

 

[dhlizard]

We could answer you if you provided an iPhone model.

 

[Intell]

We could answer you if you provided an iPhone model.

He mentions iOS 6.1.4, that would mean iPhone 5. You can't downgrade those because Apple isn't signing iOS 6.1.2 anymore and there is no way to bypass the checks they've put in place.

 

[hakr100]

Currently I have an iPhone 5 replacement phone from Apple, because my original iPhone 5, which was jailbroken, suffered from a broken power switch. The new iPhone had 6.1.3 installed, and is unjailbreakable.

----------

He mentions iOS 6.1.4, that would mean iPhone 5. You can't downgrade those because Apple isn't signing iOS 6.1.2 anymore and there is no way to bypass the checks they've put in place.

I don't really know what is meant by "signing," and "checks."

 

[Troneas]

He mentions iOS 6.1.4, that would mean iPhone 5. You can't downgrade those because Apple isn't signing iOS 6.1.2 anymore and there is no way to bypass the checks they've put in place.

i think he wants to understand the reasoning behind apple preventing others to downgrade, not so much the technicality of it.

 

[dhlizard]

He mentions iOS 6.1.4, that would mean iPhone 5. You can't downgrade those because Apple isn't signing iOS 6.1.2 anymore and there is no way to bypass the checks they've put in place.

Wasn't sure due to abstract nature of the question - the "whatever" OP tacked on (as if he might have randomly just picked firmware versions).

Apple utilizes an authentication system to prevent firmware downgrades. Due to a weakness in older handsets (3G, 3GS, iPhone 4) an exploit called Limera1n exploits that weakness. When combined with saved SHSH, it allows authentication by way of accessing a substitute server in lieu of Apple's server.

Google - Caching Apples Signature Server - good place to begin your reading/research

 

[Applejuiced]

Research what shsh blobs are and do.
Then you will understand the security measures put there by Apple so people cannot downgrade or restore to older firmware versions not signed by Apples servers.

What is a SHSH blob?

Every time you do a restore to install a firmware, your device checks if it is allowed to install that version by asking Apple (called "calling home"). Apple usually only allows installing the latest version of the firmware. So if you bought an iPhone with firmware 4.1 on it, you cannot reinstall firmware 4.1, because Apple doesn't allow you.

During this "calling home", iTunes asks an Apple server if it is allowed to install this firmware version on your phone. It sends the ECID (something like the serial number) and the firmware version to Apple. iTunes gets back a SHSH blob. That will be sent to your phone and your phone then checks if it is allowed to install this firmware.

The trick is now that you can backup the SHSH that you get from Apple. So if you want to install the firmware that Apple is signing today in a far future again, you just give iTunes (and your phone) the SHSH that you backed up.

There is NO WAY to generate SHSH blobs. If Apple doesn't issue them anymore, you cannot generate them.

Cydia server usually backs them up for you, but you can have them also additionally on your computer. In order to downgrade, you just have to change your hosts file so that when iTunes asks Apple for the SHSH, it points to some other computer, which provides the SHSH from the backup.

Many users have SHSH backups without knowing it. To find out for which firmware versions you have a backup, run the TinyUmbrella[1] tool. With TinyUmbrella[1] you can also backup your SHSH blobs, even without a jailbreak.

The old iPhone 3G doesn't have this check built-in, but since iOS 4.0 there's still a check if the SHSH is correct. This means that to downgrade an iPhone 3G to 4.1 you still need the SHSH backup. Or you have to use another tool to install the firmware (like redsn0w[2]).

Since iOS 5 and with all newer devices, Apple switched to AP-Tickets. You can't use AP-Tickets to replay them to downgrade again, so this method is now mainly useless and you can't downgrade any longer if you're missing the apticket.
Currently even if you had those unique shsh blobs saved for a particular 4S or iphone 5 you cannot use them since there is no hardware exploit like limerain that can be used on older devices such as iphone 3GS and iphone 4.

 

[darricksailo]

I'm just wondering why it is not possible to downgrade the IOS on an iPhone from 6.14 to 6.12 or whatever.

I know I can wipe a hard drive clean and install an older OS on a computer, assuming the OS supports the hardware on the computer, and usually it does, within limits or by downloading drivers.

So, I guess the question is, why can't you erase what is on the iPhone and reinstall the OS (within reason) you want?

Plain English if you can...I only have an M.A. in English lit.

Long story short, when you install the OS on a PC, it just installs from the disc/package

For an iPhone, it has to check with apple's server before the installation of the OS happens. If the server says "no", then the installation won't happen

On a PC, this form of server check doesn't exist

 

[blasto2236]

My interpretation of the OPs question is not HOW does this work, but WHY is it set up this way?

Honestly, unless you work for Apple (and even, then, probably a very specific part of Apple), it would be hard to say definitively why they choose to lock down iOS and not OSX.

 

[TriJetHero]

My interpretation of the OPs question is not HOW does this work, but WHY is it set up this way?

Honestly, unless you work for Apple (and even, then, probably a very specific part of Apple), it would be hard to say definitively why they choose to lock down iOS and not OSX.

main reason is security

the iphone is a common phone and suceptible to mallicious intend, locking it decreases the risk.

OSX is less risk as it is less common and UNIX is fairly stable. Compare this to the ease windows is hacked and the widespread use of pc's.

 

[Applejuiced]

My interpretation of the OPs question is not HOW does this work, but WHY is it set up this way?

Honestly, unless you work for Apple (and even, then, probably a very specific part of Apple), it would be hard to say definitively why they choose to lock down iOS and not OSX.

The why is basically Apples choice to make their devices secure and also to make jailbreaking harder.