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Comment by lyso | Hacker News Reader

> or at least have something on the device verify the origin and integrity of iOS images before allowing them to be installed or run

They do prevent you from downgrading your iOS image by refusing to authenticate old versions on install. I find it hard to interpret this as simply "downgrading is not supported" as Gruber does, as it is an active step on Apple's part to stop you from doing something. Installing Linux on my Macbook is also supported by Apple, but that doesn't mean they actively alter my Macbook's boot process to stop me from doing it.

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Unless things have changed with the iPhone 4, the only case in which downgrade is not possible is if the upgrade modifies the baseband firmware. At least that has been my experience. I've downgraded iOS on my phone a number of times, most recently from the latest beta of 4.1 to 4.0.2.

You may argue otherwise, but from where I stand, preventing downgrading and thus preventing jailbreaking has been a (fortunate for Apple?) side effect, rather than a goal.

This changed with the 3GS, where Apple began issuing signatures for individual devices to authorize firmware installs. This procedure is also in play for the 3rd generation iPod Touch, iPhone 4, and iPad.

The firmware install process includes a phone-home procedure that verifies with Apple that the device is authorized to install that firmware.

Once a new firmware image is released, Apple just stops issuing signatures for the previous releases and devices can no longer install it.

The procedure is vulnerable to a replay attack, and so Cydia offers a stand-in replacement for Apple's authorization server, but users who want to downgrade have to be savvy enough to save their auth token ahead of time.

OK, I stand corrected. I'm stuck with a 3G. Probably my example of downgrading from a beta to the current release is no good either - it will likely still work with the system you described, but it is not indicative of the ability to arbitrarily downgrade the OS>

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