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I've updated the iOS on my iPhone 4 to 5.0.1 the baseband was also updated to 04.11.08. I've downgraded the OS it but the baseband didn't change, can someone help please?

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For unlocks see apple.stackexchange.com/questions/32390/… –  bmike Dec 6 '11 at 14:09
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By now it is possible to down-grade your baseband - at least for the iPhone 3G and 3GS (unfortunately not 4/S) - from the iPad's 06.15 to 05.13.04 (un-lockable) thanks to the great work of the iPhone Dev Team: See this post on their blog. –  iolsmit Jul 5 '12 at 0:46
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up vote 5 down vote accepted

In general, it's not possible to downgrade the modem firmware / baseband software version on iOS, even if you jailbreak the device or use software designed to install modified software on an iPhone.

It's a popular request, so I imagine many people are working on it but nothing is out today to perform the task you ask on the iPhone 4.

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Apple signs all versions of iOS digitally. This signature is checked before installation. If no active signature is found, the version cannot be installed. Apple keeps the signature active at all times for the current version. And the immediate prior version for only a few days after the current version is released. Between this time, a user can swap back and forth freely (including the units baseband). Once that window is close, the baseband can never be reverted, not even using tools from the jailbreak community. –  cksum Mar 14 '12 at 5:08
    
Awesome - was not aware of the short window of back. Very interesting. Is the signing part of SHSH or similar, but different and independent of the OS level? –  bmike Mar 14 '12 at 6:04
    
The signing is a server-side certificate (or token) issued by Apple. It can be thought of as a trusted certificate. Once Apple revokes that certificate from a particular version of iOS, it becomes unusable. The process starts with iTunes, which queries Apple's servers before installation commences. If it can't reach Apple's servers or receives a revocation notice instead of getting a green-light, it will halt the installation. Rerouting to Cydia's servers have worked in the past, but they are no longer assured as Apple is tightening the process. –  cksum Mar 14 '12 at 8:37
    
Apple waits a few days to revoke the old certificate (just to play it safe). So the day 5.1 was released, Apple was still signing 5.0.1. That meant you could bounce between the two, fully downgrading firmware and respective baseband without trouble. But after a few days, Apple stopped signing 5.0.1, effectively closing any possibility of a downgrade (unless you had a non-A5+ device and SHSH blobs). And even if you did rely on SHSH blobs to downgrade, that does not reverse the baseband. Nothing can alter that unless you downgrade within Apple's limited window. –  cksum Mar 14 '12 at 8:41
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