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For this case, consider a phone stuck in a bootloop (not fixable through data wiping or USB jigs) with no CWM Recovery and no adb or fastboot access. Only an SD Card, the stock recovery mode, and Qualcomm's QPST serial port debugging interface.

Relevant files available are a handful of NANDroid backups of the same phone (made previously via CWM Recovery) and an "update.zip" from the provider/manufacturer that contains an OTA update.

Goal is to restore a NANDroid backup using the stock recovery's "apply update from external storage" feature.

Update: I've dug around, and as far as I understand, my stock recovery mode requires properly signed .zip files. I've made numerous .zips up on my own using various scripts and toolsets I had found, and some by tampering with the OTA update .zip I had mentioned. Attempting to apply either of them results in the exact same output that follows below. (The only one going past that point is the original OTA update.zip, whose instalation dies after the message "file_getprop: failed to stat "/system/build.prop": No such file or directory", but this is certainly part of the bigger problem with the device)

-- Install /sdcard ...
Finding update package...
Opening update package...
Verifying update package...
Instalation aborted.

Is my case hopeless without having the manufacturer's private key on hand? Can you think of any workarounds or alternatives? Could I be making a mistake in the zip-making process?

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Most stock recovery modes require proper, officially signed update.zip files. There are ways to convert NANdroid backups to update.zip files or even directly back them up as such but the hard part is actually signing the update file (there are a handful of tutorials for doing these on the Internet, and in part in the official Android documentation). The signing requires official keys which can only be generated by the manufacturers. Getting them is nigh impossible unless you have inside access or very very cooperative support.

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You can't get there from here. As you found out, almost every OEM requires images flashed via the stock firmware/recovery to be cryptographically signed by the OEM to prevent tampering. Considering that they're the only one that are going to have the private signing keys, you're not going to be able to make your own image.

The most likely path to a working phone is to get an official image from the OEM. For example, RUUs for HTC phones and ODIN images for Samsung phones. These are generally available online (in some cases, available from the OEM or carrier themselves) and have the advantage of being properly signed for flashing via either stock recovery or bootloader access. It may be possible to get an official image from the OEM support as well.

Once you have a working device again, you can root (or unlock the bootloader) and flash your preferred recovery and restore the backup you do have.

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