These last days I must be set on a Apple devices destruction mode. First I lost access to my MacBook while trying to increase its physical security – I configured it to boot from network and I lost all access to boot sequence commands. I think my model has an EFI bug because the security-mode set to full doesn’t ask for a password when I start/restart my laptop, only asks for password if I want to boot from other devices. I had to install a Snow Leopard Server to boot from a netboot image (the process works extremely well!) and fix the startup sequence… This of course after quite a few (known) attempts to reset the damn startup sequence – I even removed the NRAM battery, to no effect!
Proceeding in this “destruction” sequence, I set my iTunes to encrypt backups and I forgot the damn password (too many passwords…). Since losing that backup wasn’t a big issue, I tried just to remove the encrypted option but that doesn’t work since it requires the old password. Some web searching without any relevant results. The best clue was to mess with keychain-2.db file, located at /var/Keychains. I tried to move it but it didn’t work, so I went checking its contents, since it’s a sqlite3 database. The interesting field is located at the genp table and it is something like (your results should differ, at least the first row, which is “rowid” field):
So I deleted this row (delete from genp where rowid = 153) and reconnected my iPad to iTunes. I tried to remove the Encrypt iPad Backup option but it asked again for the password. Fill it with random junk and voila, problem solved
A new, unencrypted, backup will start. After it finishes (or you can stop it), you will be able to set a new password and the encrypted backup will start.
Most probably you will need to have your iOS device jailbroken to access that file. If you can access that file from a file system browser then you can edit it at your iTunes computer and copy back to the device (I doubt that this is possible with devices not jailbroken).
Update: This method doesn’t seem to be valid in iOS 5.x. The database has changed and the fields appear to be encrypted. Need to do some research on this.