3Gs Update 5.0.1 to 6.1.2 possible without sh Blobs?

Hi, I have a 3gs old Bootrom, iOs 5.0.1. (JB)
I want to update to 6.1.0 or 6.1.2. to JB again.
I´ve load the ipsw, but itunes tells me that this version can not be verified.

Any ideas (except using 5.0.1 until the end of days) ???