SQL Server Backup - move to new server - Media Failure
As Andrew Loree suggested, this might be due to a 3rd party SQL backup product being installed on the source instance.

In order to find out what software was used to take the backup, you can run a RESTORE HEADERONLY on the .bak file from your SQL2012 instance. That should give you some information about how the backup was created.

RESTORE HEADERONLY FROM DISK='X:\file.bak'


If the above also fails, then there would be corruption, but you've already ruled out corruption via SHA1 checksums.