Halo Content Resigner
HaloSigner is a simple utility which allows users to rehash certain types of Halo content files after they have been modified with a hex editor. Currently, campaign saves from Halo Anniversary, Halo 3, Halo 3: ODST, Halo: Reach, and Halo 4 are supported. GPDs from Halo: Reach are also supported.
Simply drag a raw content file (e.g. an mmiof.bmf) into the program and it will be rehashed.