Written by amd42
GUI by Xerax
HaloSigner resigns Halo: Reach GPDs (4D53085B.gpd) and campaign saves (mmiof.bmf) after you have modded them in a hex editor. Simply start the program, drag in your altered file, use your favorite tool to insert it back into its package, and enjoy your mods.
All research behind this was done by amd42 , Thunder Waffle, and Kontrol. No information was obtained through any 3rd-party sources.
Did I mention that it's open-source? Download VirusTotal Virus Scan HaloSigner EXE C# Source Code (GPLv3)