Should you be getting an unexpected decryption error when installing Diablo 3, check out the fix that Blizzard suggests:
This error message generally indicates that the installation files may be corrupted, damaged or incomplete. Please make sure your download has finished before attempting to install the game. Secondly, make sure the installation executable file is in the same folder as all your other Diablo III installation files. If the problem persists, you may need to delete all the files and restart the download.
Your installation folder should not have any files with the .Part extension. If it does, it indicates the downloader was closed prematurely. Restart the downloader in this case, selecting the same location the “Diablo-III-8370-enUS-Installer” folder is in, and it will check the existing files and download what’s missing. It’s important to let the downloader finish on it’s own. It will start the “Diablo III Setup.exe” file when it’s done. It can take a while so please be patient.