Calculating Checksums for Files
Calculating checksums with Checksum Validator couldn't be any easier. There are four different ways to get a file into Checksum Validator:

Drag a file onto the Checksum Validator application icon in the Finder (or in the Dock, if Checksum Validator is in the dock). This will launch Checksum Validator, if not already running. Drag a file into the Checksum Validator "Drag Well" window (Checksum Validator must already be running). Use the standard MacOS open dialog, by choosing "Open…" from the "File" menu or press ⌘-O. For certain file types you can control-click (or right click for those with a two button mouse) on a file in the Finder and choose "Open With" in the resulting context menu and choose "Checksum Validator" from the submenu. This works for the "standard" download compressions and encodings: dmg, zip, gz, tgz, tar, bin, sit, sitx and hqx.

As soon as the file is opened, Checksum Validator goes about calculating the checksum for you. Which digest it uses (MD5 or SHA-1) when the file is first loaded depends on your preferences settings. See Preferences for more information. As soon as the calculation is done, the "thinking" indicator will stop spinning and the checksum will be shown:

Which digest you need, is obviously set by the web site where you downloaded the file, since you need to calculate the checksum with the same digest they used. If you need to calculate the checksum with the other digest type just click the different type in the window and Checksum Validator will immediately go to task calculating the checksum with the other digest.

If you are creating a checksum to put on your own site, choose whichever checksum you wish. MD5 creates a shorter checksum string and is a little more common; SHA-1 creates a longer checksum string and is considered slightly more secure. Note that if you are also packaging your files as disk images you should check out the application DMG Designs at www.kingdom-era.com which not only will make creating and managing your products easier but can automatically calculate MD5 or SHA-1 checksums for you as part of the packaging process.

Note: Checksums can only be calculated on files, not on directories. Realize that packages are a special type of directory, therefore, packages also cannot have a checksum calculated for them. Packages are directories on the Mac that are designed to look like a single file, this includes almost all applications, and certain "files" like Xcode project files, GarageBand files and iMovie files, for example. You'll need to archive (zip) them (or use some other compression type of program) or create a disk image of them, and then calculate a checksum on the resulting file.