Explain checksum.

Fred Cohen (permission needed): Checksum is a way to verify the integrity of information in systems with no built-in protection. In other words, it provides a way of validating that the content of a file or a package / library is intact. This is useful since attacks or errors may occur during transmission of files. However, it requires that the package author has run a checksum function for the file / package which creates a specific hash for that version of the file. A minor change of the file content will result in a different checksum. If you have access to the original checksum of the file, you may run checksum on your own. In case the resulting checksum matches the original one, no changes have been made in the file. You can now conclude that no error or malicious injection was done during transmission of the file.