When you download a file from the internet, quite often you cannot be 100% guaranteed that the file has not been changed in some way from the original. This could either be by the site you are downloading from, corruption due to errors in the download process, an individual who has uploaded the file for you, or possibly the most dangerous, the file has been infected by malicious software.

One of the ways you can identify whether a file has been changed from its original state is to check its digital signature if it has one. Or you can check the file integrity by looking at the hash value. Put simply, every file has unique data contained in it, and when you apply a certain algorithm called a “cryptographic hash function” to it, a string value is returned which is only valid for that file in the current state. If any piece of data in the file is changed, even 1 byte, and you apply the algorithm again, the value given this time will be different to the first value. And with that information you can see if the file is not the same before trying to use it.

So if you have something like an MD5 or SHA1 hash value from a website and want to check the integrity of the file you have downloaded, what do you do? There is nothing available in Windows to help you out. What is needed is a utility to calculate a hash value using the same algorithm. This is what done ChecksumVerify App, calculate a hash value.