What Is a Checksum? Tim Fisher has more than 30 years' of professional technology experience. He's been writing about tech for more than two decades and serves as the VP and General Manager of Lifewire. A checksum is the outcome of running an algorithm, called a cryptographic hash function, on a piece of data, usually a single file.
Checksums really aren't that hard to understand or create. Let's start with a simple example, hopefully showcasing the power of checksums to prove that something has changed. The MD5 checksum for the following phrase is a long string of characters that represent that sentence.
How do checksum calculators calculate checksums? Checksum calculators use a number of algorithms including longitudinal parity check, Fletcher's checksum, Adler-32, and cyclic redundancy checks (CRCs). How do I validate multiple checksums at once?
Do the checksums not match? This can mean anything from the fact that someone has replaced the download with something malicious without you knowing, to a reason less sinister like that you opened and changed the file, or the network connection was interrupted and the file didn't finish downloading.
A checksum is the outcome of running an algorithm, called a cryptographic hash function, on a piece of data, usually a single file. Comparing the checksum that you generate from your version of the file, with the one provided by the source of the file, helps ensure that your copy of the file is genuine and error free.
JDigest is an open source checksum calculator that works in Windows as well as on macOS and Linux. Since not all checksum calculators support all possible cryptographic hash functions, be sure that any checksum calculator you choose to use supports the hash function that produced the checksum that accompanies the file you're downloading.
For our purposes here, they essentially equal each other. However, making even a slight change, like removing just the period, will produce a completely different checksum. As you can see, even a minuscule change in the file will produce a vastly different checksum, making it very clear that one is not like the other.
Tim Fisher has 30+ years' professional technology experience. He focuses on support and Microsoft topics but is an expert in all areas of tech. He's also the GM & VP of Lifewire.
The idea of a checksum or a cryptographic hash function might seem complicated and not possibly worth the effort, but we'd like to convince you otherwise! Checksums really aren't that hard to understand or create.