JChecksum is a free and open-source program that calculates MD2, MD5 and SHA checksums. Checksums or hash sums are values used to check data correctness. Although it has different names and uses (from cryptography to parity bit), the main principle is always the same.
Digital data is composed of zeros and ones. A checksum is a function (you can think of it as the result of adding all the ones) that creates a numeric value from some data, for instance, a MD5 sum from a music file. The process involves some advanced mathematical calculations, but in the end, if two files have the same size and the same checksum, they should be the same. This can be very useful in several occasions. It can be used as a fast way to find duplicate files (as opposed to comparing them byte by byte), or it can be used to find communication errors in your Internet connection (i.e., that the file you downloaded is effectively the same that was available at a website).
More advanced checksums (some variations of SHA, for instance) are even used for security concerns, namely to assure no one tampered with a file or a connection.
JChecksum can calculate MD2, MD5, SHA-1, SHA-256, SHA-384, and SHA-512, being MD5 and SHA the most currently used. The program is very easy to use, allowing drag and drop of files, a single click to copy the value to the clipboard, and automatic comparison of two values. Even if all these features seem rather obvious, they are not always found in this kind of programs and are very useful. After all, this is a program with a very simple function, which stays simple by its easy-to-use interface.
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