Hashing is taking an input string of any length and giving out an output of a fixed length. In cryptocurrencies, transactions are taken as an input and run through a hashing algorithm, such as MD5, SHA-1, or SHA-256. Bitcoin uses SHA-256 hashing. Using hashing, an examiner can tell if a computer file, whether it be cryptocurrency, a document, photo, music, even an entire hard drive, has been changed by matching hashes of subsequent copies of the data. Generally, this process cannot be reversed by using any mathematical function.