Hash functions are used to check the integrity of a file after making a copy of it. To achieve this, when a file is copied from a source to a destination (for example, when downloaded from a web server), a corresponding hash is also copied with it. This original hash, horiginal, acts as a fingerprint of the original file. After copying the file, we generate the hash again from the copied version of the file—that is, hcopied. If horiginal = hcopied—that is, the generated hash matches the original hash—this verifies that the file has not changed and none of the data was lost during the download process. We can use any cryptographic hash function, such as MD5 or SHA, to generate a hash for this purpose.
Now, let's look at symmetric encryption.