Message-Digest algorithm 5, also known as MD5, is a cryptographic hash function used commonly to verify the integrity of downloaded files as well as for password authentication. It functions as an integrity check because if any data is changed (purposefully or accidentally), then the MD5 hash returns a different value from the actual checksum. It works as a safe way to store passwords because the function is one way: a website can compute the MD5 hash of your password and store that on its server instead of the plain text password. When you log in, the server computes the MD5 of what you entered and compares it against the pre-hashed password to see if they match. That way, if a hacker compromises the server, he or she can't figure out what your original password was from the hash alone. Some crypanalysts have determined that an MD5 checksum is not completely secure, but it is still one of the most widely used hash function on the internet.