1. Key is stored
within the document. When someone attempts to open the document, the
program checks whether the key entered is the same as the stored one. If the key
doesn’t match, the program locks further processing of the document.
2. A key hash is
stored within the document. "A hash function is a
function, mathematical or otherwise, that takes a variable-length input
string (called a pre-image) and converts it to a fixed-length (generally smaller)
output string (called a hash value)." (Bruce
Schneier). When this method is employed, a key entered by a user is being
transformed into a data string of fixed length used to verify the key, but that string
cannot be used to retrieve the key itself.