Applications of Hash Functions

We now look at three examples of applications of hash functions, each one requiring a different security property.

Application requiring preimage resistance

Hash functions provide a simple and widely adopted way of implementing password storage protection. The idea is to store the passwords in a password file in ‘disguised’ form to be checked. Yet, anyone who gains access to the password file (including the system administrator) cannot recover the passwords themselves. In this application, a hash function provides an unusual type of confidentiality for stored data. The unusual aspect is that the data itself (the password) never needs to be recovered. As a result, this type of confidentiality can be provided by a cryptographic primitive, which doesn’t use a key.

In advance of a logon attempt, a user’s identity $I$ is stored in the password file next to the result of passing the user’s password $P$ through a hash function $h$. In other words, the password file consists of a table of pairs of the form $(I, h(P))$. Note that the passwords themselves are not stored in the password table.

The password-based log-on process for a user attempting to access resources on a device, which we discuss later at a greater length, is depicted in the illustration below and operates as follows:

• The user enters an identity $I$ when prompted by the log-on screen.

• The user enters a password $P$ when prompted by the log-on screen.

• The authentication application running on the device inputs the password $P$ to the hash function and computes $h(P)$.

• The authentication application running on the device looks up the table entry in the password file corresponding to identity $I$ and compares the stored value of the hashed password with the value $h(P)$ that it has just calculated in step 3. If the entries match, then the user is authenticated. Otherwise, the user is rejected.