Consider a time-consuming authentication scenario where a database records all secret keys of a large number of users. When the system authenticates a user, it first issues a challenge message to the user. The user then uses his/her key to encrypt the challenge and then returns the encrypted challenge to the system. The system then encrypts the challenge using one key in the database after another and compares the result with the received encryption. Once a match is found, the system accepts the user. Otherwise, the user is denied. This authentication protocol surely takes a lot of time and computation