On the security of key derivation functions in office

Abstract

In Microsoft Office, the file security is mainly protected by user authentication and files encryption. The cryptographic keys are usually derived from a password. Thus, password based key derivation function (PBKDF) is the core of the security scheme. However, the security of the PBKDF of Office is not yet ensured. In this paper, the PBKDF algorithm is analyzed through the game-playing approach and upper bounded of the Adversary's Advantage over the KDF and random function. Based on that, we discussed the practical safety of the Office encrypted files, and show that Office is secure when the user password is longer than 6 characters.