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The Advanced Encryption Standard (AES) is the current de facto standard block cipher for extremely confidential information. It is used by many large agencies, and popular for its computational efficiency in both software and hardware. However, the standard (Rijndael) S-box used of AES is a static and fixed matrix. Thus, a backdoor can be built into the cipher to exploit this feature. In this paper, we argue for a very simple key dependent S-box scheme which generates a dynamic S-box for each round of encryption. Our scheme uses some bits of the primary key (or expanded key) to directly manipulate the standard S-box in such a way that its content is changed but its cryptographic properties are preserved. We show using experimental tests that our proposed scheme strengthens the cipher against certain attacks, at the expense of a relatively modest one-time computational procedure during the set-up phase.