Martin Cochran
I joined Google in August of 2008 and have been working on the security and core search functionality of the Google Search Appliance. I graduated with degrees in mathematics and computer science from the University of Puget Sound and went on to get a PhD in computer science with an emphasis in cryptography from the University of Colorado in 2008.
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Message Authentication Codes (MACs) are core algorithms deployed in virtually every security protocol in common usage. In these protocols, the integrity and authenticity of messages rely entirely on the security of the MAC; we examine cases in which this security is lost.
In this paper, we examine the notion of "reforgeability" for MACs, and motivate its utility in the context of {power, bandwidth, CPU}-constrained computing environments. We first give a definition for this new notion, then examine some of the most widely-used and well-known MACs under our definition in a variety of adversarial settings, finding in nearly all cases a failure to meet the new notion. We examine simple counter-measures to increase resistance to reforgeability, using state and truncating the tag length, but find that both are not simultaneously applicable to modern MACs. In response, we give a tight security reduction for a new MAC, WMAC, which we argue is the "best fit" for resource-limited devices.
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