Synthesis of Tamper-Resistant Pin-Constrained Digital Microfluidic Biochips

Digital microfluidic biochips (DMFBs) are an emerging technology that implements bioassays through manipulation of discrete fluid droplets. Recent results have shown that DMFBs are vulnerable to actuation tampering attacks, where a malicious adversary modifies control signals for the purposes of manipulating results or causing denial-of-service. Such attacks leverage the highly programmable nature of DMFBs. However, practical DMFBs often employ a technique called pin mapping to reduce control pin count while simultaneously reducing the degrees of freedom available for droplet manipulation. Attempts to control specific electrodes as part of an attack cannot be made without inadvertently actuating other electrodes on-chip, which makes the tampering evident. This paper explores this tamper resistance property of pin mapping in detail. We derive relevant security metrics, evaluate the tamper resistance of several existing pin mapping algorithms, and propose a new security-aware pin mapper. Further, we develop integer linear programming-based methodologies for inserting indicator droplets into a DMFB in order to boost tamper resistance. Experimental results show that the proposed techniques can significantly increase the difficulty for an attacker to make stealthy changes to the execution of a bioassay.