Recent security analysis of digital micro-fluidic biochips (DMFBs) has revealed that the DMFB design flow is vulnerable to IP piracy, Trojan attacks, overproduction, and counterfeiting. An attacker can launch assay manipulation attacks against DMFBs that are used for clinical diagnostics in healthcare. Moreover, security for lab-on-chip has emerged as an important design criterion in view of the recent findings about spurious test results from Theranos Edison devices. We present encryption based on micro-fluidic multiplexers, wherein an assay is encrypted with a secret-key pattern of fluidic operations. Only an authorized user of the DMFB possesses the secret-key pattern and can get the correct assay outcome. Simulation results show that for practical assays, e.g., protein dilution, an 8-bit secret key is sufficient for overcoming threats to DMFBs.