Abstract
A cryptosystem for plaintext messages was developed with gratings and their spectral and directional optical properties. Although there were many applicable grating types and optical responses, this manuscript took an example of a binary metallic surface relief and its specular reflectance at three wavelengths to demonstrate the working principles and the capabilities of the cryptosystem. For one, a series of numbers and the grating characteristics served as the ciphertext containing plaintext messages and the information of the sender's signature. Confidential, high-density, and authentic messages could be, therefore, delivered with tiny or even virtual gratings. Second, four unique encryption/decryption keys here significantly reduced the risk of the ciphertext being easily recovered by eavesdroppers. Further manipulation of keys not only offered several strategies of enhancing the system's safety, but also allowed the coexistence of many two-party, or even multiple-entity, communications. The reflectance spectra shown here were mainly attributed to the Wood's anomaly and were numerically obtained from programs based on rigorous coupled-wave analysis. © 2010 Optical Society of America.