The secure information retrieval technologies are critical for status identification, particularly in the battlefield for friend/enemy discrimination. The materials or devices used in these technologies should be hard to find, difficult to counterfeit and as simple as possible. Moreover, if the coding information is totally position-invariant, i.e. neither sequence nor pixel based, it will greatly simplify the retrieval system. We describe an information retrieval technology, which possesses the above-mentioned features. The information is encoded by using luminescent semiconductor nanocrystals (or quantum-dots, QDs) mixed with a transparent solution, namely info-ink. When an exciting light beam shines at the info-ink, its emitting spectral features, i.e., wavelength and intensity, provide the encoded information. The info-ink could be applied on any kind of surface, for examples, document cover, top area of helmet, and even a fingernail. It is actually a thin layer of paint and requires no power supply. The retrieval device consists of an exciting light source, a mini-spectrometer and a data processing unit. However, for battlefield applications, a major problem with fluorescence-based technology is that the brightness of sunlight can overwhelm most reflected fluorescent signal. To overcome the shortcomings, the quantum dots are engineered to fluoresce at wavelengths corresponding to the absorption lines of the solar spectrum, more commonly known as Fraunhofer lines.