Secure passive tagging scheme

A method of tagging a manufactured product with a passive tag includes processing a subset of a plurality of unique combinations of at least two axis ratios, where the subset is determinable by a plurality of parameters that define a portion of a coordinate space, to determine a first particular unique combination of the at least two axis ratios. A gas having the determined particular one unique combination of at least two axis ratios is incorporated into the manufactured product. The product to be tagged may be a first product, characterized by a first particular characteristic, and a second product is characterized by a second particular characteristic different from the first particular characteristic. The unique combination of at least two axis ratios is a first unique combination. The plurality of unique combinations of at least two axis ratios is processed to determine a second particular unique combination of the at least two axis ratios, and a gas having the determined second particular unique combination of at least two axis ratios is incorporated into the second product.