1. A method of encoding a data item string for secure transmission using a physiological signal, the method comprising: separating the data item string into a defined number of component segments; constructing a polynomial equation based on the data item string such that each component segment of the data item string is a different coefficient of the polynomial equation; identifying a plurality of signal features from a physiological signal; creating a plurality of ordered pairs based on the plurality of identified signal features from the physiological signal, where each ordered pair includes a first element corresponding to an index of a different one of the identified signal features and a second element corresponding to an output of the polynomial equation computed using the first element of the ordered pair as an input; and generating a data package including the plurality of ordered pairs.