In order to use handwriting as a universal manmachine interface, we assume a special pen device, called data-embedding pen, which can embed binary data into handwriting as a sequence of invisible ink drops along the handwriting in a real-time manner. This paper describes the assumed hardware, applications, and required technologies of the data-embedding pen. Especially, an accurate stroke recovery algorithm is proposed for retrieving the data embedded in drawing order. In the algorithm, the embedded data itself is fully utilized to improve the accuracy of the recovery. A simulation experiment showed that the algorithm can attain high accuracy on the stroke recovery and the data retrieval.