Method of invisibly embedding into a text document generated by a licensed software the License Identification Signature of the software wherein each original copy of the software has been assigned a unique Software License Code (SLC) and a unique License Identification Key (LIK) by a Licensing Authority, such a method comprising the steps of computing a Document Hash Value (DHV) of the text document (12), getting from the DHV a License Identification Code (LIC) of the text document by using the LUK (14), and embedding (18) invisibly into the text document the License Identification Signature (LIS) formed by the SLC, DHV and LIC after the LIS has been encoded by using character attributes.