Abstract: Each copy of software, such as software used by a digital computer to create a document and the software used by printers, facsimile machines or digital copiers to produce or reproduce documents, is assigned a unique identifying code pattern which is printed on all documents produced with that software by a high resolution printer. The unique identifying code pattern is a plurality of spaced apart marks having a size no greater than about 150 dpi and preferably about 300 dpi, and is therefore, at best, barely noticeable to the human observer. The "invisible signature" permits detection of documents made by unauthorized copies of software or by unauthorized or improper use of authorized copies. Preferably, the unique identifying code is replicated multiple times over the document using an error correcting code to assure that at least one replication will be clear of matter selected for printing by the software. A high resolution scanner extracts and identifies the code patterns printed on the document.

Abstract: Each copy of software is assigned a unique identifying code pattern which is printed on all documents produced with that software by a high resolution printer. The unique identifying code pattern is a plurality of spaced apart marks having a size no greater than about 300 dpi, and is therefore, at best, barely noticeable to the human observer. The "invisible signature" is also reproduced on documents made by unauthorized copies of software which can therefore be traced. Preferably, the unique identifying code is replicated multiple times over the document using an error correcting code to assure that at least one replication will be clear of matter selected for printing by the software. A high resolution scanner extracts and identifies the code patterns printed on the document.