1. 1 A Functional Taxonomy for Software Watermarking Jas Nagra, Clark Thomborson University of Auckland Christian Collberg University of Arizona

2. 30 January 2002SW WM Taxonomy 2 Why Build a Taxonomy? The first step in wisdom is to know the things themselves; this notion consists in having a true idea of the objects; objects are distinguished and known by classifying them methodically and giving them appropriate names. Therefore, classification and name-giving will be the foundation of our science. Carolus Linnæus, Systema Naturæ, 1735 (from Lindqvist and Jonsson, “How to Systematically Classify Computer Security Intrusions”, 1997.)

3. 30 January 2002SW WM Taxonomy 3 Software Watermarking Copyleft … Watermarked Software Watermarking Process Unprotected Software Watermark: “ Copyleft …”  Authorship, Copyright  Ownership  Validity  Licensed Uses Recognisable mark; no change to function.

4. 30 January 2002SW WM Taxonomy 4 Taxonomy by Technology  Static software watermarks: either in –Data (e.g. const string “Copyright …”) or in –Code (e.g. ordering of basic blocks). –Static SW watermarks are recognisable by a static analysis, e.g. grepping for strings. The recognition may involve cryptography.  Dynamic software watermarks: –I/O behaviour (Copyright notice; “Easter Egg”), –Data structure, or –Execution trace (opcodes, addresses, …).

5. 30 January 2002SW WM Taxonomy 5 Easter Eggs  A special I/O sequence (a “secret key”) reveals the Egg: a recognition system is distributed with the watermarked software.  They’re fun to hide and to hunt.  The hiding process is a security concern, as are the Eggs.  See www.eeggs.com

6. 30 January 2002SW WM Taxonomy 6 Goals of our Taxonomy  Names should be –Precise // Concise // Already familiar;  Classification tree should be –Complete & unambiguous; –Well-balanced (not many criteria); and –Useful to (Technologists // Lawyers)  (Experts // Novices). Two of our goals are internally contradictory, so we won’t find an optimal solution!

7. 30 January 2002SW WM Taxonomy 7 Ambiguity of “Watermark”  Should a watermark be visible to end-users? –Yes [Kaplan 1996] –Not usually [Lacy, Quackenbush, Reibman & Snyder 1998] –In some applications [Cox & Linnartz 1998] –No [Miller, Cox, Linnartz & Kalker 1999] –Yes, if it’s a “visible watermark”; no, if it’s an “invisible watermark” [Kutter & Hartung 2000]  Our technological taxonomy has focussed our attention on technical detail (how it works) rather than on design goals (what it’s supposed to do).

8. 30 January 2002SW WM Taxonomy 8 Ambiguity of “Watermark” (cont.)  Should a watermark be robust (survive common transformations such as copying, moving to a different location, compression)? –Yes [Kutter & Hartung 2000] –Yes or no, depending on the application [Miller, Cox, Linnartz & Kalker 1999]  Carelessly drafted laws might forbid (or require) us to destroy (or preserve) any watermark used for “digital rights management”!  The absence of robustness does not guarantee fragility!!

9. 30 January 2002SW WM Taxonomy 9 Robustness and Fragility  List of Transformations: 1.Copying within a protection domain 2.Copying into another domain 3.Deriving a substantially-similar work 4.Copying a short excerpt  A robust watermark will survive any transform with a small number.  A fragile watermark will be destroyed by any transform with a large number.  Some watermarks are neither fragile nor robust! (For example: WMs destroyed by some “class 1” transforms, but surviving some in class 4.)

10. 30 January 2002SW WM Taxonomy 10 Resolution of the Ambiguities  Should a watermark be visible to end-users? –Yes, if it’s an assertion (e.g. of copyright or authorship) or an affirmation (e.g. of authenticity). –No, if its purpose is to permit or prevent an unauthorised use.  Should a watermark be robust? –Yes, for affirmations and preventions. –No, for assertions and permissions.

11. 30 January 2002SW WM Taxonomy 11 Types of Software Watermarks  Visible robust watermarks: useful for assertion (of copyright or authorship)  Invisible robust watermarks: useful for prevention (of unlicensed use)  Visible fragile watermarks: useful for affirmation (of authenticity or validity)  Invisible fragile watermarks: useful for permission (of licensed uses).

12. 30 January 2002SW WM Taxonomy 12 Authorship Marks (Assertion)  An assertion of authorship, and its related copyright and moral rights, can be made in what we call an “Authorship Mark”.  Visibility is desired, otherwise the end-user won’t be given notice.  Robustness is desired, otherwise the authorship mark would not be present in a substantially similar work.

13. 30 January 2002SW WM Taxonomy 13 Fingerprint Marks (Prevention)  A publisher, distributor or other agent of the author might embed a unique “fingerprint mark” on each copy they sell or license.  Fingerprints should be robust, ideally surviving even in modestly-sized excerpts.  Fingerprints should be invisible, otherwise they will be easily removed by pirates, and they may annoy the end-user.  Fingerprints will prevent unauthorised use, when suitable detection & response systems are in place.  Used in the “Content Protection System Architecture” proposal from 4C Entity.

14. 30 January 2002SW WM Taxonomy 14 License Marks (Permission)  Fragile marks, which are destroyed or suitably modified whenever a copy is made, allow us to design a system that permits licensed use. For example: an object with a “copy-2” mark can be transformed into two objects with “copy-0” marks.  License Marks are most useful in conjunction with Fingerprint Marks: the Fingerprint Mark indicates what sort of License Mark is required.  License Marks should be invisible, so that they may resist attacks by pirates.  LWMs (License Watermarks) are used with FWMs (Fingerprint Watermarks) in the Content Protection for Recordable Media proposal from 4C Entity.

15. 30 January 2002SW WM Taxonomy 15 Validity Marks (Affirmation)  Visible, fragile marks can affirm, to the end-user, that the software has not been modified in any important way since manufacture.  VWMs for software are typically designed to be fragile except for verbatim copying; a single-bit change will invalidate the VWM.  Cryptographic signature algorithms are used to implement VWMs in Java and in Microsoft’s Authenticode.

16. 30 January 2002SW WM Taxonomy 16 A Fifth Function?  Any watermark is useful for the transmission of unrelated information (espionage, humour, …).

17. 30 January 2002SW WM Taxonomy 17 Our Functional Taxonomy Goal: “… wisdom … by classifying [watermarks] methodically and giving them appropriate names.”