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

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.)

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.

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, …).

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

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!

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).

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!!

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.)

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.

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).

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.

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.

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.

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.

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

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