Joel Zlotnick, Jordan Brough and Troy Eberhardt
Reverse engineering of document artwork is a critical prepress activity for sophisticated counterfeiters that print by offset lithography or other non-digital techniques. The first three parts of this series describe methods that counterfeiters employ to reverse engineer the individual plate artwork and the press configuration used to print a genuine document, and recommend design methods that are specifically targeted to make the traditional counterfeiting process as difficult as possible.[1, 2, 3] This Part 4 paper expands on prior work by rethinking the role of split fountain printing in security design.
In conventional security printing, split fountains are used to produce seamless colour transitions. Within this monochromatic design paradigm, split fountains transition ink opacity instead of colour. This enhances artwork complexity by allowing additional increases in the number of available opacity levels possible when overprinting plate artwork, and by severing associations between individual ink opacities and specific printing plates.
Part 1, 2 and 3
Previous work in this series identified design techniques that interrupt counterfeiting prepress processes by concealing the number of plates used to print a document, which obscures the individual plate images and impedes accurate redrawing of artwork. The strategy described in Part 2 of this series proposed printing a translucent ink of the same colour from each of four press fountains, resulting in a monochromatic composite image containing four discrete ink opacity levels. Darker areas are created by applying multiple layers of translucent ink, instead of covering more of the substrate surface with ink. The concept was improved upon in Part 3 by varying the opacity of the monochromatic ink in each of the four fountains, which allows up to fifteen discrete density levels instead of just four. The strategy described in Part 3 left two improvements to be desired: first, a further increase in the number of possible opacity levels even beyond fifteen, and second, the delinking of ink formulation from plate artwork so that each fountain no longer contains just one unique ink. Both goals can be achieved with split fountain printing – the subject of this paper – but split fountains must be fundamentally rethought if they are to be adapted for this new function.
Like previous parts of this series, this paper proposes a theoretical model that will require tailoring for actual press work. Some considerations that are important in real printing environments, such as quality control in split fountains and how the order of overprinted artwork affects the appearance of a final image, have been simplified here. With due respect to their importance, this paper does not address quality control, inspection, plate registration and other factors that are material in real printing workflows.
Split fountains redux
The conventional use of split fountain printing is to blend one colour of ink with another on press before a plate is inked, which creates a seamless colour transition in the printed artwork. Some examples are shown in Figures 1 and 2, which show colour transitions only in the horizontal direction; this is a consequence of printing press mechanics, and horizontal transitions can be seen in the majority of security documents.
Despite this limitation, split fountain press hardware is customisable across a broad range of other variables. For example, the pure inks can be placed in various horizontal locations, and in bands of varying widths, across each fountain. The transition areas across which the pure inks are blended can also be of varying widths and placement. The number of inks and ink transitions can also vary, from two pure inks up to five or more inks in a single fountain. With such a large number of variables, there are many novel ways in which split fountains can be adapted to fight artwork reverse engineering. Several papers in this series will customise these variables to achieve specific visual effects and security goals. For now, the scope of this paper introduces just the first of several split fountain applications, in which the press hardware is configured the same way in all ink stations and only the placement of a certain specific gamut of inks within that hardware is considered.
Two split fountain functions
The strategies described in Part 2 and Part 3 of this series use only one colour of ink, so it is not obvious how a technique for colour transition (as demonstrated in Figures 1 and 2) could be applied in a monochromatic design. Since the ink formulations described in Part 3 differ by opacity, not colour, a split fountain could be used to transition between two ink opacities instead of between two colours. This is the fundamental adaptation that converts split fountains into a tool for combating artwork replication, and in this usage split fountains serve two functions.
First, each split fountain can print a continuum of ‘spot’ ink opacities that vary continuously as the pure inks blend across the horizontal width of the split, producing a theoretically infinite number of discrete opacities within each individual fountain. When all plates are printed, the combined artwork displays many more opacity levels than the fifteen discrete opacities possible using the strategy described in Part 3 of this series.
Second, split fountains permit inks of many opacities (instead of only one) to be delivered from each individual printing plate, and also permit inks of the same opacity to be delivered from different printing plates. The link between ink opacity and plate artwork is therefore severed. If no relationship associates each pure ink to only one plate, then studying small areas of print in a finished document does not provide insight about the inks in other parts of that fountain, or the artwork on other parts of that plate. If the individual plates cannot be decrypted, the artwork cannot be replicated accurately.
One possible configuration
Assume the availability of a four-station offset press, with split fountain hardware mounted on all four stations, where the split fountain hardware is configured the same way in all four fountains. The only element that is changed between fountains is the placement of various pure ink formulations. The four sets of individual plate artwork shown in Figures 3 through 6 each utilise a split fountain transition between two inks of the same colour, but different (theoretical) opacities.
The first fountain contains inks of 8% opacity at the edges and 30% opacity at the centre, the second 30% opacity at the edges and 8% at the centre, the third 18% opacity at the edges and 44% at the centre and the fourth 44% opacity at the edges and 18% at the centre. These opacities are contrived for ease of explanation in this example, and do not reflect real-world formulations – but at the same time the relative percentages are not arbitrary and were chosen for reasons that will be addressed in later work. For now, the composite artwork including all four plates is shown in full in Figure 7, and magnified in Figure 8. Inks with exaggerated opacity differences were chosen to make the transitions easy to see in Figures 3 through 6. However, Part 3 of this series discussed some benefits of using four inks that are more similar in opacity (18%, 22%, 27% and 33% instead of 8%, 18%, 30% and 44%, for example). Split fountain transitions between inks of more similar opacities could be appropriate in a real security document, because it would more effectively mask the presence of the splits.
Consider how the two split fountain functions described above are met. The transitions in Figures 3 through 6 each produce a continuum of ‘spot’ opacities that span between each pure ink. When the plates combine in Figure 7, the number of density combinations is theoretically infinite, and in any practical case still exceeds the maximum of fifteen discussed in Part 3 of this series, so the first function is achieved. The second function has also been accomplished, since each of the four pure inks (8%, 18%, 30% and 44%) is deployed on two different plates in the set shown in Figures 3 through 6, and each plate prints a similar range of opacities to the other plates. To see the security advantages, attempt to work backwards from Figure 7 to isolate the four original plates. Not only is colour denied as a point of reference, as it was in Parts 2 and 3 of this series, but ink opacity has also been denied because each ink opacity in this configuration is no longer associated to just one printing plate. Counterfeiters attempting to circumvent replicating the plate artwork by trying to reproduce the density values in Figure 7 directly will find that attack considerably more difficult and will be further pushed to simulate rather than replicate artwork, assuming the difficulty does not derail the attack completely.
Finally, the split fountains can be seen clearly in Figures 3 through 6, but are disguised in Figure 7. In this example the concealment of the splits is intentional, because a counterfeiter that cannot identify the presence, placement or composition of the splits cannot replicate the plate artwork or the press setup. However, alternative design strategies that deliberately display the presence of the splits in specific ways do have advantages, some of which will be taken up in subsequent papers.
Stars, squares and artwork
The artwork shown in Figures 3 through 8 complies with the basic monochromatic design strategies described in Part 2 of this series. These strategies include a restricted colour gamut, the same artwork style on every plate, darkening areas by overprinting multiple plates instead of widening lines, and limiting use of continuous artwork patterns.
Additionally, the artwork in Figures 3 through 7 consists of two components that illustrate different concepts related to the presence of the split fountains: five large star graphics and a background pattern composed of squares. The stars are less complicated than would be appropriate for a real security pattern, and have been included primarily because they provide an easy way to illustrate how the printed appearance of identical artwork changes depending upon its horizontal location within the split fountain design. Figure 9 shows how the five stars at the top, which have been extracted from Figure 7, were created by the overlap of the same basic plate artwork printed with inks of different opacities. Each star at the top of Figure 9 looks different from the others because the ink opacities printed from each plate are different for each star location, as a result of the split fountains. This is also true for the squares in the background pattern, which, unlike the stars, were more deliberately designed to deter reverse engineering.
The background design in Figure 7 derives from an arbitrary image of a sky with clouds that contains lighter and darker areas, and has been converted to a pattern of overlapping non-identical squares. Darker areas of the background pattern are created not by widening lines or covering more of the paper with ink, but by layering multiple plate images. The appearance of each composite square in Figure 7 depends on how many plates, and which plates, were overprinted in that particular location based on which ink opacities must be combined to approximate that part of the cloud image. Although each square in Figures 3 through 6 covers approximately the same surface area, each composite square in Figure 7 shows different interior artwork and a complex edge contour where they overlap. The generation of the composite squares from plate overlap is shown magnified in Figure 10. Unlike the hex elements that form the background pattern in Part 2 and Part 3 of this series, which have hard edges and are bounded by unprinted white space, the squares shown in Figure 10 have erratic contours that extend into the white borders. This produces irregular overlap between the squares, increasing artwork complexity while still inhibiting tracing of patterns across the design.
These developments change the microscopic appearance of the squares in several ways. First, in Parts 2 and 3 of this series, the density of the cutouts and lines in the interior artwork of each of the hex elements was always limited to one of the discrete density values available (four or fifteen, respectively). In contrast, the areas of the squares shown in Figures 7 and 8 that do not match between different plate images – specifically, the erratic edge contours and the patterns of jagged or curved lines through the interiors – show opacities that change dynamically across the width of the design, and are no longer limited to a short list of certain values. This has the visual effect of changing the contrast between the opacity of the main body of each square, and the edges and lines. For example, consider only the lines inside the squares shown in Figure 8. In some cases, the lines show high contrast with the body of the square, and in other cases, the lines show low contrast and are barely visible, and the same effect can be seen where the square edges overlap.
The split fountain strategy described increases artwork complexity beyond what is possible using the design strategies described in Parts 2 and 3 of this series, and can make reverse engineering of document artwork even more complex and tedious. Additionally, split fountain printing offers a large number of variables that can provide additional options for split fountains to prevent artwork reverse engineering. Although the strategy described in this paper includes a goal of concealing the presence and placement of the splits, other design strategies that showcase and emphasise the presence of the splits also have advantages and can provide added security against certain counterfeiting strategies. Future work will explore additional design possibilities within the specific split fountain configuration already described in this paper, as well as techniques for optimising split fountain variables not considered here, such as the width, location and quantity of split fountains in a four-station press.
MORE ABOUT THE AUTHORS
Joel Zlotnick is employed by the US Department of State, Bureau of Consular Affairs, Counterfeit Deterrence Laboratory as a physical scientist. He conducts research on how design strategies can help maximise the security value of printing technologies and security features, and develops training programmes on counterfeit detection.
Jordan Brough is employed by the Homeland Security Investigations Forensic Laboratory as a forensic document examiner, specialising in adversarial analysis and counterfeit deterrence. Jordan spends his time examining suspect documents and consulting with United States security document designers.
Troy Eberhardt is employed by the US Immigration & Customs Enforcement Homeland Security Investigations Forensic Laboratory. He supervises the Research and Development Section at the laboratory, which specialises in identifying and mitigating vulnerabilities within travel documents.
1 Zlotnick, J., Brough, J. and Eberhardt, T. (2016). Interrupting traditional counterfeiting workflow, Part 1: Colour and split fountains. Keesing Journal of Documents & Identity, Vol. 49, pp. 14-19.
2 Zlotnick, J., Brough, J. and Eberhardt, T. (2016). Interrupting traditional counterfeiting workflow, Part 2: Monochrome colour gamut. Keesing Journal of Documents & Identity, Vol. 51, pp. 22-27.
3 Zlotnick, J., Brough, J. and Eberhardt, T. (2017). Interrupting traditional counterfeiting workflow, Part 3: Security design and ink opacity. Keesing Journal of Documents & Identity, Vol. 53, pp. 22-28.