Joel Zlotnick, Jordan Brough and Troy Eberhardt

This is the last of this paper series, which has introduced innovative security design strategies that interrupt reverse engineering[1] of security document artwork. Prior papers described the overprinting of translucent offset inks of the same colour[2, 3] to prevent individual plate images from being isolated from other plate images, and the repurposing of split fountain printing[4, 5, 6, 7] to increase the complexity of monochromatic security designs. This Part 8 paper considers a final split fountain variable, the inclusion of three or more pure inks per fountain, and concludes this series on monochromatic security design.

This paper series has proposed security design strate­gies that conceal the plate artwork and press setup used to print a genuine security document. If a counter­feiter is unable to identify individual plate images, then accurate replication of the artwork on each plate is not possible. Monochromatic security design, customisation of ink opacities, increasing print density by layering images instead of widening lines, and use of split fountain printing for opacity transitions instead of colour blending are some of the subtopics this series has already addressed. This Part 8 paper explores the use of more than two pure inks in a single fountain.

As in prior work, the examples presented are theoretical and would require adaptation prior to use in real printing workflows. Some assumptions are made for simpler explanation of the examples, particularly that the opacities of overprinted ink layers are linearly additive regardless of print order, the specific ink opacities involved, and ink layer thickness. While quality control is an important consideration, it is also not addressed here.

Three inks in one fountain
In conventional offset split fountain security printing, a seamless colour transition is created between two or more inks that are blended on press, but printed from a single printing plate. In most split fountain security designs only two distinct inks are involved: one colour at the left and right edges of the print, and a second colour in the middle. Split fountains can also be con­figured with more than two ink colours. For example, each split fountain in Figures 1 and 2 contains three inks. Incorporation of even more colours is possible, subject to the mechanical limitations of the split fountain hardware on press.

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Figure 1: Security print containing a split fountain incorporating three ink colours, which transitions from green to pink to blue and back to green.
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Figure 2: Security print containing two split fountain plates, each incorporating three ink colours. The first transitions from brown to pink to orange and back to brown, and the second transitions from green to blue to olive and back to green.

Although Figures 1 and 2 illustrate split fountain colour blends, in this series split fountain printing is a tool for creating opacity transitions instead of colour transitions. Prior work in this series considered split fountains with only two inks of different opacities, but here the first example explores a configuration of three inks per ink station (see Figures 3 through 10), each with a different opacity. The second example (Figures 11 through 17) demonstrates how several design variables explored throughout this paper series can be integrated at the same time.

Figure 3: Opacities of pure inks in each of four multiple split fountain ink stations, expressed as various combinations of inks of 8%, 18%, 30% and 44% opacity. Each simulates an asymmetric split fountain opacity transition that appears longer on one side than the other.

To stage the first example, Figure 3 shows the distri­bution of three inks of the same colour but of different opacities across the roller widths of each of four ink stations in a hypothetical offset printing press. For example, the first station prints an ink of 8% opacity at its left edge, inks of 44% opacity and 30% opacity in the centre, and 8% again at the right edge. The split fountains feature identical placement and transition width, such that the split fountain hardware configu­ration is identical in all four ink stations.

The opacities shown in Figure 3 are only theoretical, and simplified for purposes of explanation. Real ink formulations would need to be identified and cus­tomised for actual printing workflows. Further, Figure 3 shows just one possible arrangement of inks and split fountains. A large number of possible combinations of variables could be conceived that would produce a huge variety of visual effects, and these examples are not intended to capture all possibilities.

Asymmetric transition speeds
Several facets of split fountain printing were evaluated in Parts 4 through 7 of this series. In Part 7, the over­printing of certain split fountains was described as causing the apparent speed of the split fountain transition to change in different horizontal locations. In Part 7 the left and right sides of each split transition on press, and the left and right sides of the composite split fountain images in the print, were mirror images of one another. In contrast, the configuration presented in Figure 3 mimics an asymmetric split fountain effect where the left and right sides of the opacity transition appear to be of different widths. That the left and right halves of the split are unequal and occur over different distances presents a simulation problem that is subtly different from simulation of a symmetric colour transition in a conventional split fountain, and further conceals the press configuration by masking one of the inks.

Returning to Figure 3, consider the first ink station. On the left, an ink of 8% opacity transitions to an ink of 44% opacity across about one third of the roller width. Two additional split fountains then transition the 44% opacity ink to an ink of 30% opacity and then to another ink of 8% opacity, but the full transition from 44% to 8% on the right side takes up about two thirds of the roller width. Although the speed of the opacity transition from 44% to 30% and the transition from 30% to 8% are not the same under close inspection, the general effect is of a rapid change from 8% to 44% on the left, and a more gradual change from 44% to 8% on the right. The 30% opacity ink blends into the centre of the transition from 44% to 8%, producing the appearance of an asymmetrical split fountain that transitions quickly on its left side, but slowly on the right. The other three stations are similar; for example, the second station shows a long transition from 18% to 44% over the left two thirds of the width, but a short transition from 44% to 18% over the right third. As stated, the ink opacities used in the example are arbitrary, so a more purposeful selection of the specific ink formulations in each position could make the ink blend on the longer side of the split transition even more subtle.

Simulating flat tones
Prior work in this series described the use of paired ink stations with identical split fountain hardware configurations but opposite ink placements, such that overprinting the same plate artwork from each station in the pair could simulate three discrete flat tones across the full width of a printed design. In contrast, the distribution of ink opacities in the configuration shown in Figure 3 cannot simulate flat tones across the full width using combinations of two plates (though all four plates together still approximate 100% opacity). Instead, the configuration shown in Figure 3 allows the simulation of the seven discrete flat tones shown in Figure 4, though most only span half of the roller width continuously, and only in specific locations that depend on which plate images are combined.

Figure 4: Overlaps of two and four of the plates from Figure 3. The first two combinations simulate flat tones through the left half, the third and fourth simulate flat tones through the centre, and the fifth and sixth simulate flat tones at the right. All four plates printed together simulate a flat tone across the full width.

Figure 4 shows that overprinting the first and fourth stations simulates a flat tone of 52% (8% + 44%) across the left half of the roller, and the overprinting of the second and third stations simulates a flat tone of 48% (18% + 30%), also on the left side. Similarly, over­printing the first and second stations, or the third and fourth stations, produces flat tones of 74% and 26% through the centre. Overprinting the first and third stations, or second and fourth stations, simulates flat tones of 38% and 62% across the right half. Finally, printing the same artwork from all four plates approxi­mates 100% opacity, which is the only flat tone simula­tion in this configuration that spans the full roller width. The seven flat tone opacities simulated in Figure 4 (26%, 38%, 48%, 52%, 62%, 74% and 100%) derive from the true press configuration in Figure 3, which only contains inks of 8%, 18%, 30% and 44% opacity.

Three plate combinations
Where Figure 4 illustrates combinations of two (or all four) plates that can simulate flat tones, Figure 5 shows four combinations of three ink stations that simulate the presence of additional asymmetric split fountains, but cannot simulate flat tones in this configuration. Consider the overprinting of identical artwork from the first, second and third stations in Figure 3, as shown at the top of Figure 5. This combination produces a simulated opacity transition that progresses rapidly from 56% at the left to 92% over about a third of the width, and then takes the remaining two thirds of the width to transition first from 92% to 82%, and then from 82% to 56%. As described previously, the apparent speed of change from 92% to 82% is not quite the same as from 82% to 56%, but the 82% opacity ink tends to blend into the middle of the opacity decrease from 92% to 56%. The appearance mimics a rapid transition from 56% to 92% at the left, and a slower transition from 92% to 56% at the right. The plate combinations illustrated in Figure 5 all feature a similar effect, with two combinations showing a faster opacity transition on the left side, and two showing a faster transition at the right.

Figure 5: Overlaps of three plates from Figure 3. These combinations cannot simulate flat tones given the specific placement of inks in Figure 3 but do simulate additional asymmetric opacity transitions like those shown in Figure 3.

One artwork concept
Four split fountain transitions involving three inks each (Figure 3) and the combinations produced by their overlap (Figures 4 and 5) can be integrated into a design as shown in Figures 6 through 9, and the composite artwork in Figure 10. Each of the plate artwork designs in Figures 6 through 9 is associated with one of the split fountain stations shown in Figure 3. The complete design in Figure 10 is composed entirely of overprinted artwork from the eleven combinations shown in Figures 4 and 5, and it does not show any individual plate in isolation to inhibit reverse engineering. The variety of image densities displayed in different areas of Figure 10 is a consequence of creative overprinting of only four plate images, but creates the illusion that many unique plates and spot inks were used. Working backwards from Figure 10 to replicate the artwork in Figures 6 through 9 and the press setup shown in Figure 3 would be complicated, since the design deliberately obfuscates the press setup.

Figure 6: Three-ink split fountain artwork composed of an ink of 8% opacity at the edges, and 44% and 30% opacities in the middle, from the first fountain in Figure 3. The transition from 8% to 44% on the left appears to be faster than the transition from 44% to 8% on the right.

The waves at the bottom of the artwork are the only part of the design that features long, horizontally continuous shapes, and have been included to allow some overt split fountain opacity transitions to be inspected. Although contiguous connected shapes were identified as a reverse engineering risk in prior work, in Figure 10 they highlight the presence of split fountain transition effects that would be difficult to simulate using only one plate and no halftones. In Figure 10, these effects include not just simulation of flat tones and simulated split fountain opacity transitions (Figures 4 and 5) that do not exist in the true press setup (Figure 3), but also some asymmetric simulated opacity transitions that progress more rapidly on one side than the other. Although a counterfeiter could simulate these effects using a halftone process, such an approach sacrifices the line integrity of the solid artwork shapes and produces a counterfeit that is easy to detect with a magnifier.

Figure 7: Three-ink split fountain artwork composed of an ink of 18% opacity at the edges, and 30% and 44% opacities in the middle, from the second fountain in Figure 3. The transition from 18% to 44% on the left appears to be slower than the transition from 44% to 18% on the right.
Figure 8: Three-ink split fountain artwork composed of an ink of 30% opacity at the edges, and 18% and 8% opacities in the middle, from the third fountain in Figure 3. The transition from 30% to 8% on the left appears to be slower than the transition from 8% to 30% on the right.
 
Figure 9: Three-ink split fountain artwork composed of an ink of 44% opacity at the edges, and 8% and 18% opacities in the middle, from the fourth fountain in Figure 3. The transition from 44% to 8% on the left appears to be faster than the transition from 8% to 44% on the right.
Figure 10: Composite artwork produced by overprinting of Figures 6 through 9. The waves at the bottom contain several overt asymmetric split fountains that simulate both fast and slow opacity transition speeds between opacities of varying contrast. Compare to Figure 17.

Integrating all variables
Parts 2 through 7 of this series introduced a variety of techniques and variables for creating security document artwork resistant to reverse engineering. In each paper, a single technique was featured in isolation, without consideration of other variables. Though security art­work resistant to reverse engineering can be developed using just one of the presented techniques, the second example here (Figures 11 through 17) illustrates the implementation of several techniques simultaneously.

Figure 11: Opacities of pure inks in each of four multiple split fountain ink stations, expressed as various combinations of inks of 8%, 18%, 30% and 44% opacity. Each station features its own unique ink placements and split fountain hardware configuration. Compare to Figure 3.

Figure 11 shows an alternative press configuration where each ink station is unique, and features its own configuration of ink opacities, ink placements, split fountain locations, split fountain transition widths and numbers of splits and pure inks. The configuration in Figure 11 produces a gamut of visual effects when the ink stations are overprinted (Figure 12), including overt and concealed split fountains, fast and slow simulated opacity transitions, and regions of both high and low opacity contrast. Individual plate artwork derived from the press configuration shown in Figure 11 is shown in Figures 13 through 16, with the composite artwork as Figure 17.

Figure 12: Overlap of split fountain inks from Figure 11 to create complex combinations of light and dark opacity, including both abrupt and gradual opacity changes. No flat tones are present. The eleven complex false split fountain effects shown here replace the combinations shown in Figures 4 and 5.
Figure 13: Three-ink split fountain artwork composed of an ink of 44% opacity at the edges, and 8% and 30% opacities in the middle, from the first fountain in Figure 11. Compare the placement of the inks and the widths of the split fountain transitions to the other plates in Figures 14 through 16.

Compare Figure 11 to Figure 17 to see how the placement, nature and visibility of the opacity transitions in the composite artwork is a function of the press configuration, and how a press and inks can be configured to support a highly complex security artwork design.

Figure 14: Three-ink split fountain artwork composed of an ink of 18% opacity at the edges, and 44% and 8% opacities in the middle, from the second fountain in Figure 11. Compare the placement of the inks and the widths of the split fountain transitions to the other plates in Figures 13, 15 and 16.
Figure 15: Two-ink split fountain artwork composed of an ink of 30% opacity at the edges, and 18% opacity in the middle, from the third fountain in Figure 11. Compare the placement of the inks and the widths of the split fountain transitions to the other plates in Figures 13, 14 and 16.
Figure 16: Two-ink split fountain artwork composed of an ink of 8% opacity at the edges, and 30% opacity in the middle, from the fourth fountain in Figure 11. Compare the placement of the inks and the widths of the split fountain transitions to the other plates in Figures 13 through 15.
Figure 17: Composite artwork produced by overprinting of Figures 13 through 16. The contributing plates have been configured to create this dynamic, high-contrast effect, which includes several overt asymmetric split fountains that simulate both fast and slow opacity transition speeds. Compare to Figure 10.

Conclusion
This is the last of an eight-paper series on monochro­matic security design that has described techniques that can make document artwork resistant to reverse engineering. Although this is the conclusion of this series, additional strategies regarding the prevention of artwork reverse engineering will be discussed in future research. While it was an important beginning and has presented new modes of thinking about counterfeiting risk, monochromatic design is restrictive simply because it is monochromatic. This series has treated colour as a liability because sophisticated counterfeiters can exploit colour when reverse engineering document artwork. However, use of colour in a security document affects not just security, but also a document’s identity and aspects of public acceptance. Accordingly, future work will reintroduce colour elements back into security document designs, allowing for expanded colour design flexibility while retaining the security advantages of the monochromatic design techniques developed in this paper series.

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.

References

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.

4 Zlotnick, J., Brough, J. and Eberhardt, T. (2017). Interrupting traditional counterfeiting workflow, Part 4: Split fountains redux. Keesing Journal of Documents & Identity, Vol. 54, pp. 31-35.

5 Zlotnick, J., Brough, J. and Eberhardt, T. (2018). Interrupting traditional counterfeiting workflow, Part 5: False split fountains. Keesing Journal of Documents & Identity, Vol. 55, pp. 26-31.

6 Zlotnick, J., Brough, J. and Eberhardt, T. (2018) Interrupting traditional counterfeiting workflow, Part 6: Split fountain position. Keesing Journal of Documents & Identity, Vol. 56, pp. 26-32.

7 Zlotnick, J., Brough, J. and Eberhardt, T. (2019) Interrupting traditional counterfeiting workflow, Part 7: Split fountain width. Keesing Journal of Documents & Identity, Vol. 58, pp. 30-36.