Evaluation of the Effectiveness of Protecting Three-Dimensional Printers against Acoustic Infiltration

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review Article (electronics-2692881): Evaluation of the Effectiveness of Protecting 3D Printers against Acoustic Infiltration

In the article, there were shown analysis of acoustic emissions produced by a 3D printer during its operation and were assessed the effectiveness of masking acoustic signals produced by a 3D printer with noise. Based on the obtained spectrograms, it was found that the levels of the above-mentioned emissions do not depend on the complexity of the printed objects. The review is comprehensive and clear. However, some revisions should be done to further improve the paper quality. The comments and suggestions are as follows:

Comments on the manuscripts:

1.      In the abstract part, there is no good summary of the content and conclusions of this study, and it is suggested to further enrich the abstract.

2.      In the introduction part, 3D printers use additive manufacturing and are often used in many enterprises, design companies and design offices are mentioned. However, there are many kinds of 3D printing, and this paper does not indicate which 3D printing technology is the object of research in this paper.

3.      The introduction part does not introduce the research status of 3D printers against acoustic infiltration in detail, and there are too few references.

4.      Extrusion 3D printing technology has a widely applications in construction, ceramics and other fields, can you talk about the feasibility and necessity of extrusion 3D printing technology in against acoustic infiltration, the following papers can be cited: DOI:10.1016/j.addma.2022.102944; DOI:10.1016/j.conbuildmat.2022.127156.

5.      The second section describes ‘Materials and Methods’, but this section has no material content, so the title is inappropriate.

6.      The conclusion does not highlight the key points, it is suggested to further refine.

Comments on the Quality of English Language

The language should be improved.

Author Response

Dear Reviewer

Thank you for providing the detailed and constructive review for our paper which allowed to increase a scientific level of this paper. We tried to reply for each point mentioned in the review. Our corrections were highlighted in manuscript by blue and red colours. We hope that the new version of our manuscript looks better and meets your requirements.

Here we’d like to reply for each point mentioned in the review:

1. In the abstract part, there is no good summary of the content and conclusions of this study, and it is suggested to further enrich the abstract.

We agree with your opinion. We have improved the abstract according your suggestion. Our response is included in the text of manuscript.

2. In the introduction part, 3D printers use additive manufacturing and are often used in many enterprises, design companies and design offices are mentioned. However, there are many kinds of 3D printing, and this paper does not indicate which 3D printing technology is the object of research in this paper.

You are right. The introduction doesn’t include information about different technologies of 3D printing. We carried out review such technologies and we pointed which printing technology is in our interest.

We added below text in our manuscript:

3D printing is a complex definition for many technologies that differ in manufacturing processes, materials used and the type of objects that can be made from them. Based on the literature analysis, the main 3D printing technologies can be characterized as follows:

• FDM (Fused Deposition Modeling)/FFF (Fused Filament Fabrication) – 3D printing from thermoplastics. This is the most popular technology on the home and semi-professional printer market. The printing material is plastic in the form of a monofilament. This monofilament is heated to the appropriate temperature during the 3D printing process. After obtaining a semi-liquid consistency, the monofilament is put on layer by layer (as a result of the printing platform moving down or by moving the printing head up) using the printing head. Bonding occurs under the influence of temperature. FDM printers can use materials such as ABS or PLA. The operation of printer is based on the working of three stepper motors, which are the source of sound signals.
• SLA (stereolithography) – 3D printing from resins hardened with a laser beam. This technology uses a resin hardened under the influence of light as a printing material. The semi-liquid resin is located in a special container in which the work table is placed. The material hardens during the action of the laser beam. The table with resin is lowered by one layer and the beam reproduces the desired shape in the XY plane.
• DLP (Digital Light Processing) – 3D printing from resins hardened with a projector beam. In this technology, the resin is hardened by the light of the projector, which is placed under the resin container. The entire layer is hardened at once.
• PolyJet / MJP (PolyJet / Multi-Jet Printing) – 3D printing from resins hardened with UV light. Printing is done in a similar way to printing on paper using a laser or ink. The print head moving over the platform (work table) sprays a layer which is at the same time hardened by ultraviolet radiation.
• CJP (Color Jet Printing) – 3D printing from gypsum powders in full color. A layer of powder is placed on the surface of the work table, and the printer head sprays colored ink and binder in appropriate places. The print is chemically hardened [1-2].
• Binder Jetting – 3D printing from sand or powdered metal, ceramics or sand. This printing technology does not require the use of supporting structures. The process of creating objects resembles traditional inkjet printing. The Binder Jetting method is used in industry to create, for example, foundry moulds [3].
• SLS (Selective Laser Sintering) – 3D printing from powdered plastics selectively glued and welded using a laser beam [3].
• SLM/DMLS (Selective Laser Melting/Direct Metal Laser Melting) – 3D printing from powdered metals melted using a laser beam.
• EBM (Electron Beam Melting) – 3D printing from powdered metals melted using an electron beam.
• MJF (Multi-Jet Fusion) – 3D printing from powdered polyamide, which is selectively glued and welded at high temperature.
• LOM (Laminated Object Manufacturing) – 3D printing involving three-dimensional printing using layers of paper with glue and polyester laminate, cut by using a laser.
• Bioprinting – 3D printing from living biological material. In this way, fragments or even entire organs can be obtained [3].

The above mentioned 3D printing technologies use different printing materials and different methods of hardening them. The article further focuses on the analysis and possibilities of masking sound signals correlated with printed shapes for the Zortrax M200 Plus printer, which uses FDM technology and PLA printing material. The choice of the printer resulted from its popularity and availability.

3. The introduction part does not introduce the research status of 3D printers against acoustic infiltration in detail, and there are too few references.

According to your suggestion, we revised the Introduction section. Specifically, we added the following text in the introduction.

The article focuses on issues related to the acoustic emission of printers () in the side channel. This issue has been discussed, among others, in [8,10,14,24]. Various possibilities for improving the safety of printers against acoustic infiltration were presented, but none of the solutions indicated a method involving the use of color noise masking.

In [8], an innovative acoustic side-channel attack model for FDM technology was presented. The shape of the printed object (a key) was reproduced with quite high accuracy. It was found that to improve protection against acoustic infiltration and to make G code that control the operation of a printer, reconstruction process more difficult, randomness of the motor movement could be introduces into this code. However, the disadvantage of such a countermeasure will be the delay in the overall printing time.

In [10], the selected 3D printer with the ME3DP (Material Extrusion 3D Printing) technology was tested to analyse the existing risk. Simple objects were printed during its operation to identify the stepping motor operation that are emission sources in the secondary channels. The carried out analyses were focused on finding the correlation between the registered emissions and the printer head (printhead) movements when printing an object. It was shown that the analysis of the registered sensitive signal runtimes and their spectrograms allows us to recreate the printed object shape. The studies focused on measuring the acoustic signals that came from the stepping motors. The analysis of the possibilities of identifying the shapes of printed objects was based on the visual method.

In [14] it was pointed out that most of the research works were focused on protecting the intellectual property (IP) of the product after it has been built. However, there is presence of persistent threat to the confidentiality of the product during the manufacturing process. Additive manufacturing is based on the tight integration of the cyber and physical domains. One of the challenges is preventing confidentiality breach due to physical-to-cyber domain attacks, where hackers can analyze various analog emissions from the side-channels to steal the cyber-domain information. This information theft is based on the assumption that the attacker can estimate the relationship between analog emissions (acoustics, power, electromagnetic, etc.,) and the cyber-domain data (such as G-code). Therefore, a solution incorporated into computer-aided manufacturing tools was proposed, such as slicing algorithm and the tool-path planning which are in the cyber-domain. This novel methodology provides the physics and data-driven leakage models for acoustic side channels, defines various design variables (orientation and speed), provides an optimization algorithm, and incorporates it in the digital process chain. For various reference 3D models, an average reduction of mutual information of approximately 25% was achieved.

The problem of acoustic emissions from dot-matrix printers was also considered [26]. An attack was presented that allowed for the reconstruction of the text of prescriptions printed in English. In the experiments conducted, 72% of printed words were recovered, and even up to 95% if contextual knowledge of the text was assumed, with the microphone at a distance of 10 cm from the printer.

4. Extrusion 3D printing technology has a widely applications in construction, ceramics and other fields, can you talk about the feasibility and necessity of extrusion 3D printing technology in against acoustic infiltration, the following papers can be cited:

DOI:10.1016/j.addma.2022.102944

DOI:10.1016/j.conbuildmat.2022.127156.

3D printers do not always have to be used to print newly designed patterns that constitute an industrial or commercial secret. 3D printers can be used to print for the use of new printing materials. In this way, the properties of such materials are examined from the point of view of e.g. solidification rate, cooling rate, elasticity, brittleness, etc. Therefore we mentioned about it in the Introduction and added some referenced proposed by you (DOI:10.1016/j.addma .2022.102944, DOI:10.1016/j.conbuildmat.2022.127156).

However, attention should be paid to the fact that the operation of the 3D printer could be important information.

Quan Jiang, Qiang Liu, Si Wu, Hong Zheng, Wenliang Sun, Modification effect of nanosilica and polypropylene fiber for extrusion-based 3D printing concrete: Printability and mechanical anisotropy, Additive Manufacturing, Volume 56, August 2022, DOI:10.1016/j.addma.2022.102944;

Qiang Liu, Quan Jiang, Mojia Huang, Jie Xin, Pengfei Chen, Si Wu, Modifying effect of anionic polyacrylamide dose for cement-based 3DP materials: Printability and mechanical performance tests, Construction and Building Materials, 330 (2022), 127156, https://doi.org/10.1016/j.conbuildmat.2022.127156;

5. The second section describes ‘Materials and Methods’, but this section has no material content, so the title is inappropriate.

We have changed the title of the section into “Methods”.

6. The conclusion does not highlight the key points, it is suggested to further refine.

We improved the Conclusions and we added several sentences which better highlighted the key points of conducted analyses.

It should be noted that the types and levels of noise used to mask acoustic signals do not slow down the printing process itself, as these parameters do not affect the operation of the printer. It can only be assumed that in the case of extremely high noise levels (of the order of at least one hundred or several dozen dB), printer vibrations could occur, which would potentially affect the print quality. A certain drawback of the proposed method is the fact that it is not cost-free - it requires certain financial outlays for the purchase of electroacoustic devices listed in section 2.2.

Another inconvenience is the need to assess the effectiveness of the solution in a specific manufacturing environment by recording acoustic signals generated during a test print of an object: without masking and with masking with noise of the recommended type and intensity. To carry out this assessment, basic knowledge in the field of digital signal processing and physical phenomena occurring in the subject area of research is necessary.

Tests on the effectiveness of masking acoustic signals using color noise were carried out for the Zortrax M200 Plus 3D printer. The extensive experience of the authors of the article in area of protection of information against electromagnetic penetration allows to conclude that the use of a given solution for one type of device does not necessarily guarantee success in suppressing sensitive electromagnetic emissions of another type of such device. This is related to both the design of the device and its hardware configuration. A similar phenomenon can be observed in the case of 3D printers. The possibility of wider use of the method presented in this article must therefore be verified for other types of 3D printers. This is related to the possible occurrence of acoustic signals for such printers in the range of other frequencies. This approach is necessary to find a universal solution (for as many different types of printers as possible) to counteract non-invasive information acquisition using acoustic infiltration.

Comments on the Quality of English Language

The language should be improved.

We conducted extensive editing of English language and style. We realize that, despite all our efforts, this article may not be language and style error-free. Therefore, the final version, after possible acceptance, will be corrected by the proofreading service in the MDPI.

 

We look forward to hearing from you in due time regarding our submission and to responding to any further questions and comments you may have.

Best Regards

Authors

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript entitled “Evaluation of the Effectiveness of Protecting 3D Printers against Acoustic Infiltration”, authors demonstrate a mechanism to mask the acoustic emission from a 3D printer. In particular, they used several types of noises, which are labeled with colors. I found the manuscript technically sound and interesting. Nevertheless, there are some points that should be addressed before recommend publishing this work. My more specific comments follow:

 

1.    At the beginning of Section 2, the authors begin by saying "Noise is generally considered to be an undesirable phenomenon, especially when its intensity is high", from which the reader can expect the authors to present a way to eliminate printer noise, not making it worse by adding a “masking noise”.

 

2.    The Introduction section can reach a broader readership if the authors cite and discuss some recent applications, not limiting to a marginal comment without citations. Authors can cite, for example,

a.    DOI: 10.3390/ma16020880

b.    DOI: 10.3390/s21238014

c.    DOI: 10.1038/s41586-022-05234-7

 

3.    The conclusions should be improved to summarize the work as a whole and not by partial comments about each achievement.

Author Response

Dear Reviewer

Thank you for providing the detailed and constructive review for our paper which allowed to increase a scientific level of this paper. We tried to reply for each point mentioned in the review. Our corrections were highlighted in manuscript by blue and red colours. We hope that the new version of our manuscript looks better and meets your requirements.

Here we’d like to reply for each point mentioned in the review:

1. At the beginning of Section 2, the authors begin by saying "Noise is generally considered to be an undesirable phenomenon, especially when its intensity is high", from which the reader can expect the authors to present a way to eliminate printer noise, not making it worse by adding a “masking noise”.

Thank you for this remark. Of course you are right. A noise is not positive element of our environment. We try to use different kind of devices which are characterized by low or very low level of noise. The same situation applies to electromagnetic compatibility (EMC) where electronic devices have to meet requirements EMC that is to say measured electromagnetic emissions can’t be higher than limits levels described by normalized documents. But in area of protection of information against electromagnetic penetration there are situation when additionally sources of electromagnetic emissions are inserted,  e.g. when a mobile phone has to be jammed. Similar phenomenon can be applied in case of 3D printer when the acoustic signal correlated with the operation of stepper motors has to be jammed.

Therefore at the beginning of Chapter 2 there were placed several sentences which explained such situation that while noise is generally considered undesirable, it can be useful in certain circumstances.

2. The Introduction section can reach a broader readership if the authors cite and discuss some recent applications, not limiting to a marginal comment without citations. Authors can cite, for example, DOI: 10.3390/ma16020880, DOI: 10.3390/s21238014, DOI: 10.1038/s41586-022-05234-7

According to your suggestion, we revised the Introduction section. Specifically, we added the text which describe different 3D printing technologies. This allowed to add suggested references. We carried out also discussion selected items of references.

Magdalena Żukowska, Maryam Alsadat Rad, Filip Górski, Additive Manufacturing of 3D Anatomical Models — Review of Processes, Materials and Applications, Materials 2023, 16, 880, https://doi.org/10.3390/ma16020880

Jéssica A. P. Ribeiro, Hugo R. D. Filgueiras, Arismar Cerqueira Sodré Junior, Felipe Beltrán-Mejía, Jorge Ricardo Mejía-Salazar, 3D-Printed Quasi-Cylindrical Bragg Reflector to Boost the Gain and Directivity of cm- and mm-Wave Antennas, Sensors 2021, 21, 8014, https://doi.org/10.3390/s21238014

Cheng Zeng, Maya Winters Faaborg, Ahmed Sherif, Martin J. Falk, Rozhin Hajian, Ming Xiao, Kara Hartig, Yohai Bar-Sinai, Michael P. Brenner, Vinothan N. Manoharan, 3D-printed machines that manipulate microscopic objects using capillary forces, Nature, Vol. 611, pp. 68–73 (2022), doi: 10.1038/s41586-022-05234-7

3. The conclusions should be improved to summarize the work as a whole and not by partial comments about each achievement.

We improved the Conclusions and we added several sentences which better highlighted the key points of conducted analyses.

It should be noted that the types and levels of noise used to mask acoustic signals do not slow down the printing process itself, as these parameters do not affect the operation of the printer. It can only be assumed that in the case of extremely high noise levels (of the order of at least one hundred or several dozen dB), printer vibrations could occur, which would potentially affect the print quality. A certain drawback of the proposed method is the fact that it is not cost-free - it requires certain financial outlays for the purchase of electroacoustic devices listed in section 2.2.

Another inconvenience is the need to assess the effectiveness of the solution in a specific manufacturing environment by recording acoustic signals generated during a test print of an object: without masking and with masking with noise of the recommended type and intensity. To carry out this assessment, basic knowledge in the field of digital signal processing and physical phenomena occurring in the subject area of research is necessary.

Tests on the effectiveness of masking acoustic signals using color noise were carried out for the Zortrax M200 Plus 3D printer. The extensive experience of the authors of the article in area of protection of information against electromagnetic penetration allows to conclude that the use of a given solution for one type of device does not necessarily guarantee success in suppressing sensitive electromagnetic emissions of another type of such device. This is related to both the design of the device and its hardware configuration. A similar phenomenon can be observed in the case of 3D printers. The possibility of wider use of the method presented in this article must therefore be verified for other types of 3D printers. This is related to the possible occurrence of acoustic signals for such printers in the range of other frequencies. This approach is necessary to find a universal solution (for as many different types of printers as possible) to counteract non-invasive information acquisition using acoustic infiltration.

We look forward to hearing from you in due time regarding our submission and to responding to any further questions and comments you may have.

 

Best Regards

Authors

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript studies a very interesting topic regarding the information safety of 3D printing. The sounds made by the 3D printer in the fabrication process may reflect the geometric shape of the printed objects, which could result in info disclosure. To address this potential issue, the manuscript presents a way to protect the info against acoustic infiltration. The authors took a geometry containing complex contours as an example and extracted the spectrograms of 3D printer acoustic signals. They then introduced different types of noises with various sound levels and analyzed how randomly the spectrograms of 3D printer acoustic signals can be messed up. Eventually, they found that the violet noise is the most effective one to serve the job. The manuscript requires major revision. Please consider my following questions.

1.       The study in this manuscript cannot represent all 3D printing manners. Instead, it particularly works on direct ink writing and fused deposition modeling, which highly relies on the control of three motors. For other 3D printers with photo-projection, the shape of objects will be extremely hard to detect through acoustic information. Therefore, the authors should narrow down their study toward specific 3D printing techniques.

2.       For figure 2: The selection of complex geometry was not clear and not rationalized well. For example, how to quantify the degrees of geometric complexity regarding straight lines and curvatures? Will it be about inclined angles, distance, and curvature types?

3.       Page 7, line 186: I believe there is a typo, F – A (not G).

4.       For the extracted spectrograms of the two acoustic signals in Section 3, is it possible to build a modeling relationship between the geometry and the spectro-curve? The current studies in the manuscript are confined to the two selected objects. It is essential to predict the object shape by a generalized model.

5.       When discussing the impact of different types of noises with various sound levels on destroying the original 3D printing acoustic signals, how did the authors determine the effectiveness of different noise inputs and eventually state the best is the violet noise? I don’t want to “see” it from the figures qualitatively but want to know it quantitatively. If the correlation analysis in Section 3.4.2 is the approach, I recommend re-arrange the section better.

Comments on the Quality of English Language

English should be polished. For example, the sentence on page 3, line 90.

Author Response

Dear Reviewer

Thank you for providing the detailed and constructive review for our paper which allowed to increase a scientific level of this paper. We tried to reply for each point mentioned in the review. Our corrections were highlighted in manuscript by blue and red colours. We hope that the new version of our manuscript looks better and meets your requirements.

Here we’d like to reply for each point mentioned in the review:

1. The study in this manuscript cannot represent all 3D printing manners. Instead, it particularly works on direct ink writing and fused deposition modeling, which highly relies on the control of three motors. For other 3D printers with photo-projection, the shape of objects will be extremely hard to detect through acoustic information. Therefore, the authors should narrow down their study toward specific 3D printing techniques.

We agree with your opinion. We have improved the Introduction according your suggestion. Our response is included in the text of manuscript.

We added below text in the Introduction:

3D printing is a complex definition for many technologies that differ in manufacturing processes, materials used and the type of objects that can be made from them. Based on the literature analysis, the main 3D printing technologies can be characterized as follows:

• FDM (Fused Deposition Modeling)/FFF (Fused Filament Fabrication) – 3D printing from thermoplastics. This is the most popular technology on the home and semi-professional printer market. The printing material is plastic in the form of a monofilament. This monofilament is heated to the appropriate temperature during the 3D printing process. After obtaining a semi-liquid consistency, the monofilament is put on layer by layer (as a result of the printing platform moving down or by moving the printing head up) using the printing head. Bonding occurs under the influence of temperature. FDM printers can use materials such as ABS or PLA. The operation of printer is based on the working of three stepper motors, which are the source of sound signals.
• SLA (stereolithography) – 3D printing from resins hardened with a laser beam. This technology uses a resin hardened under the influence of light as a printing material. The semi-liquid resin is located in a special container in which the work table is placed. The material hardens during the action of the laser beam. The table with resin is lowered by one layer and the beam reproduces the desired shape in the XY plane.
• DLP (Digital Light Processing) – 3D printing from resins hardened with a projector beam. In this technology, the resin is hardened by the light of the projector, which is placed under the resin container. The entire layer is hardened at once.
• PolyJet / MJP (PolyJet / Multi-Jet Printing) – 3D printing from resins hardened with UV light. Printing is done in a similar way to printing on paper using a laser or ink. The print head moving over the platform (work table) sprays a layer which is at the same time hardened by ultraviolet radiation.
• CJP (Color Jet Printing) – 3D printing from gypsum powders in full color. A layer of powder is placed on the surface of the work table, and the printer head sprays colored ink and binder in appropriate places. The print is chemically hardened [1-2].
• Binder Jetting – 3D printing from sand or powdered metal, ceramics or sand. This printing technology does not require the use of supporting structures. The process of creating objects resembles traditional inkjet printing. The Binder Jetting method is used in industry to create, for example, foundry moulds [3].
• SLS (Selective Laser Sintering) – 3D printing from powdered plastics selectively glued and welded using a laser beam [3].
• SLM/DMLS (Selective Laser Melting/Direct Metal Laser Melting) – 3D printing from powdered metals melted using a laser beam.
• EBM (Electron Beam Melting) – 3D printing from powdered metals melted using an electron beam.
• MJF (Multi-Jet Fusion) – 3D printing from powdered polyamide, which is selectively glued and welded at high temperature.
• LOM (Laminated Object Manufacturing) – 3D printing involving three-dimensional printing using layers of paper with glue and polyester laminate, cut by using a laser.
• Bioprinting – 3D printing from living biological material. In this way, fragments or even entire organs can be obtained [3].

The above mentioned 3D printing technologies use different printing materials and different methods of hardening them. The article further focuses on the analysis and possibilities of masking sound signals correlated with printed shapes for the Zortrax M200 Plus printer, which uses FDM technology and PLA printing material. The choice of the printer resulted from its popularity and availability.

2. For figure 2: The selection of complex geometry was not clear and not rationalized well. For example, how to quantify the degrees of geometric complexity regarding straight lines and curvatures? Will it be about inclined angles, distance, and curvature types?

Researches and analyzes carried out in [11] showed that the movements of the print head along the X and Y axes and along straight lines forming a 45-degree angle with the X and Y axes are easily identifiable. This means that the level of sensitivity to acoustic infiltration process for such movements of printing head is high. Results of the carried out tests show that this operating mode of the Zortrax M200 Plus 3D printer is a source of acoustic signals are characterized by the highest infiltration susceptibility. This was the starting point for research and analyzes in the field of counteracting the process of non-invasive acquisition of information about printed shapes. At the same time, it can be assumed that the effectiveness of the solution in masking acoustic signals with a high level of sensitivity will also be effective in the case of signals for which this level is low.

3. Page 7, line 186: I believe there is a typo, F – A (not G).

Yes, thank you. We corrected this mistake.

4. For the extracted spectrograms of the two acoustic signals in Section 3, is it possible to build a modeling relationship between the geometry and the spectro-curve? The current studies in the manuscript are confined to the two selected objects. It is essential to predict the object shape by a generalized model.

Thank you for an important remark. We are thinking about the problem. It would be a good solution to propose a generalized model on this step of tests which would allow to predict the object shape. But in our opinion this is too early. The aim of the studies was to test whether sounds produced by the Zortrax M200 Plus printer could be effectively masked by the appropriate type of noise. The choice of noise color had to meet two conditions: 1) masking sensitive signals in a specific frequency range and 2) minimizing the level of generated noise. The determinant of the mentioned masking effectiveness was the visual assessment of spectrograms and the comparison of the values of correlation coefficients. Finding a model relationship between the geometry and the spectral curve is another issue that is beyond the scope of this article.

5. When discussing the impact of different types of noises with various sound levels on destroying the original 3D printing acoustic signals, how did the authors determine the effectiveness of different noise inputs and eventually state the best is the violet noise? I don’t want to “see” it from the figures qualitatively but want to know it quantitatively. If the correlation analysis in Section 3.4.2 is the approach, I recommend re-arrange the section better.

The assessment of the effectiveness of using color noise in counteracting the acoustic infiltration process was carried out in two stages. After recording sensitive acoustic signals correlated with the operation of stepper motors, a frequency-time analysis was performed. The mentioned signals were visualized in the form of spectrograms, which allowed to a visual assessment of the possibility of reproducing printed shapes by a 3D printer. Additionally, the obtained spectrograms showed the frequency characteristics of acoustic signals. Thus, a range of signal frequencies was determined that allows determining the movements of the print head and making a preliminary selection of the type of noise that would be most effective in masking acoustic signals coming from the 3D printer.

Since visual assessment is not always an effective assessment. Such evaluation depends on many factors related to the visual condition of the assessor, an analytical assessment was carried out using the correlation coefficient between the pattern signal and the measured signal. To determine the degree of similarity, a typical division of the correlation coefficient value into sub-ranges indicating the level of similarity was used. For this purpose, the division presented in Table 2 was selected, which is most often used in technical sciences (especially in the area of information protection), as opposed to social or medical sciences. This is a "stricter" division due to the area of application. Therefore, values above 0.2 should not be accepted.

 

Comments on the Quality of English Language

English should be polished. For example, the sentence on page 3, line 90.

We corrected this paragraph.

The basic parameter determining the type of noise is the power spectral density ?(?). Depending on its characteristics, white noise and the so-called color noises are distinguished. The characteristics of popular noises in acoustics are given on Figure 3.

We conducted extensive editing of English language and style. We realize that, despite all our efforts, this article may not be language and style error-free. Therefore, the final version, after possible acceptance, will be corrected by the proofreading service in the MDPI.

 

We look forward to hearing from you in due time regarding our submission and to responding to any further questions and comments you may have.

Best Regards

Authors

 

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This paper focused on a novel and practical approach to maintaining the security of information related to 3D printing, and it has found a promising method using colored noise for acoustic masking. While the article outlines a promising approach for securing 3D printing processes through acoustic masking with colored noise, it's important to consider potential drawbacks and limitations:

 

1.       Complexity of Implementation: Implementing acoustic masking with colored noise might be more complex than it seems. It may require specialized equipment and expertise in signal processing to achieve the desired level of security.

 

2.       Effect on Printing Speed: Depending on the intensity and type of noise used for masking, it could potentially slow down the printing process. This might be a significant drawback for users who require fast 3D printing.

 

3.       Compatibility: The effectiveness of this method may vary depending on the type and model of 3D printer. The article mentions the need for further research on different types of printers, suggesting that the approach may not be universally applicable.

 

 

It's important to carefully evaluate these potential drawbacks and limitations in the context of the specific 3D printing application and security requirements to determine whether this method is a suitable solution.

Author Response

Dear Reviewer

Thank you for providing the detailed and constructive review for our paper which allowed to increase a scientific level of this paper. We tried to reply for each point mentioned in the review. Our corrections were highlighted in manuscript by blue and red colours. We hope that the new version of our manuscript looks better and meets your requirements.

Here we’d like to reply for each point mentioned in the review:

1. Complexity of Implementation: Implementing acoustic masking with colored noise might be more complex than it seems. It may require specialized equipment and expertise in signal processing to achieve the desired level of security.

Thank you for this remark.

Protection of information is an extremely important activity from the point of view of many areas of human activity. It concerns personal data, health, information security at the enterprise and state levels. An important area related to information protection is counteracting the acquisition of this information through non-invasive ways. For this purpose, any measures that prove effective in counteracting acoustic infiltration should be used. The aim of the article is to show that achieving the required level of security does not always require financial outlays that are unattainable for the average 3D printer user. Of course, knowing already know the solutions that are the result of the analyzes presented in the article. However, in order to present a proposal for a security method, knowledge of digital signal processing and physical phenomena occurring in the studied areas is necessary.

About the above we mentioned in the conclusions.

2. Effect on Printing Speed: Depending on the intensity and type of noise used for masking, it could potentially slow down the printing process. This might be a significant drawback for users who require fast 3D printing.

Thank you for this remark. Our solution doesn’t have influence on speed of printing. But we can read about methods which could protect information against acoustic infiltration. Such solution are presented e.g. [8].

We added below text in the Introduction.

In [8], an innovative acoustic side-channel attack model for FDM technology was presented. The shape of the printed object (a key) was reproduced with quite high accuracy. It was found that to improve protection against acoustic infiltration and to make G code that control the operation of a printer, reconstruction process more difficult, randomness of the motor movement could be introduces into this code. However, the disadvantage of such a countermeasure will be the delay in the overall printing time.

3. Compatibility: The effectiveness of this method may vary depending on the type and model of 3D printer. The article mentions the need for further research on different types of printers, suggesting that the approach may not be universally applicable.

Thank you. You are right. Protection of information against acoustic penetration (also against electromagnetic and vibration penetration) requires special approach.

We added below text in the conclusion:

Tests on the effectiveness of masking acoustic signals using color noise were carried out for the Zortrax M200 Plus 3D printer. The extensive experience of the authors of the article in area of protection of information against electromagnetic penetration allows to conclude that the use of a given solution for one type of device does not necessarily guarantee success in suppressing sensitive electromagnetic emissions of another type of such device. This is related to both the design of the device and its hard-ware configuration. A similar phenomenon can be observed in the case of 3D printers. The possibility of wider use of the method presented in this article must therefore be verified for other types of 3D printers. This is related to the possible occurrence of acoustic signals for such printers in the range of other frequencies. This approach is necessary to find a universal solution (for as many different types of printers as possible) to counteract non-invasive information acquisition using acoustic infiltration.

 

We look forward to hearing from you in due time regarding our submission and to responding to any further questions and comments you may have.

Best Regards

Authors

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

This revision is acceptable.

Comments on the Quality of English Language

This revision is acceptable.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript looks better and can be accepted in its current form.