1. My Smartphone Knows What You Print Exploring Smartphone-based Side-channel Attacks Against 3D Printers
Article by: Chen Song, Feng Lin, Zongjie Ba, Kui Ren, Chi Zhou, Wenyao Xu
Presented by: Hui Yie Teh

2. 3D Printing
3D printing (a.k.a. additive printing) is widely used to manufacture products that are highly intellectual property (IP) sensitive
This is because 3D printing is:
Efficient
Allows creativity
Affordable
Used in industries such as:
Medical
Aerospace
Architecture
Education
Nowadays, 3D printing is commonly used to manufacture products that are highly intellectual property (IP) sensitive
This is because they are:
Efficient as they save time and cost (less waste material)
Allows creativity (allows flexible creation of complex geometric construction)
Affordable
They are used in a wide range of industries such as the medical, aerospace, architecture and education sectors
Extra notes:
Intellectual property: intangible property that is the result of creativity such as patents and copyright e.g. design of the product
Affordable: 3D printers and the materials are affordable – few hundred dollars ($200 - $700?)
Image from:

3. Problem
Any common person with a smartphone can enter within the range of a 3D printer and collect side-channel data
Completely inconspicuous cause of ubiquity of smartphones
No need for physical contact with the printer
Attacker does not have to be in the surrounding area
However, these 3D printers produce sound and other side-channels when constructing an object
So any common person with a smartphone can enter within the range of a 3D printer and start collecting side-channel data
Side-channel data - any information gained from the physical domain e.g. sound, electromagnetic leaks and timing information
This is unsuspicious as mobile phones are widely used and it is completely normal for people to leave their phones everywhere (on table where printer is at)
No need physical contact with printer and the attacker does not have to be around to collect data

4. Motivation Purpose of this paper is to:
study side-channel attacks against 3D printers using smartphones
to see if the IP information can be inferred through these side-channel attacks
IP protection in the physical domain is still largely unexplored
Has been discussed before in the cyber domain
Therefore, this paper studies the side-channel attacks against 3D printers using smartphones and to see if there can be any IP leakage from these side-channel attacks
Also attacks against 3D printers in the physical domain is still largely unexplored as compared to attacks in the cyber domain has been discussed before, giving rise to encryption and watermark to protect IP in the cyber domain
To explain a little more about the physical domain and the cyber domain, I’ll talk about the background of 3D printing

5. Background Cyber domain: 3D digital design data
Object model is created in a Computer Aided Design (CAD) software
Converted to STL file – standard object file
Computer Aided Manufacturing (CAM) generates toolpath file
Toolpath file (G-code)
Physical domain: 3D physical object
3D printer manufactures the object
3D printing is broken down into the 2 mentioned domains
The design an object is created in the cyber domain by firstly using computer aided design software. It is then converted into a standard object file and from that, the computer aided manufacturing software generates the toolpath file.
The most common toolpath file is G-code
So in the G-code, it includes all the operational instructions of the printing process
To put it simply, they are instructions on where to move, how fast to move, and what path to follow as well as the printer settings such as the temperature, coordinates
Thus, all IP information is in the G-code and if it can be inferred, there will be IP leakage and product replication
As for the physical domain, it is the actual construction of the object
Extra notes:
CAD: the use of computer systems to aid in the creation, modification, analysis or optimization of a design.
STL file: standard object file where the model is represented by the surface geometry composed of triangular facets
CAM: the use of software to control machine tools and related ones in the manufacturing of workpieces, : module slices the model into uniform layers and generates the toolpath file

6. Background
In this study, a Fused Deposition Modelling (FDM) 3D printer is used
4 primitive printer operations:
Layer movement - whether the printer prints in the same layer or change to the next layer.
Header movement - whether header prints object or aligns position
Axial movement - whether nozzle moves in X or Y axis in the X-Y plane
Directional movement - which direction the nozzle moves in X or Y axis
FDM is a type of 3D printer that uses a material such as a thermoplastic filament, which is heated to its melting point and then extruded, layer by layer, to create a 3D object.
Layer movement: whether the printer prints in the same layer or change to the next layer.
Header movement: whether header prints object or aligns position
Axial movement: whether nozzle moves in X or Y axis in the X-Y plane
Directional movement: which direction the nozzle moves in X or Y axis (left/right for X-axis or up/down for Y-axis)

7. Background Acoustic and magnetic side-channels
Acoustic side-channel helps predict the axial movement well, but not the directional movement.
Each axis is controlled by one motor and each motor produces different sounds
The directional movement is controlled by a rotor and electromagnets which produces similar sounds, but distinguishable patterns in the magnetic field
Magnetic side-channel is used to predict directional movement instead
Preliminary studies done in the article found that the acoustic side-channel(sound) predict the axial movement well, but not the directional movement.
This is because each axis is controlled by one motor which produces distinct sounds.
However, the directional movement is controlled by a rotor and electromagnets in the motor (as seen in the image above) which produces similar sounds. But it has distinct patterns in the magnetic field.
Therefore, magnetic side-channel is used to extract information about the directional movement instead

8. Idea Place a smartphone nearby a 3D printer
Have a data recording application on the smartphone
Reconstruct G-code based on the side-channel information gathered
The idea is to have a smartphone nearby a 3D printer that collects acoustic and magnetic side-channel data
Through a recording application on the smartphone
This is to see if able to reconstruct G-code based on the side-channel information gathered

9. Details 3D printer used in the study: Smartphone used in the study:
Ultimaker 2 Go
Smartphone used in the study:
Nexus 5
Built-in sensors such as microphones and magnetometer sensor
Uses the magnetic-enhanced IP side-channel attack framework for 3D printers via smartphones in the physical domain
This study uses the Ultimaker 2 Go 3D printer
And Nexus 5 smartphone with built in sensors such as microphones and magnetometer sensors
The phone is placed 20cm away from the 3D printer
The attack is then conducted using the magnetic-enhanced IP side-channel attack framework.

10. Details
The framework is visualized as shown and it has 3 modules that are highlighted:
Side-channel information : where we gather data
Printing plan extraction: where we extract information of the primitive operations based on the side-channel data gathered
IP reconstruction: reconstruct the G-code based on the previous module
Will go through each one in more detail
The proposed magnetic-enhanced IP side-channel attack framework

11. Side-channel Information
Data acquisition
Smartphone recording application that continuously collect side channel data
Pre-processing
Remove light ambient noises using the
Savitzky-Golay filter
Separated into frames with fixed size
Has 2 parts
Data acquisition part which is done through the recording app that continuously collects the magnetic and acoustic side-channel data at the same time
Pre-processing part which removes light ambient noises such as such as white noise using the Savitzky-Golay filter
It is then separated into frames of fixed sizes
Extra notes:
Savitzky-Golay filter: A digital filter that can be applied to a set of digital data points for the purpose of smoothing the data, that is, to increase the signal-to-noise ratio without greatly distorting the signal.

12. Printing Plan Extraction
Predict the printer operations based on the data frames collected
Parameter extraction step for each data frame:
Layer movement analysis
Header movement analysis
Axial movement analysis
Directional movement analysis
Then, all information is integrated
Support Vector Machines (SVM) is used to predict the primitive movement
Through the data frames collected, we then use that to predict the primitive printer operations.
As mentioned before, have 4 primitive operations
Layer, header and axial movement analysis uses the acoustic side-channel data as the sounds produced by these movements are clearly distinguishable and can be extracted from the acoustic side-channel data
Directional movement extraction is based on the magnetic side-channel for reasons as explained before
Then the information is integrated and the SVM machine learning algorithm is used as classifiers to predict the primitive movement operations.
Extra notes:
Feature extraction is used to enhance the SVM model. Two types of features extracted:
Temporal features: Time domain features which are simple to extract and have easy physical interpretation, like: the energy of signal, zero crossing rate, maximum amplitude, minimum energy, etc.
Spectral features: Frequency based features which are obtained by converting the time based signal into the frequency domain using the Fourier Transform, like: fundamental frequency, frequency components, etc
Layer movement analysis
the actuation system for the platform is very different from the one for the nozzle because it contains a deformed bar instead of the belt. Thus, platform movement (Z-plane) generates unique acoustic signal and the acoustic channel model can be applied
Header movement analysis
During extrusion of material, header prints with regular printing speed. When header performs a quick alignment, the material is no longer extruded and a much faster speed is applied to avoid stringing effect. Fast movements generate acoustic signal with significant pattern. Thus, use acoustic channel model.
Axial movement analysis:
Acoustic channel model since the sound produced by the two motors(one for X and one for Y) are different
Directional movement analysis:
Use magnetic channel model cause the directional movement is controlled by the motors which use electromagnets to change the direction and this produces similar sounds.

13. IP reconstruction Obtain predicted printer operation parameters
Converted into G-code
- Lastly, the IP reconstruction is done by having the predicted printer operations parameters converted into G-code

14. Results Primitive operation models:
For the primitive operation predictions, it is seen that almost all the primitive operations were predicted with high accuracy of 90% and above, However, there is one that is slightly below 90% which is the X-left directional movement
A frame size of 200ms is chosen for the evaluation.
As to why this is chosen, the authors carried out the study with different frame sizes and found that smaller frame size lowers the classification accuracy because that means there are less characteristic information contained in each frame. However, bigger frame sizes will affect the quality of the printing process.
Thus, the frame size of 200ms chosen as a trade-off between model accuracy and resolution of the printing process
Primitive operation models:
Almost all primitive operations were classified more than 90% correctly, except for the X-left directional movement which has an accuracy of 89.9%
Based on the frame size of 200ms
Chosen as a trade-off between model accuracy and resolution of the printing process

15. Results Simple design Complex design
Manage to replicate successfully with an average Mean Tendency Error (MTE) of 5.87% for 4 layers
Complex design
The complex shape of the object is still successfully replicated, though a little less accurately with an average MTE of 9.67%
The few outliers caused by misclassifications, mostly by previous X directional movements
A simple and complex design is reconstructed to test the feasibility of the attack. It is repeated in several layers and then compared with the original shape (red = original shape, black = reconstructed shape)
For the simple design, the low average MTE of 5.87% indicate that the attack is successful and the original design IP was accurately replicated
As for the complex design, it is a little less accurate with an increase in MTE of about 4%. But the complex shape of the object is still replicated
Few outliers, mostly in the Y axis. They are most likely generated by the misclassifications of the previous X directional movements
Extra notes:
MTE: assesses the geometrical reconstruction based on the relative shape difference
The X directional movement are less accurate than the Y directional movements. X-left and X-right gives the worst performance, with 89.9% and 91.2% accuracy. They do a little more poorly compared to Y-up and Y-down, up to almost 5% difference in accuracy

16. Limitations Distance Ambient noise Printing speed Carry-on attack
Advanced shapes
The authors discussed the limitations of their study
Distance: It is seen that the average MTE increases by up to 30% when the distance doubled from 20cm to 40cm. Thus, in order to keep the accuracy of the framework, the phone needs to be relatively near.
Ambient noise: light ambient noises were filtered out using the Savitzky-Golay filter but strong ones (or unexpected ones) are not taken into account in the experiment
Different printing speeds, carry-on attack and advanced shapes replication such as circles and arcs are also not done in the study
Extra notes:
Distance: Need to keep the phone relatively near since magnetic side-channel diminish a lot with distance (need it to be ~20cm)
Carry-on attack: can’t be done since mobile phones have to be stationary during the data collection part
Advanced shapes: such as circle, ellipse and arc might cause complications in identifying the printing state (whether material is extruded or not)because the nozzle motion trajectory becomes convoluted.

17. Criticisms
Explored a practice enhancement to allow the attack to be orientation- independent
Other FDM 3D printers
3D printer variation
e.g. selective laser sintering and selective laser melting
Other nearby 3D printers
Did well explaining the limitations and next steps they will take in the future
Also, good that they did a practice enhancement to obtain orientation-independent magnetic data. So to carry out the attack, the smartphone don’t need to be placed in the same orientation as the one in the study
However, I noted that this study only uses one FDM printer model, would be good to see how this attack framework perform in other models e.g. MakerBot (it might affect the accuracy of the X directional movement)
Also the attack model from this study cannot be used in other 3D printers e.g. selective laser sintering as their structure varies from the FDM printers
This paper does not mention what happens if there are other 3D printers nearby. This might affect the side-channel data collected and they did not clarify what happens if it’s so
?Have fixed frame size in the experiments. But to capture smaller movements, we need smaller frame size. However, for faster speeds, we need larger frame size. A trade-off is needed depending on the situation and they can maybe incorporate adaptive frame size to increase the accuracy of the attack model.
Extra notes:
Orientation independent attack: Used for magnetic side-channel only since acoustic side-channel propagates in a sphere. Done by Euler’s rotation theorem
selective laser sintering that uses a laser as the power source to sinter powdered material (typically metal), aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. Sintering: process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.
Smaller frame size: so that the temporal resolution of the features extracted can be increased
Larger frame size: to increase the frequency resolution for better spectral features

18. Thank you
Questions?