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On the Relationship between Generalization and Robustness to Adversarial Examples

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One of the most intriguing phenomenons related to deep learning is the so-called adversarial examples. These samples are visually equivalent to normal inputs, undetectable for humans, yet they cause the networks to output wrong results. The phenomenon can be framed as a symmetry/asymmetry problem, whereby inputs to a neural network with a similar/symmetric appearance to regular images, produce an opposite/asymmetric output. Some researchers are focused on developing methods for generating adversarial examples, while others propose defense methods. In parallel, there is a growing interest in characterizing the phenomenon, which is also the focus of this paper. From some well known datasets of common images, like CIFAR-10 and STL-10, a neural network architecture is first trained in a normal regime, where training and validation performances increase, reaching generalization. Additionally, the same architectures and datasets are trained in an overfitting regime, where there is a growing disparity in training and validation performances. The behaviour of these two regimes against adversarial examples is then compared. From the results, we observe greater robustness to adversarial examples in the overfitting regime. We explain this simultaneous loss of generalization and gain in robustness to adversarial examples as another manifestation of the well-known fitting-generalization trade-off.
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symmetry
S
S
Article
On the Relationship between Generalization and Robustness
to Adversarial Examples
Anibal Pedraza * , Oscar Deniz and Gloria Bueno


Citation: Pedraza, A.; Deniz, O.;
Bueno, G. On the Relationship
between Generalization and
Robustness to Adversarial Examples.
Symmetry 2021,13, 817. https://
doi.org/10.3390/sym13050817
Academic Editor: Jan Awrejcewicz
Received: 23 March 2021
Accepted: 30 April 2021
Published: 7 May 2021
Publisher’s Note: MDPI stays neutral
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
VISILAB, University of Castilla La Mancha, ETSII, 13071 Ciudad Real, Spain; oscar.deniz@uclm.es (O.D.);
gloria.bueno@uclm.es (G.B.)
*Correspondence: anibal.pedraza@uclm.es
Abstract:
One of the most intriguing phenomenons related to deep learning is the so-called adver-
sarial examples. These samples are visually equivalent to normal inputs, undetectable for humans,
yet they cause the networks to output wrong results. The phenomenon can be framed as a symme-
try/asymmetry problem, whereby inputs to a neural network with a similar/symmetric appearance
to regular images, produce an opposite/asymmetric output. Some researchers are focused on de-
veloping methods for generating adversarial examples, while others propose defense methods.
In parallel, there is a growing interest in characterizing the phenomenon, which is also the focus of
this paper. From some well known datasets of common images, like CIFAR-10 and STL-10, a neural
network architecture is first trained in a normal regime, where training and validation performances
increase, reaching generalization. Additionally, the same architectures and datasets are trained in
an overfitting regime, where there is a growing disparity in training and validation performances.
The behaviour of these two regimes against adversarial examples is then compared. From the re-
sults, we observe greater robustness to adversarial examples in the overfitting regime. We explain
this simultaneous loss of generalization and gain in robustness to adversarial examples as another
manifestation of the well-known fitting-generalization trade-off.
Keywords:
machine learning; computer vision; deep learning; adversarial examples; adversarial
robustness; overfitting
1. Introduction
Research in machine learning has experienced a great advance since the advent of
deep learning. This methodology is able to learn meaningful features to classify, generate or
detect objects in images, audio or any kind of signal. The results obtained with this frame-
work are outstanding, although their behaviour remains like a black box. Also, striking
errors appear in some specific cases, like in the case of the so-called adversarial examples.
Adversarial examples are carefully perturbed inputs to a machine learning system
that, even though they seem very similar to the original examples, produce a response in
which the output is incorrect. For example, Figure 1shows an image which, with a small
perturbation, is classified as a screw rather than a whistle, which is obviously an error for
us humans [1].
Note that the phenomenon can be interpreted as a symmetry/asymmetry problem,
whereby inputs to a neural network with a similar/symmetric appearance to regular
images, produce an opposite/asymmetric output.
Several attacks (algorithm to produce adversarial examples) and also defenses (method-
ologies to make networks more robust against adversarial examples) have been developed.
On the one hand, the most successful attack methods are the ones proposed in [
2
4
]. Most
of them compute variations on the input images to modify the gradient computed in
the network, so that the output points to a different class. As for defense methods, their
purpose is to modify the training methodology, or the network architecture, so that the
Symmetry 2021,13, 817. https://doi.org/10.3390/sym13050817 https://www.mdpi.com/journal/symmetry
Symmetry 2021,13, 817 2 of 13
produced models are more robust to these perturbations. Some of the most popular are:
Adversarial Training [
5
], which proposes data augmentation with adversarial examples to
train the network on their features; Pixel Defend [
6
] which modifies the image pixels to fit
them to the training pixel distribution and “undo” the perturbations; and the so-called De-
fensive Distillation [
7
]. In the latter method, the encoded and predicted class probabilities
of a neural network classifier are used to train a new network that increases its robustness
with respect to the original.
Figure 1.
Adversarial example. From (
left
) to (
right
): original image (classified as “whistle”),
adversarial noise added, resulting adversarial image (classified as “screw”).
Apart from research into defense and attack methods, there is a body of research
focused on characterizing the phenomenon of adversarial examples. Some of the research
in this line have suggested the well-known generalization versus memorization trade-
off as a cause for this problem [
8
]. Training a model to achieve good generalization
(high performance on unseen examples) decreases robustness to adversarial examples and
vice versa.
Let us consider a thought experiment in which the test subset is composed by samples
generated by crafting adversarial examples from the training set. Even though those
adversarial examples are perceptually close to the training set, they are nonetheless valid
test (i.e., unseen) samples. On the other hand, overfitting to the training samples is general
bad for generalization. However, since the adversarial examples can be arbitrarily close
to the original samples, overfitting should also have a positive effect on these adversarial
examples. In fact, following the same reasoning, as the number of training samples tend to
infinity *any* adversarial example should benefit from overfitting. Thus, there are indeed
reasons that suggest that at least for certain cases robustness to adversarial examples can
benefit from overfitting.
Some works, like in [
9
], argue that adversarial examples manifestation crafted on
different architectures and disjoints datasets indicates that overfitting is not related to
the cause of this phenomenon. However, Refs. [
10
,
11
] support that
L2
weigh decay and
regularization can help to increase robustness to adversarial examples, supporting the
implication of overfitting. Other works show that adversarial examples exploit the features
that are learned by the model [
12
]. In the latter work, it is stated that neural networks learn
their own features to classify the data. The features selected by neural networks are usually
not the same as the human perception patterns. Moreover, they study the presence of robust
and non-robust features, and propose a methodology to build datasets taking this into
account. Forcing the models to learn the robust features increases adversarial robustness.
Other findings to explain this phenomenon point in the direction of the non-linearity of the
layers employed in the network architectures [1].
In [
13
], it is shown that overfitted models have stronger adversarial robustness, at the
cost of lower generalization. However, their methodology presented some problems,
since they did not study the evolution of adversarial robustness during all the epochs
in the training process, only in the first and last epoch. Moreover, a comparison with a
non overfitting regime is not considered. In this paper, we extend their experimentation,
considering the progression of adversarial example robustness in two training regimes:
normal (validation-error reduction) and overfitting, monitoring the whole training process
to study the robustness trends.
Symmetry 2021,13, 817 3 of 13
Finally, Ref. [
14
] performs a comprehensive study of robustness for different models
in the ImageNet dataset. This work considers robustness and accuracy in final models,
that is, in the last epoch of a training process, where maximum accuracy is obtained for
the validation dataset. As a result, a comparison is performed among different families
of architectures. The main conclusion is that models in which validation accuracy is
lower and architecture complexity is also limited (Alexnet, MobileNet) are more robust
in comparison to more complex networks in which validation accuracy is higher (such as
Inception or DenseNet).
In our work, we focus on the trade-off that is established between accuracy and
robustness at different training epochs for the same model. Moreover, two different
regimes are forced in the training process. One, in which a “normal” training is performed
(as in [
14
], looking for the maximum validation accuracy) and another in which the model
is trained in an overfitting regime (validation accuracy drops at certain point, while training
accuracy keeps increasing). With the latter regime, the method is able to obtain a model
that is more robust against adversarial examples, at the cost of decreased test performance.
The contributions of this paper are the following. Extensive experimentation has been
performed to show the different behaviour of neural networks when they are trained in
both a normal and overfitted regime. Different attack methods are considered, from the
most popular and robust in recent research. To support the experimentation, some metrics
are calculated to compute the adversarial robustness of the models, in order to show the
relationship of these metrics to the hypothesis presented in [
13
], which is also supported in
our work.
This paper is organized as follows: Section 2presents the datasets employed in this
work and an introduction is given to the different attack methods and robustness metrics.
Section 3explains in detail the different experiments that are carried out, analyzing the
obtained results. Finally Section 4outlines the main conclusions.
2. Material and Methods
In this section, the materials employed in this work are detailed. In this case, CIFAR-10
and STL-10, common reference datasets in computer vision, are selected to perform the
study. The main attack methods developed in current research are studied and defined
below. Also, relevant model robustness metrics and image distance metric are commented.
2.1. Datasets
In work with adversarial examples, it is common to use datasets like MNIST [
15
],
which covers a collection of grayscale handwritten digits with 28
×
28 pixel size, suitable
for automatic recognition system development. Other variants are proposed in [
16
], which
was developed to build a similar result than the obtained originally, with a much more
extensive test set. Finally, others like Fashion-MNIST (developed by the Zalando company
in [
17
]), are being increasingly used. This one consists of thumbnails of grayscale cloth
images, in a similar structure and size than the previous ones.
However, in this work, we start directly with more real-world like images, suitable
for a wider range of applications. For this reason, the STL-10 dataset [
18
] has been chosen
as the main dataset for this work. The images have a limited size of 96
×
96
×
3 pixels
(RGB color), and were taken from labeled examples for the ImageNet by [
19
] dataset,
with samples from 1000 different categories. An example is shown in Figure 2.
As a first step, initial experimentation is also performed with a smaller and similar
dataset as CIFAR-10, from [
20
], to show if the hypothesis in this work is promising in
an early step. Both datasets represent 10 different categories from common objects and
animals from real-world images: airplane, bird, car, cat, deer, dog, horse, monkey, ship and
truck. Note that images in CIFAR-10 are smaller (32
×
32
×
3) than the ones of STL-10,
as shown in Figure 2. Finally, note that in CIFAR-10 there are 50,000 images for training
and 10,000 for testing, while STL-10 dataset has 5000 images available for training and
8000 for testing, which makes it more challenging.
Symmetry 2021,13, 817 4 of 13
(a) CIFAR-10 (b) STL-10
Figure 2. Samples from the datasets in this work.
2.2. Methods
There are different methods to craft adversarial examples from a given input, so-called
attacks. Most of them are based on the gradient variation response so that the model
classifies the image with a different output. They can be targeted or untargeted. That is,
whether they force the adversarial to be predicted as a specific class or not. In the latter case,
the algorithms usually select the easiest to fool or a random class. A brief description of the
adversarial attack methods used in the experiments for comparison can be found in [
21
].
The main methods that are considered in this work are: FGSM [
9
], Carlini & Wagner [
2
]
and PGD [4] (derived from Basic Iterative Method [22]).
2.3. Metrics
Another important aspect of our work is the ability to measure the adversarial ro-
bustness of a model. These algorithms need a trained model and some test examples (the
more examples, the better the precision, but more computational time is required). For this
purpose, different metrics are proposed:
Loss Sensitivity: proposed in [
8
] it computes the local loss sensitivity estimated
through the gradients of the prediction. It measures the effect of the gradients for each
example, which translates into a measure of how much an example is memorized.
The larger the value, the greater the overfitting to this particular example.
Empirical Robustness: as described by [
23
], it is equivalent to computing the minimal
perturbation value that the attacker must introduce for a successful attack, as estimated
with the FGSM method.
CLEVER: elaborated in [
24
], this metric computes a lower bound on the distortion
metric that is used to compute the adversarial. It is an attack-agnostic metric in which,
the greater the value, the more robust a model is supposed to be.
Also, it is important to compare the difference between the crafted adversarial exam-
ples and the original examples. For this purpose, the main metrics that are used are based
on the
Lp
norm, calculated over the difference between the original and the adversarial
example. The most common variants of this metric are shown in Table 1.
Symmetry 2021,13, 817 5 of 13
Table 1. Distance metrics derived from the Lpnorm.
LpNorm Calculation Explanation
L0non-zero elements number of perturbed pixels
L2Euclidean distance distance in the image space
Llargest value highest perturbation at any pixel
3. Experimental Results and Discussion
Two different experimental set-ups are proposed. The first one is performed in a no-
overfitting regime and the second one in an overfitting regime. For each experiment, a cross-
validation procedure is set up. The training dataset is distributed into 5 training/validation
folds. The models are trained using 4/5th parts of the training set and validated with the
remaining fifth. This procedure is repeated 5 times, rotating the part that is left out for
validation. For each run, an optimal model is selected according to the accuracy on the
validation dataset. Then, adversarial examples are crafted on the test set for each method
against the selected snapshot. To evaluate the results, the Adversarial Successful Rate (ASR)
is computed, which is defined as “1—
accuracy
” obtained by the model for each adversarial
set. The accuracy for the original test samples is also provided. Also, the distance metrics
and adversarial robustness metrics described above are computed to show the difference
in the behaviour between both regimes. This process is performed for each fold of the
cross-validation scenario.
In order to show the results for the different runs, each line in the plots represents the
mean value obtained at each epoch. Additionally, vertical lines are included to represent
one standard deviation of the means. Analogously, when results are provided numerically
in tables, they also contain the mean and standard deviation.
3.1. No Overfitting
The first set of experiments is performed using appropriate parameters to reduce
the validation error. This is usually the main goal when training a model for a specific
task. In consequence, the generated model is expected to generalize, being able to perform
accurately for samples that are not present in the training dataset, while they keep on the
same distribution, known as the test set.
In the case of the CIFAR-10 dataset, training has been performed using a LeNet
architecture as described in [
25
], implemented in a Keras-Tensorflow backend. Training
was performed for 35 epochs and with a learning rate of 10
5
. The performance is shown
in Figure 3. The important key is to check that both training and validation accuracies grow
with the same positive progression.
Considering the results for the previous dataset, in the case of the STL-10 dataset,
the same set-up and parameters are employed. Performance is shown in Figure 3. As in
the previous case, it is observed that the achieved models are trained with no overfitting.
So, in both cases, they are suitable to continue with the experimentation.
Regarding the performance on the training process, it should be noted that the objec-
tive is to clearly differentiate between normal and overfitting regimes, independently of
the absolute values in validation accuracy, for example. As suggested in the experimental
proposal, when the model is trained on a normal regime, the validation accuracy increases
progressively, which is achieved here. However, in the overfitting regime, this accuracy
should reach a peak and then start dropping, as observed in further experimentation.
In both cases, the images used for adversarial generation are correct predictions for the
given model, so there is no drawback in discarding some of the images due to low relative
nominal test performance. Specifically, the STL-10 dataset has a wider difference between
training and validation accuracy, but the trends persist.
Symmetry 2021,13, 817 6 of 13
(a) CIFAR-10 (b) STL-10
Figure 3. Training progress for normal training.
In this case, the models trained in this non-overfitting regime have been tested against
the CW, FGSM and PGD attack methods for both datasets (CIFAR-10 and STL). In all cases,
they have been set up to perform untargeted attacks (with no predefined destination class)
using
L2
distance metric to generate the adversarial examples. Moreover, the minimum
confidence value parameter is given to CW, and standard maximum perturbation of 0.3 are
set-up for PGD and FGSM.
The epoch with the best validation accuracy is selected to craft the adversarial ex-
amples. Then the rest of models are applied over the same adversarials. Here we are
leveraging a concept known as adversarial transferability [
26
]. The architecture remains
the same but with increasing/decreasing levels of generalization through the training
process. For this reason, the adversarials are considered to be highly transferable and,
therefore, suitable to study their effects on different stages of the training and with different
parameter conditions (overfitted vs regularized)
As the training is performed in a normal regime, the 35th and last epoch is usually the
one with the best validation accuracy, see Figure 3. The adversarial examples generated
with each method are classified with the snapshot models trained with all the epochs. Then,
the accuracy in each AE (Adversarial Example) set is obtained. Figure 4shows the test
accuracy curve and the Attack Success Rate (ASR), for the attacks that have been performed.
This metric represents the percentage of adversarials that are misclassified by the model.
(a) CIFAR-10 (b) STL-10
Figure 4. Progression in the no-overfitting regime.
As shown in the performance of the CW attack for these models, the adversarial
success rate is lower in the first epochs (from the 5th epoch onwards, as the previous
ones should not be considered as the models are not stable), when the model is still a bad
classifier. However, when the test accuracy increases, the performance of the model in the
Symmetry 2021,13, 817 7 of 13
AE set drops significantly. There is a remarkable trend that shows that, when the model is
well trained for generalization, the effect of adversarial perturbations is stronger.
Regarding the differences between the CW attacks and FGSM/PGD, the former is
the best in terms of the quality of the generated adversarials. That is, they are the most
similar to the originals images (in terms of
Lp
metrics). However, in terms of robustness,
they can be discovered with small changes in the decision boundary, as will be shown with
the overfitting models (and also happens here with the normal regime). Depending on the
training state, a better generalization can also point to adversarials that fall in the same
distribution if they are very close. However, the other two attacks obtain a greater success
rate because the adversarials are crafted with a larger distance from the test distribution,
as observed for both L2(Euclidean distance) and L(maximum perturbation) metrics.
Table 2shows the performance of the CW method with the CIFAR-10 models. First,
the
L0
metric shows the total number of pixels modified by mean. The maximum value
would be 1024 (32
×
32), so more than 98% of the pixels have some alteration from
the original image. However, they are modified with very low values, as shown by
L2
(Euclidean) distance and especially by the very low maximum perturbation indicated by
L
metric. This metric indicates the maximum perturbation added to the intensity of any
pixel in the image, considering that they are represented in a range of 0–1.
Table 2. Average distances for the adversarial sets without overfitting for CIFAR-10.
Method L0L2L
CW 1024.00 ±0.00 0.43 ±0.48 0.07 ±0.06
FGSM 1017.92 ±25.30 8.93 ±0.48 0.30 ±0.00
PGD 1018.99 ±20.87 8.10 ±0.57 0.30 ±0.00
Table 3shows the AE distances with respect to the original test set for STL-10. The PGD
and FGSM AE sets consistently fool the model during the whole training process, as seen
in the previous curve. This is produced because, as this table points out, the adversarials
generated by these methods are farther away (from the original test samples) in comparison
with the examples generated by CW. With
L2
distances ranging from 23–26, against 0.97 on
average by CW, this means that the adversarials are too far. Instead, CW examples achieve
good ASR with potentially undetectable adversarials. Considering the size of the images in
this dataset (98
×
98 = 9604), more than the 90% of the pixels are modified in comparison
with the original test images (see
L0
distances in Table 3). The maximum perturbation,
calculated in
L
distance, is 0.30 for FGSM-PGD (which is expected as it is bounded
by a parameter of the adversarial methods), while CW has a smaller value, with 0.06.
These results also point in the same direction as the previous statements, supporting the
notion that the improvement in generalization also leads to worsening results for AEs that
are closer to the originals. In other words, as the models achieve better generalization,
robustness to the best AEs (which are very close to the original) decreases.
Table 3. Average distances for the adversarial sets without overfitting for STL-10.
Method L0L2L
CW 9216.00 ±0.00 0.97 ±1.42 0.06 ±0.06
FGSM 8969.59 ±387.89 26.41 ±1.39 0.30 ±0.00
PGD 9049.55 ±262.61 23.37 ±1.17 0.30 ±0.00
The dataset and trained models are also evaluated in terms of their adversarial robust-
ness, using the metrics explained in Section 2.3. For the CLEVER metric, Figure 5shows
the evolution of the adversarial robustness score, depending on the epoch considered. It
also shows how the robustness score increases in the sequence of epochs, but in a slight
amount for the CIFAR dataset. In the case of STL, the models would be robust to higher
Symmetry 2021,13, 817 8 of 13
perturbations, as the values are closer to 1. However, adversarials with further distance
metrics would be sufficient to fool these models.
(a) CIFAR-10 (b) STL-10
Figure 5. CLEVER metrics without overfitting.
Another attack-agnostic metric is evaluated for the trained snapshots. Loss sensibility
measures robustness against changes in the gradients. As shown in Figure 6, the effect is
increased when the model is more accurate in the test data. However, it reaches a peak
around 1.4, which is a relatively low value.
Finally, Empirical robustness shows the minimal theoretical perturbation needed to
fool the model (in a FGSM attack). As shown in Figure 6, the value remains constant for
the whole training process, with a low decreasing tendency.
(a) Loss sensibility (b) Empirical robustness for FGSM attack
Figure 6. Adversarial metrics in CIFAR-10 dataset without overfitting.
3.2. Overfitting
This set of experiments is performed using parameters that force the model to overfit
the input data. This is usually avoided when training a model for a specific task, and in
fact, several techniques have been developed to prevent that (data augmentation, dropout
layers, ...). With this regime, the generated model closely fits the training data, exhibiting
poor performance for unseen images.
In this case, the network architecture remains the same, but the learning rate is set at
0.0024, which is found to be a suitable value to induce the models to overfit. In general,
the overfitting susceptibility of a model depends on several factors (dataset, network
architecture, level of regularization applied, learning rate, number of epochs...). In principle,
letting training run for a large number of epochs should be a way to get to overfitting.
However, the computational cost of this is very high. Therefore we decided to run our
experiments with a fixed number of epochs and vary the learning rates. A high learning
Symmetry 2021,13, 817 9 of 13
rate helped us reach, in a reasonable time, a behaviour similar to what would be obtained
for a large number of epochs with a low learning rate. The performance for both datasets is
shown in Figure 7.
(a) CIFAR-10 (b) STL-10
Figure 7.
Training curve for the overfitting regime. Notice that beyond epoch 6 the validation
accuracy tends to decrease while training accuracy keeps increasing.
In comparison to Figure 3, there is a supposed improvement in the performance
for both CIFAR-10 and STL-10, since stabilized values of training accuracy around 0.9
and 1.0 are obtained. However, this is induced by overfitting. The models exhibit good
performance in the training set, and bad performance on the validation set. The accuracy
on the latter tends to decay from the fifth epoch, which is a clue that the models are in an
overfitting state. In comparison with the no-overfitting regime, the performance curves do
not follow the same tendency in training and validation.
The models trained in overfitting regime have been also tested against the CW, FGSM,
PGD attack methods. As in the previous experiment, they have been set up to perform
untargeted attacks using L2distance metric to generate the adversarial examples.
In the same way, the epoch with the best validation accuracy is selected to craft the
adversarial examples over the test set. In this case, this is usually at the 5th to 10th epoch.
Then, the architecture is tested in all the snapshots for the crafted adversarials. Figure 8
shows the test accuracy curve in comparison with the ASR of the adversarial attacks.
(a) CIFAR-10 (b) STL-10
Figure 8. Attack Success Rate vs Test Accuracy in overfitting regime.
Again, PGD and FGSM adversarial examples consistently fool the model during the
whole process. All the methods have greater ASR values for the epoch that was selected
to generate the adversarial examples, as expected. This is observed as greater standard
deviation in some specific initial epochs during the different runs. The CW method
Symmetry 2021,13, 817 10 of 13
is discovered as the most informative method for the purpose of this work. For both
datasets, the behaviour is similar. In contrast with the normal regime shown in
Figure 4,
the overfitting regime is able to stabilize the adversarial impact measured by the ASR
around 50–60% and does not show an incremental rate on the adversarial performance,
as the “normal” training did. This is a clear sight to support the benefit of an overfitting
regime since the effect of adversarial examples was much more deep in the normal regime.
Table 4shows the metrics extracted from the adversarial examples in the CIFAR-10
dataset. In this case, fewer pixels (as indicated by
L0
metric) and with less perturbation, are
needed to produce the adversarials. However, they have a smaller impact on the accuracy
of the models.
Table 4. Average distances for the adversarial sets with overfitting for CIFAR-10.
Method L0L2L
CW 1024.00 ±0.00 0.49 ±1.15 0.06 ±0.08
FGSM 1017.19 ±27.86 8.64 ±0.52 0.30 ±0.00
PGD 1019.08 ±20.05 7.62 ±0.45 0.30 ±0.00
The behaviour is the same for the STL-10 dataset, whose distance metrics with respect
to the original test set are shown in Table 5.
Table 5. Average distance metric for the adversarial dataset with overfitting for STL-10.
Method L0L2L
CW 9216.00 ±0.00 0.82 ±2.37 0.06 ±0.07
FGSM 8951.35 ±408.80 26.04 ±1.47 0.30 ±0.00
PGD 9021.16 ±289.93 23.32 ±1.29 0.30 ±0.00
Considering the size of the images in STL-10 (98
×
98 = 9604), more than the 90%
of the pixels are modified in comparison with the original test images. The maximum
perturbation is 0.3 for FGSM-PGD, while CW has a smaller value, with 0.06. For this regime,
the behaviour of the attack methods is similar to the non-overfitting setting. Perturbations
are much less detectable for the CW method than the other methods, no matter how the
model is trained.
For the CLEVER metric, Figure 9shows the values for this robustness score. As it is ob-
served, in the overfitting regime models are much stronger. In the case of
CIFAR-10
dataset,
values are similar than the ones shown in Figure 5, but a larger standard deviation points
out that the values are greater in more cases. The greatest impact is observed in
STL-10
dataset. In this case, CLEVER values are not in a range of 0.4–1.0 (as is the normal regime).
They increase exponentially to from 1.0 to 5.0 in the last epochs. In consequence, the metric
is supporting our proposal that this scenario is beneficial against adversarial examples.
Symmetry 2021,13, 817 11 of 13
(a) CIFAR-10 (b) STL-10
Figure 9. CLEVER metrics with overfitting.
Regarding the other metrics (see Figure 10) Loss sensibility follows the same pat-
tern with values that escalate from 0.6–1.4 to 2.0–9.0, measures that overfitting has been
produced indeed.
Finally, Empirical robustness shows how the minimal perturbation needed to fool the
model remains constant but with a tendency to increase its value throughout the epochs
(in contrast to the no-overfitting regime).
(a) Loss sensibility (b) Empirical robustness for FGSM attack
Figure 10. Adversarial metrics in CIFAR-10 dataset with overfitting.
4. Conclusions
In the first experiment, a training without overfitting is performed, so the model
generalizes well on the validation set and, supposedly, on data from the test set and similar
datasets. When a model is trained on this regime, we detect that adversarial robustness
drops with the training epochs at the same time that test accuracy increases. However,
the second experiment does not show this behaviour. With overfitting, the ASR remains
stable at lower values and decreases when the training epochs make performance decrease
on the test set.
With the experimentation performed in this work, opposite behaviours are observed
for the overfitting and no overfitting regimes. The former remains stable to adversarial
robustness, even reducing the possible radius of affecting perturbations. In consequence,
robustness to adversarial examples is shown to increase (as opposed to test set accuracy).
This is also evident from the metrics of adversarial robustness considered. In the latter
case, test accuracy increases but adversarial robustness decreases, for which an exploding
incidence of adversarials is confirmed, with a greater space of perturbations to be used for
adversarial attack methods.
Symmetry 2021,13, 817 12 of 13
Our results support the notion that the phenomenon of adversarial examples seem to
be linked to the well-known fitting-generalization trade-off.
Author Contributions:
Funding acquisition, O.D. and G.B.; investigation, A.P.; methodology, A.P.;
project administration, G.B.; supervision, O.D.; validation, G.B.; writing—original draft, A.P.;
writing—review and editing, O.D. and G.B. All authors have read and agreed to the published
version of the manuscript.
Funding:
This work was partially funded by projects TIN2017-82113-C2-2-R by the Spanish Ministry
of Economy and Business and SBPLY/17/180501/000543 by the Autonomous Government of Castilla-
La Mancha; as well as the Postgraduate Grant FPU17/04758 from the Spanish Ministry of Science,
Innovation, and Universities.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
AE Adversarial Example
ASR Adversarial Successful Rate
CIFAR Canadian Institute For Advanced Research
CLEVER Cross-Lipschitz Extreme Value for nEtwork Robustness
CW Carlini & Wagner
FGSM Fast Gradient Sign Method
MNIST Modified National Institute of Standards and Technology
PGD Projected Gradient Descent
STL Self-Taught Learning
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