ArticlePDF Available

# On the Relationship between Generalization and Robustness to Adversarial Examples

Authors:

## Abstract and Figures

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.
Content may be subject to copyright.
symmetry
S
S
Article
On the Relationship between Generalization and Robustness
Anibal Pedraza * , Oscar Deniz and Gloria Bueno


Citation: Pedraza, A.; Deniz, O.;
Bueno, G. On the Relationship
between Generalization and
Symmetry 2021,13, 817. https://
doi.org/10.3390/sym13050817
Accepted: 30 April 2021
Published: 7 May 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional afﬁl-
iations.
conditions of the Creative Commons
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:
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:
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.
left
) to (
right
): original image (classiﬁed as “whistle”),
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
Symmetry 2021,13, 817 12 of 13
Our results support the notion that the phenomenon of adversarial examples seem to
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:
CLEVER Cross-Lipschitz Extreme Value for nEtwork Robustness
CW Carlini & Wagner
MNIST Modiﬁed National Institute of Standards and Technology
STL Self-Taught Learning
References
1. Szegedy, C.; Zaremba, W.; Sutskever, I.; Bruna, J.; Erhan, D.; Goodfellow, I.; Fergus, R. Intriguing properties of neural networks.
arXiv 2013, arXiv:1312.6199.
2. Carlini, N.; Wagner, D. Towards evaluating the robustness of neural networks. In Proceedings of the 2017 IEEE Symposium on
Security and Privacy (SP), San Jose, CA, USA, 22–24 May 2017; pp. 39–57.
3.
Chen, P.Y.; Sharma, Y.; Zhang, H.; Yi, J.; Hsieh, C.J. EAD: Elastic-net attacks to deep neural networks via adversarial examples.
In Proceedings of the Thirty-Second AAAI Conference on Artiﬁcial Intelligence, New Orleans, LA, USA, 2–7 February 2018.
4.
Madry, A.; Makelov, A.; Schmidt, L.; Tsipras, D.; Vladu, A. Towards deep learning models resistant to adversarial attacks. arXiv
2017, arXiv:1706.06083.
5.
Tramèr, F.; Kurakin, A.; Papernot, N.; Goodfellow, I.; Boneh, D.; McDaniel, P. Ensemble adversarial training: Attacks and defenses.
arXiv 2017, arXiv:1705.07204.
6.
Song, Y.; Kim, T.; Nowozin, S.; Ermon, S.; Kushman, N. Pixeldefend: Leveraging generative models to understand and defend
against adversarial examples. arXiv 2017, arXiv:1710.10766.
7.
Papernot, N.; McDaniel, P.; Wu, X.; Jha, S.; Swami, A. Distillation as a defense to adversarial perturbations against deep neural
networks. In Proceedings of the 2016 IEEE Symposium on Security and Privacy (SP), San Jose, CA, USA, 23–25 May 2016;
pp. 582–597.
8.
Arpit, D.; Jastrzebski, S.; Ballas, N.; Krueger, D.; Bengio, E.; Kanwal, M.S.; Maharaj, T.; Fischer, A.; Courville, A.; Bengio, Y.; et al.
A closer look at memorization in deep networks. In Proceedings of the 34th International Conference on Machine Learning,
Sydney, Australia, 6–11 August 2017; Volume 70, pp. 233–242.
9. Goodfellow, I.J.; Shlens, J.; Szegedy, C. Explaining and harnessing adversarial examples. arXiv 2014, arXiv:1412.6572.
10. Galloway, A.; Taylor, G.W.; Moussa, M. Predicting adversarial examples with high conﬁdence. arXiv 2018, arXiv:1802.04457.
11.
Kubo, Y.; Trappenberg, T. Mitigating Overﬁtting Using Regularization to Defend Networks Against Adversarial Examples.
In Proceedings of the Canadian Conference on Artiﬁcial Intelligence, Kingston, ON, Canada, 28–31 May 2019; Springer:
Berlin/Heidelberg, Germany, 2019; pp. 400–405.
12.
Ilyas, A.; Santurkar, S.; Tsipras, D.; Engstrom, L.; Tran, B.; Madry, A. Adversarial examples are not bugs, they are features. arXiv
2019, arXiv:1905.02175.
13.
Deniz, O.; Vallez, N.; Bueno, G. Adversarial Examples are a Manifestation of the Fitting-Generalization Trade-off.
In Proceedings of the
International Work-Conference on Artiﬁcial Neural Networks, Gran Canaria, Spain,
12–14 June 2019;
Springer: Berlin/Heidelberg, Germany, 2019; pp. 569–580.
Symmetry 2021,13, 817 13 of 13
14.
Su, D.; Zhang, H.; Chen, H.; Yi, J.; Chen, P.Y.; Gao, Y. Is Robustness the Cost of Accuracy?—A Comprehensive Study on the
Robustness of 18 Deep Image Classiﬁcation Models. In Proceedings of the European Conference on Computer Vision (ECCV),
Munich, Germany, 8–14 September 2018; pp. 631–648.
15.
Bottou, L.; Cortes, C.; Denker, J.S.; Drucker, H.; Guyon, I.; Jackel, L.D.; Le Cun, Y.; Muller, U.A.; Säckinger, E.; Simard, P.; et al.
Comparison of classiﬁer methods: A case study in handwritten digit recognition. In Proceedings of the 12th IAPR International
Conference on Pattern Recognition, Conference B: Computer Vision & Image Processing, Jerusalem, Israel, 9–13 October 1994;
Volume 2, pp. 77–82.
16.
Yadav, C.; Bottou, L. Cold Case: The Lost MNIST Digits. In Advances in Neural Information Processing Systems (NIPS) 32;
Curran Associates, Inc.: Red Hook, NY, USA, 2019.
17.
Xiao, H.; Rasul, K.; Vollgraf, R. Fashion-mnist: A novel image dataset for benchmarking machine learning algorithms. arXiv
2017
,
arXiv:1708.07747.
18.
Coates, A.; Ng, A.; Lee, H. An analysis of single-layer networks in unsupervised feature learning. In Proceedings of the Fourteenth
International Conference on Artiﬁcial Intelligence and Statistics, Ft. Lauderdale, FL, USA, 11–13 April 2011; pp. 215–223.
19.
Deng, J.; Dong, W.; Socher, R.; Li, L.J.; Li, K.; Li, F.-F. Imagenet: A large-scale hierarchical image database.
In Proceedings of the
2009 IEEE Conference on Computer Vision and Pattern Recognition, Miami Beach, FL, USA, 20–25 June 2009; pp. 248–255.
20.
Krizhevsky, A. Learning Multiple Layers of Features from Tiny Images; Technical Report; University of Toronto: Toronto, ON, Canada,
2009.
21.
Pedraza, A.; Deniz, O.; Bueno, G. Approaching Adversarial Example Classiﬁcation with Chaos Theory. Entropy
2020
,22, 1201.
[CrossRef] [PubMed]
22. Kurakin, A.; Goodfellow, I.; Bengio, S. Adversarial examples in the physical world. arXiv 2016, arXiv:1607.02533.
23.
Moosavi-Dezfooli, S.M.; Fawzi, A.; Frossard, P. Deepfool: A simple and accurate method to fool deep neural networks.
In Proceedings of the
IEEE Conference on Computer Vision and Pattern Recognition, Las Vegas, NV, USA, 26 June–1 July 2016;
pp. 2574–2582.
24.
Weng, T.W.; Zhang, H.; Chen, P.Y.; Yi, J.; Su, D.; Gao, Y.; Hsieh, C.J.; Daniel, L. Evaluating the robustness of neural networks:
An extreme value theory approach. arXiv 2018, arXiv:1801.10578.
25.
LeCun, Y.; Bottou, L.; Bengio, Y.; Haffner, P. Gradient-based learning applied to document recognition. Proc. IEEE
1998
,
86, 2278–2324. [CrossRef]
26.
Papernot, N.; McDaniel, P.; Goodfellow, I. Transferability in machine learning: From phenomena to black-box attacks using
... Obviously, the existing regression models have two problems: one is that the temperature information is not clear and sufficient; second, the timing modeling performance of fitting tools still needs to be improved. In this paper, prior knowledge will be used to extract the temperature features and reduce the data dimension; deep learning is used as a fitting tool to strengthen the performance in time series regression, to enhance the robustness of the established model [11]. This paper will verify the advantages of deep learning technology and explore the optimal data cost required to establish the model. ...
Article
Full-text available
A cable-stayed bridge is a typical symmetrical structure, and symmetry affects the deformation characteristics of such bridges. The main girder of a cable-stayed bridge will produce obvious deflection under the inducement of temperature. The regression model of temperature-induced deflection is hoped to provide a comparison value for bridge evaluation. Based on the temperature and deflection data obtained by the health monitoring system of a bridge, establishing the correlation model between temperature and temperature-induced deflection is meaningful. It is difficult to complete a high-quality model only by the girder temperature. The temperature features based on prior knowledge from the mechanical mechanism are used as the input information in this paper. At the same time, to strengthen the nonlinear ability of the model, this paper selects an independent recurrent neural network (IndRNN) for modeling. The deep learning neural network is compared with machine learning neural networks to prove the advancement of deep learning. When only the average temperature of the main girder is input, the calculation accuracy is not high regardless of whether the deep learning network or the machine learning network is used. When the temperature information extracted by the prior knowledge is input, the average error of IndRNN model is only 2.53%, less than those of BPNN model and traditional RNN. Combining knowledge with deep learning is undoubtedly the best modeling scheme. The deep learning model can provide a comparison value of bridge deformation for bridge management.
Article
Background and objective: Training a deep convolutional neural network (CNN) for automatic image classification requires a large database with images of labeled samples. However, in some applications such as biology and medicine only a few experts can correctly categorize each sample. Experts are able to identify small changes in shape and texture which go unnoticed by untrained people, as well as distinguish between objects in the same class that present drastically different shapes and textures. This means that currently available databases are too small and not suitable to train deep learning models from scratch. To deal with this problem, data augmentation techniques are commonly used to increase the dataset size. However, typical data augmentation methods introduce artifacts or apply distortions to the original image, which instead of creating new realistic samples, obtain basic spatial variations of the original ones. Methods: We propose a novel data augmentation procedure which generates new realistic samples, by combining two samples that belong to the same class. Although the idea behind the method described in this paper is to mimic the variations that diatoms experience in different stages of their life cycle, it has also been demonstrated in glomeruli and pollen identification problems. This new data augmentation procedure is based on morphing and image registration methods that perform diffeomorphic transformations. Results: The proposed technique achieves an increase in accuracy over existing techniques of 0.47%, 1.47%, and 0.23% for diatom, glomeruli and pollen problems respectively. Conclusions: For the Diatom dataset, the method is able to simulate the shape changes in different diatom life cycle stages, and thus, images generated resemble newly acquired samples with intermediate shapes. In fact, the other methods compared obtained worse results than those which were not using data augmentation. For the Glomeruli dataset, the method is able to add new samples with different shapes and degrees of sclerosis (through different textures). This is the case where our proposed DA method is more beneficial, when objects highly differ in both shape and texture. Finally, for the Pollen dataset, since there are only small variations between samples in a few classes and this dataset has other features such as noise which are likely to benefit other existing DA techniques, the method still shows an improvement of the results.
Article
Learning discriminative features with adversarial behaviors can be extremely challenging to build a robust learning model. This is partly evidenced by the difficulties in training robust maximum-margin models (e.g., ArcFace and CosFace) that cannot discriminate decision boundaries between perturbed samples perfectly. One potential approach is to design a loss function that achieves robust generalization by learning high-density features in the latent space to discriminate between adversarial and legitimate samples effectively. Therefore, we propose an Ensemble Maximum-Margin Softmax (EMMS) method to construct a robust generalization that yields reliable models. Specifically, EMMS is designed to address the limitation in maximum-margin methods and induce high-density discriminative features for clean and adversarial settings. The empirical experiments using the CIFAR and SVHN datasets show that EMMS is more robust in terms of accuracy and error rates than other peer techniques. The outcomes have revealed that EMMS could improve the robustness of the model compared with ArcFace and CosFace under various types of attacks.
Article
Full-text available
Adversarial examples are one of the most intriguing topics in modern deep learning. Imperceptible perturbations to the input can fool robust models. In relation to this problem, attack and defense methods are being developed almost on a daily basis. In parallel, efforts are being made to simply pointing out when an input image is an adversarial example. This can help prevent potential issues, as the failure cases are easily recognizable by humans. The proposal in this work is to study how chaos theory methods can help distinguish adversarial examples from regular images. Our work is based on the assumption that deep networks behave as chaotic systems, and adversarial examples are the main manifestation of it (in the sense that a slight input variation produces a totally different output). In our experiments, we show that the Lyapunov exponents (an established measure of chaoticity), which have been recently proposed for classification of adversarial examples, are not robust to image processing transformations that alter image entropy. Furthermore, we show that entropy can complement Lyapunov exponents in such a way that the discriminating power is significantly enhanced. The proposed method achieves 65% to 100% accuracy detecting adversarials with a wide range of attacks (for example: CW, PGD, Spatial, HopSkip) for the MNIST dataset, with similar results when entropy-changing image processing methods (such as Equalization, Speckle and Gaussian noise) are applied. This is also corroborated with two other datasets, Fashion-MNIST and CIFAR 19. These results indicate that classifiers can enhance their robustness against the adversarial phenomenon, being applied in a wide variety of conditions that potentially matches real world cases and also other threatening scenarios.
Article
Full-text available
It has been suggested that adversarial examples cause deep learning models to make incorrect predictions with high confidence. In this work, we take the opposite stance: an overly confident model is more likely to be vulnerable to adversarial examples. This work is one of the most proactive approaches taken to date, as we link robustness with non-calibrated model confidence on noisy images, providing a data-augmentation-free path forward. The adversarial examples phenomenon is most easily explained by the trend of increasing non-regularized model capacity, while the diversity and number of samples in common datasets has remained flat. Test accuracy has incorrectly been associated with true generalization performance, ignoring that training and test splits are often extremely similar in terms of the overall representation space. The transferability property of adversarial examples was previously used as evidence against overfitting arguments, a perceived random effect, but overfitting is not always random.
Article
Full-text available
Adversarial perturbations of normal images are usually imperceptible to humans, but they can seriously confuse state-of-the-art machine learning models. What makes them so special in the eyes of image classifiers? In this paper, we show empirically that adversarial examples mainly lie in the low probability regions of the training distribution, regardless of attack types and targeted models. Using statistical hypothesis testing, we find that modern neural density models are surprisingly good at detecting imperceptible image perturbations. Based on this discovery, we devised PixelDefend, a new approach that purifies a maliciously perturbed image by moving it back towards the distribution seen in the training data. The purified image is then run through an unmodified classifier, making our method agnostic to both the classifier and the attacking method. As a result, PixelDefend can be used to protect already deployed models and be combined with other model-specific defenses. Experiments show that our method greatly improves resilience across a wide variety of state-of-the-art attacking methods, increasing accuracy on the strongest attack from 63% to 84% for Fashion MNIST and from 32% to 70% for CIFAR-10.
Article
Full-text available
We present Fashion-MNIST, a new dataset comprising of 28x28 grayscale images of 70,000 fashion products from 10 categories, with 7,000 images per category. The training set has 60,000 images and the test set has 10,000 images. Fashion-MNIST is intended to serve as a direct drop-in replacement for the original MNIST dataset for benchmarking machine learning algorithms, as it shares the same image size, data format and the structure of training and testing splits. The dataset is freely available at https://github.com/zalandoresearch/fashion-mnist.
Article
The robustness of neural networks to adversarial examples has received great attention due to security implications. Despite various attack approaches to crafting visually imperceptible adversarial examples, little has been developed towards a comprehensive measure of robustness. In this paper, we provide a theoretical justification for converting robustness analysis into a local Lipschitz constant estimation problem, and propose to use the Extreme Value Theory for efficient evaluation. Our analysis yields a novel robustness metric called CLEVER, which is short for Cross Lipschitz Extreme Value for nEtwork Robustness. The proposed CLEVER score is attack-agnostic and computationally feasible for large neural networks. Experimental results on various networks, including ResNet, Inception-v3 and MobileNet, show that (i) CLEVER is aligned with the robustness indication measured by the $\ell_2$ and $\ell_\infty$ norms of adversarial examples from powerful attacks, and (ii) defended networks using defensive distillation or bounded ReLU indeed achieve better CLEVER scores. To the best of our knowledge, CLEVER is the first attack-independent robustness metric that can be applied to any neural network classifier.
Conference Paper
A great deal of research has focused on algorithms for learning features from un- labeled data. Indeed, much progress has been made on benchmark datasets like NORB and CIFAR by employing increasingly complex unsupervised learning al- gorithms and deep models. In this paper, however, we show that several very sim- ple factors, such as the number of hidden nodes in the model, may be as important to achieving high performance as the choice of learning algorithm or the depth of the model. Specifically, we will apply several off-the-shelf feature learning al- gorithms (sparse auto-encoders, sparse RBMs and K-means clustering, Gaussian mixtures) to NORB and CIFAR datasets using only single-layer networks. We then present a detailed analysis of the effect of changes in the model setup: the receptive field size, number of hidden nodes (features), the step-size (stride) be- tween extracted features, and the effect of whitening. Our results show that large numbers of hidden nodes and dense feature extraction are as critical to achieving high performance as the choice of algorithm itselfso critical, in fact, that when these parameters are pushed to their limits, we are able to achieve state-of-the- art performance on both CIFAR and NORB using only a single layer of features. More surprisingly, our best performance is based on K-means clustering, which is extremely fast, has no hyper-parameters to tune beyond the model structure it- self, and is very easy implement. Despite the simplicity of our system, we achieve performance beyond all previously published results on the CIFAR-10 and NORB datasets (79.6% and 97.0% accuracy respectively).
Conference Paper
Several machine learning models, including neural networks, consistently mis- classify adversarial examples—inputs formed by applying small but intentionally worst-case perturbations to examples from the dataset, such that the perturbed in- put results in the model outputting an incorrect answer with high confidence. Early attempts at explaining this phenomenon focused on nonlinearity and overfitting. We argue instead that the primary cause of neural networks' vulnerability to ad- versarial perturbation is their linear nature. This explanation is supported by new quantitative results while giving the first explanation of the most intriguing fact about them: their generalization across architectures and training sets. Moreover, this view yields a simple and fast method of generating adversarial examples. Us- ing this approach to provide examples for adversarial training, we reduce the test set error of a maxout network on the MNIST dataset.
Article
Recent work has demonstrated that neural networks are vulnerable to adversarial examples, i.e., inputs that are almost indistinguishable from natural data and yet classified incorrectly by the network. In fact, some of the latest findings suggest that the existence of adversarial attacks may be an inherent weakness of deep learning models. To address this problem, we study the adversarial robustness of neural networks through the lens of robust optimization. This approach provides us with a broad and unifying view on much of the prior work on this topic. Its principled nature also enables us to identify methods for both training and attacking neural networks that are reliable and, in a certain sense, universal. In particular, they specify a concrete, general guarantee to provide. These methods let us train networks with significantly improved resistance to a wide range of adversarial attacks. This suggests that adversarially resistant deep learning models might be within our reach after all.