matches or surpasses the adversarially trained networks with the same structure without any model
training.[
14
]. Inspired by Fu et al., our method obtained subnetworks with different network structures
and remaining ratios to promote the adversarial transferable diversity for the DDF. By weakening the
transferability between ensemble states, we improve the initiative of the DDF against the adversary.
2 Method
In the framework of dynamic defense, we represent the dynamic stochastic ensemble with adversarial
robust lottery ticket subnetworks. [
14
] proved the poor adversarial transferability between the scratch
tickets under a single structure. Drawing inspiration from prior works, we further explore the
adversarial transferable diversity from the different fundamental structures and remaining ratios.
2.1 The Dynamic Defense Framework and Adversarial Transferable Diversity
The DDF is a randomized defense strategy to protect ensemble gradient information, and the essential
requirements for it are randomness and diversity to promote the ensemble’s adversarial robustness[
12
].
It presents a model ensemble defense method with randomized network parameter distribution
specialty, which causes an unknowable act of the defender. The output of dynamic stochastic
ensemble model fens containing Inumber of models is defined as follows:
fens(x, θ) =
I
X
i=1
f(x, θ)(1)
The randomness is achieved by transferring the ensemble states with ensemble variables
θ
. The
DDF demands the construction of diversified ensemble statuses with a heterogeneous model library.
Relevant studies highlight that diverse network structure plays a crucial role in ensemble defense[
15
].
In our solution, we evaluated the heterogeneousness and diversity between ensemble subnetworks by
the poor adversarial transferability of the attacks.
2.2 Adversarial Robust Lottery Subnetwork
For purpose of testifying the multi-sparsity adversarial robust lottery subnetworks can achieve better
adversarial transferable diversity under different network structures. We picked four representative
network structures, ResNet18, ResNet34, WideResNet32, and WideResNet38[
16
,
17
], as the basic
architecture of our experiments and gained the sparse lottery ticket from original dense networks.
Following [
14
], we applied adversarial training to gain robustness of our subnetworks during pruning.
It can be expressed as a min-max problem as Eq.2.
arg min
λX
i
max
kδk≤εl(f(ˆ
λω, xi+δ), yi)s.t. kˆ
λk0≤k(2)
Where lpresents the loss function, fis a randomly initialized network with random weights, and
δ
is the perturbation with maximum value
ε
. In order to satisfy the sparsity of the subnetworks, we
set a learnable weight
λ
and a binary weight
ˆ
λ∈ {0,1}
that correspond to its dimensions[
18
,
19
].
ˆ
λ
is meant to activate a small number of primary weights
ω
. With the primary network parameters
weighted by λ∈(0,1),fcan be effectively trained by small perturbations added to the input xi.
2.3 Dynamical Ensemble for The Lottery Subnetworks
Through our method, we obtained fourty subnetworks with different basic structures and sparsity.
Based on the robust lottery subnetwork library, we define the randomized ensemble attribute parameter
θ=θ(α, n, s), which determines the ensemble states. It can be achieved in the following steps:
(A)Construct a robust lottery subnetworks library with adversarial transferable diversity, including
forty sparse subnetworks. Each of ResNet18/ResNet34/WideResNet32/WideResNet38 owns ten.
(B)Set the range for
α
and s. We brought four basic structures into the selection rather than
the entire library, increasing the possibility of including more structures. It realized by
α=
2