List of English Publication

@inproceedings{nokey,

title = {Taking Advantage of Rational Canonical Form for Faster Ring-LWE based Encrypted Controller with Recursive Multiplication},

author = {Donghyeon Song and Yeongjun Jang and Joowon Lee and Junsoo Kim },

url = {https://ieeexplore.ieee.org/document/11312706},

doi = {10.1109/CDC57313.2025.11312706},

isbn = {979-8-3315-2627-6},

year  = {2025},

date = {2025-12-12},

urldate = {2025-12-12},

booktitle = {2025 IEEE 64th Conference on Decision and Control (CDC)},

publisher = {IEEE Control Systems Society},

address = {Rio de Janeiro, Brazil},

abstract = {This paper aims to provide an efficient implementation of encrypted linear dynamic controllers that perform recursive multiplications on a Ring-Learning With Errors (RingLWE) based cryptosystem. By adopting a system-theoretical approach, we significantly reduce both time and space complexities, particularly the number of homomorphic operations required for recursive multiplications. Rather than encrypting the entire state matrix of a given controller, the state matrix is transformed into its rational canonical form, whose sparse and circulant structure enables that encryption and computation are required only on its nontrivial columns. Furthermore, we propose a novel method to “pack” each of the input and the output matrices into a single polynomial, thereby reducing the number of homomorphic operations. Simulation results demonstrate that the proposed design enables a remarkably fast implementation of encrypted controllers.},

keywords = {Controller encryption, Homomorphic encryption, Rational Canonical Form},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{nokey,

title = {A polynomial-based QCQP solver for encrypted optimization},

author = {Sebastian Schlor and Andrea Iannelli and Junsoo Kim and Hyungbo Shim and Frank Allgöwer},

url = {https://ieeexplore.ieee.org/document/11312321},

doi = {10.1109/CDC57313.2025.11312321},

isbn = {979-8-3315-2627-6},

year  = {2025},

date = {2025-12-12},

urldate = {2025-12-12},

booktitle = {2025 IEEE 64th Conference on Decision and Control (CDC)},

publisher = {IEEE Control Systems Society},

address = {Rio de Janeiro, Brazil},

abstract = {In this paper, we present a novel method for solving a class of quadratically constrained quadratic optimization problems using only additions and multiplications. This approach enables solving constrained optimization problems on private data since the operations involved are compatible with the capabilities of homomorphic encryption schemes. To solve the constrained optimization problem, a sequence of polynomial penalty functions of increasing degree is introduced, which are sufficiently steep at the boundary of the feasible set. Adding the penalty function to the original cost function creates a sequence of unconstrained optimization problems whose minimizer always lies in the admissible set and converges to the minimizer of the constrained problem. A gradient descent method is used to generate a sequence of iterates associated with these problems. For the algorithm, it is shown that the iterate converges to a minimizer of the original problem, and the feasible set is positively invariant under the iteration. Finally, the method is demonstrated on an illustrative cryptographic problem, finding the smaller value of two numbers, and the encrypted implementability is discussed.},

keywords = {Homomorphic encryption, Optimization},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{nokey,

title = {Conversion of Controllers to Have Integer State Matrix for Encrypted Control: Non-Minimal Order Approach},

author = {Joowon Lee and Donggil Lee and Seungbeom Lee and Junsoo Kim and Hyungbo Shim},

url = {https://ieeexplore.ieee.org/abstract/document/10383200},

doi = {10.1109/CDC49753.2023.10383200},

isbn = {979-8-3503-0124-3},

year  = {2023},

date = {2023-12-14},

urldate = {2023-12-14},

booktitle = {2023 62nd IEEE Conference on Decision and Control (CDC)},

pages = {5091-5096},

publisher = {IEEE Control Systems Society},

address = {Marina Bay Sands, Singapore},

abstract = {To implement an encrypted dynamic controller based on homomorphic encryption that operates for an infinite time horizon, it is essential for every component of the controller's state matrix to be an integer. In this paper, we tackle the challenge of converting a pre-designed controller into a new one with an integer state matrix while preserving its control performance in the closed-loop system. This enables encrypted dynamic systems to be realized without re-encryption and approximation of control parameters, compared to the previous results. To achieve this, we propose two approaches and provide sufficient conditions on the design parameter for each. The first approach is to design the new controller as an estimator of the original closed-loop system, and the conditions on the estimator gain are derived. Our second approach is to formulate a problem of finding certain polynomials, whose solution leads to the design of the new controller. In a special case when the numerator of the plant transfer function is a constant, we provide a constructive method to obtain such solution.},

keywords = {Controller encryption, Homomorphic encryption, Linear Systems},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{nokey,

title = {Method for Running Dynamic Systems Over Encrypted Data for Infinite Time Horizon without Bootstrapping and Re-Encryption},

author = {Junsoo Kim and Hyungbo Shim and Henrik Sandberg and Karl H. Johansson},

url = {https://ieeexplore.ieee.org/abstract/document/9682828},

doi = {10.1109/CDC45484.2021.9682828},

isbn = {978-1-6654-3659-5},

year  = {2021},

date = {2021-12-13},

urldate = {2021-12-13},

booktitle = {Proc. of 2021 IEEE 60th Conference on Decision and Control},

pages = {5614-5619},

publisher = {IEEE},

address = {Austin, Texas, USA},

abstract = {In this paper, we propose a method for dynamic systems to operate over homomorphically encrypted data for an infinite time horizon, where we do not make use of reset, re-encryption, or bootstrapping for the encrypted messages. The given system is first decomposed into the stable part and the anti-stable part. Then, the stable part is approximated to have finite impulse response, and by a novel conversion scheme, the eigenvalues of the state matrix of the anti-stable part are approximated to algebraic integers. This allows that the given system can be implemented to operate for an infinite time horizon using only addition and multiplication over encrypted data, without re-encrypting any portion of data. The performance error caused by the approximation and quantization can be made arbitrarily small, with appropriate choice of parameters.},

keywords = {Controller encryption, Homomorphic encryption},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{KimShimHan20,

title = {Design Procedure for Dynamic Controllers based on LWE-based Homomorphic Encryption to Operate for Infinite Time Horizon},

author = {Junsoo Kim and Hyungbo Shim and Kyoohyung Han},

doi = {https://doi.org/10.1109/CDC42340.2020.9304509},

isbn = {978-1-7281-7447-1},

year  = {2020},

date = {2020-12-18},

urldate = {2020-12-18},

booktitle = {Proc. of 2020 IEEE 59th Conference on Decision and Control},

pages = {5463-5468},

publisher = {IEEE},

address = {Jeju, Korea},

abstract = {The design of encrypted controllers, which perform control operation directly over encrypted signals via homomorphic cryptosystems, should consider both security of the control data and performance of the controller. Considering the use of Learning With Errors (LWE) based cryptosystem, in this paper, we present a design procedure for encrypted linear dynamic controllers. Providing a guideline for choosing the parameters of the cryptosystem as well as quantization, the procedure guarantees both the desired level of security and performance. Receiving the encrypted signals of the plant input as well as the output, the proposed controller is able to perform the dynamic operation over encrypted data for infinite time horizon, without the use of decryption or reset of the state. Thanks to additively and multiplicatively homomorphic property of the LWE-based cryptosystem, information of both control signals and gain matrices is protected by encryption.},

keywords = {Controller encryption, Homomorphic encryption},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{LeeKimShim20b,

title = {Distributed Aggregation Over Homomorphically Encrypted Data under Switching Networks},

author = {Donggil Lee and Junsoo Kim and Hyungbo Shim},

doi = {https://doi.org/10.1109/CDC42340.2020.9304350},

isbn = {978-1-7281-7447-1},

year  = {2020},

date = {2020-12-18},

urldate = {2020-12-18},

booktitle = {Proc. of 2020 IEEE 59th Conference on Decision and Control},

pages = {5495-5500},

publisher = {IEEE},

address = {Jeju, Korea},

abstract = {This paper presents a distributed algorithm for additive aggregation over encrypted data. Our algorithm allows networked systems to compute the sum of all individual data without revealing each node's private contribution. To conceal the private data, the local computations on each node and the communications with neighbor nodes are conducted by utilizing the additively homomorphic cryptosystem. As a consequence, the leader node which has the secret key of the cryptosystem recovers the correct outcome after decryption in a finite time. Moreover, the leader cannot learn the private data of the other nodes. Furthermore, the proposed algorithm can operate on encrypted data for an infinite time horizon even when the network varies intermittently with time.},

keywords = {Distributed aggregation, Homomorphic encryption},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{LeeKimShim20,

title = {Zero-Dynamics Attack on Homomorphically Encrypted Control System},

author = {Joowon Lee and Junsoo Kim and Hyungbo Shim},

doi = {https://doi.org/10.23919/ICCAS50221.2020.9268374},

isbn = {978-89-93215-20-5},

year  = {2020},

date = {2020-10-14},

urldate = {2020-10-14},

booktitle = {Proc. of 20th International Conference on Control, Automation and Systems (ICCAS)},

pages = {385-390},

publisher = {IEEE},

address = {Busan, Korea},

abstract = {Against recent cyber-attack strategies on networked control systems which commonly utilize information of control data, the notion of encrypted control system has been introduced, to protect private data in the network layer by encryption. However, even though the adversary cannot learn the information from the encrypted control signals or parameters, it is known that their values can be manipulated by the adversaries, based on homomorphic property of the cryptosystem. In this paper, we demonstrate that the injection of zero-dynamics attack is possible even for encrypted control systems. By injecting an attack signal, generated with knowledge of the plant model, directly on the encrypted controller output being transmitted to the actuator, we show that it disrupts the plant state while it is undetectable from the input and output of the controller. Simulation results are presented to demonstrate the effectiveness of the proposed attack.},

keywords = {Controller encryption, Cyber-physical systems, Homomorphic encryption, Zero-dynamics},

pubstate = {published},

tppubtype = {inproceedings}

}

@inproceedings{KimLeeShimCheonKimKimSong16,

title = {Encrypting Controller using Fully Homomorphic Encryption for Security of Cyber-Physical Systems},

author = {Junsoo Kim and Chanhwa Lee and Hyungbo Shim and Jung Hee Cheon and Andrey Kim and Miran Kim and Yongsoo Song},

doi = {10.1016/j.ifacol.2016.10.392},

year  = {2016},

date = {2016-09-08},

booktitle = {Proc. of 6th IFAC Workshop on Distributed Estimation and Control in Networked Systems},

volume = {49},

number = {22},

pages = {175-180},

address = {Tokyo, Japan},

abstract = {In order to enhance security of cyber-physical systems, it is important to protect the signals from sensors to the controller, and from the controller to the actuator, because the attackers often steal and compromise those signals. One immediate solution could be encrypting the signals, but in order to perform computation in the controller, they should be decrypted before computation and encrypted again after computation. For this, the controller keeps the secret key, which in turn increases vulnerability from the attacker. In this paper, we introduce the fully homomorphic encryption (FHE), which is an advanced cryptography that has enabled arithmetic operations directly on the encrypted variables without decryption. However, this also introduces several new issues that have not been studied for conventional controllers. Most of all, an encrypted variable has a finite lifespan, which decreases as an arithmetic operation is performed on it. Our solution is to run multiple controllers, and orchestrate them systematically. Also, in order to slow down the decrease of the lifespan, a tree-based computation of sequential matrix multiplication is introduced. We finally demonstrate the effectiveness of the proposed algorithm with quadruple water tank example.},

keywords = {Controller encryption, Cyber-physical system, Homomorphic encryption, Security},

pubstate = {published},

tppubtype = {inproceedings}

}