arXiv subjects

Kiminao Kogiso

Publications and source records attributed to Kiminao Kogiso.

17 recordsLinked to original sources

arXiv2026

Analysis of Dynamic-Key LWE-Based Encrypted Control Systems for Asymptotic Stability and Numerical Safety

This study analyzes dynamic-key Learning-with-Errors (LWE)-based encrypted state-feedback control systems with time-varying encoders and decoders. Using a Lyapunov-based approach, we derive conditions on the time-varying encoder and decoder parameters that ensure both asymptotic stability and numerical safety by preventing overflow. The validity of the derived conditions is confirmed through numerical examples.

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arXiv2026

Dynamic-Key Post-Quantum Encrypted Control Against System Identification Attacks

This study proposes post-quantum encrypted control systems based on dynamic-key Learning with Errors (LWE) encryption schemes. The proposed method develops update maps that simultaneously update the private key and ciphertexts within the LWE framework, enabling dynamic-key encrypted control resistant to system identification attacks. The growth of errors induced by homomorphic operations is analyzed, and sufficient parameter conditions guaranteeing correct decryption at each control step are clarified. Furthermore, a design procedure for the encrypted control systems is presented based on security metrics such as sample-identifying complexity and deciphering time. A numerical example demonstrates that the proposed control systems achieve secure control against the considered system identification attack.

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arXiv2025

Quantization and Security Parameter Design for Overflow-Free Confidential FRIT

This study proposes a systematic design procedure for determining the quantization gain and the security parameter in the Confidential Fictitious Reference Iterative Tuning (CFRIT), enabling overflow-free and accuracy-guaranteed encrypted controller tuning. Within an encrypted data-driven gain tuning, the range of quantization errors induced during the encoding (encryption) process can be estimated from operational data. Based on this insight, explicit analytical conditions on the quantization gain and the security parameter are derived to prevent overflow in computing over encrypted data. Furthermore, the analysis reveals a quantitative relationship between quantization-induced errors and the deviation between the gains obtained by CFRIT and non-confidential Fictitious Reference Iterative Tuning (FRIT), clarifying how parameter choice affects tuning accuracy. A numerical example verifies the proposed procedure by demonstrating that the designed parameters achieve accurate encrypted tuning within a prescribed tolerance while preventing overflow. In addition, the admissible region of parameter combinations is visualized to examine the characteristics of feasible and infeasible regions, providing practical insights into parameter design for encrypted data-driven control.

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arXiv2025

Confidential FRIT via Homomorphic Encryption

Edge computing alleviates the computation burden of data-driven control in cyber-physical systems (CPSs) by offloading complex processing to edge servers. However, the increasing sophistication of cyberattacks underscores the need for security measures that go beyond conventional IT protections and address the unique vulnerabilities of CPSs. This study proposes a confidential data-driven gain-tuning framework using homomorphic encryption, such as ElGamal and CKKS encryption schemes, to enhance cybersecurity in gain-tuning processes outsourced to external servers. The idea for realizing confidential FRIT is to replace the matrix inversion operation with a vector summation form, allowing homomorphic operations to be applied. Numerical examples under 128-bit security confirm performance comparable to conventional methods while providing guidelines for selecting suitable encryption schemes for secure CPS.

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arXiv2025

Malleability-Resistant Encrypted Control System with Disturbance Compensation and Real-Time Attack Detection

This study proposes an encrypted PID control system with a disturbance observer (DOB) using a keyed-homomorphic encryption (KHE) scheme, aiming to achieve control performance while providing resistance to malleability-based attacks. The controller integrates a DOB with a PID structure to compensate for modeling uncertainties by estimating and canceling external disturbances. To enhance security, the system is designed to output error symbols when ciphertexts are falsified during decryption or evaluation, enabling real-time detection of malleability-based signal or parameter falsification. To validate the proposed method, we conduct stage positioning control experiments and attack detection tests using an industrial linear stage. The results show that the encrypted DOB-based PID controller outperforms a conventional encrypted PID controller in terms of tracking accuracy. Furthermore, the system successfully detects two types of malleability-based attacks: one that destabilizes the control system, and another that degrades its performance. The primary contributions of this study are: (i) the implementation of a KHE-based encrypted DOB-PID controller, (ii) the improvement of control performance under uncertainties, and (iii) the experimental demonstration of attack detection capabilities in encrypted control systems.

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arXiv2025

Faithful and Privacy-Preserving Implementation of Average Consensus

We propose a protocol based on mechanism design theory and encrypted control to solve average consensus problems among rational and strategic agents while preserving their privacy. The proposed protocol provides a mechanism that incentivizes the agents to faithfully implement the intended behavior specified in the protocol. Furthermore, the protocol runs over encrypted data using homomorphic encryption and secret sharing to protect the privacy of agents. We also analyze the security of the proposed protocol using a simulation paradigm in secure multi-party computation. The proposed protocol demonstrates that mechanism design and encrypted control can complement each other to achieve security under rational adversaries.

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arXiv2023

Secure Motion-Copying via Homomorphic Encryption

This study aims to develop an encrypted motion-copying system using homomorphic encryption for secure motion preservation and reproduction. A novel concept of encrypted motion-copying systems is introduced, realizing the preservation, edition, and reproduction of the motion over encrypted data. The developed motion-copying system uses the conventional encrypted four-channel bilateral control system with robotic arms to save the leader's motion by a human operator in the ciphertext in a memory. The follower's control system reproduces the motion using the encrypted data loaded from the secure memory. Additionally, the developed system enables us to directly edit the motion data preserved in the memory without decryption using homomorphic operation. Finally, this study demonstrates the effectiveness of the developed encrypted motion-copying system in free motion, object contact, and spatial scaling scenarios.

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arXiv2023

Encrypted Simultaneous Control of Joint Angle and Stiffness of Antagonistic Pneumatic Artificial Muscle Actuator by Polynomial Approximation

This study proposes an encrypted simultaneous control system for an antagonistic pneumatic artificial muscle (PAM) actuator toward developing a cybersecure and flexible actuator. First, a novel simultaneous control system design is considered for the joint angle and stiffness of a PAM actuator in a model-based design approach, facilitating the use of an encrypted control method. The designed controller includes a contraction force model expressed as rational polynomial functions, which makes it difficult to encrypt the controller. To overcome this difficulty, a least absolute shrinkage and selection operator (LASSO)-based polynomial approximation is employed for a rational controller. The resulting polynomial controller is then transformed into a matrix-vector product form, which enables the use of a specific homomorphic encryption scheme to develop an encrypted simultaneous control system for the PAM actuator. Finally, this study quantitatively evaluates the tracking control performance of the original, approximated, and encrypted controllers. The experimental results show that the proposed encrypted controller achieves simultaneous tracking of the joint angle and stiffness with a tracking error of less than 2.7 %.

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arXiv2023

Input-Output History Feedback Controller for Encrypted Control with Leveled Fully Homomorphic Encryption

Protecting the parameters, states, and input/output signals of a dynamic controller is essential for securely outsourcing its computation to an untrusted third party. Although a fully homomorphic encryption scheme allows the evaluation of controller operations with encrypted data, an encrypted dynamic controller with the encryption scheme destabilizes a closed-loop system or degrades the control performance due to overflow. This paper presents a novel controller representation based on input-output history data to implement an encrypted dynamic controller that operates without destabilization and performance degradation. Implementation of this encrypted dynamic controller representation can be optimized via batching techniques to reduce the time and space complexities. Furthermore, this study analyzes the stability and performance degradation of a closed-loop system caused by the effects of controller encryption. A numerical simulation demonstrates the feasibility of the proposed encrypted control scheme, which inherits the control performance of the original controller at a sufficient level.

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arXiv2023

Optimal Controller and Security Parameter for Encrypted Control Systems Under Least Squares Identification

Encrypted control is a framework for the secure outsourcing of controller computation using homomorphic encryption that allows to perform arithmetic operations on encrypted data without decryption. In a previous study, the security level of encrypted control systems was quantified based on the difficulty and computation time of system identification. This study investigates an optimal design of encrypted control systems when facing an attack attempting to estimate a system parameter by the least squares method from the perspective of the security level. This study proposes an optimal $H_2$ controller that maximizes the difficulty of estimation and an equation to determine the minimum security parameter that guarantee the security of an encrypted control system as a solution to the design problem. The proposed controller and security parameter are beneficial for reducing the computation costs of an encrypted control system, while achieving the desired security level. Furthermore, the proposed design method enables the systematic design of encrypted control systems.

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arXiv2023

Optimal Security Parameter for Encrypted Control Systems Against Eavesdropper and Malicious Server

A sample identifying complexity and a sample deciphering time have been introduced in a previous study to capture an estimation error and a computation time of system identification by adversaries. The quantities play a crucial role in defining the security of encrypted control systems and designing a security parameter. This study proposes an optimal security parameter for an encrypted control system under a network eavesdropper and a malicious controller server who attempt to identify system parameters using a least squares method. The security parameter design is achieved based on a modification of conventional homomorphic encryption for improving a sample deciphering time and a novel sample identifying complexity, characterized by controllability Gramians and the variance ratio of identification input to system noise. The effectiveness of the proposed design method for a security parameter is demonstrated through numerical simulations.

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arXiv2023

Cyber-Secure Teleoperation With Encrypted Four-Channel Bilateral Control

This study developed an encrypted four-channel bilateral control system that enables posture synchronization and force feedback for leader and follower robot arms. The encrypted bilateral control system communicates encrypted signals and operates with encrypted control parameters using homomorphic encryption. We created two-axis robot arms and identified them to obtain a nonlinear model consisting of a linear system and a nonlinear disturbance. Disturbance and reaction-force observers and a proportional-derivative controller were designed to construct a networked robot-manipulation system. The controllers in the constructed bilateral control system were securely implemented on dedicated computers using a controller encryption technique. The experimental results demonstrate that the developed bilateral control system can facilitate encrypted communication and controller, synchronize the posture, and feed the reaction force back. Through experimental validation, the encrypted four-channel bilateral control system enables the inheritance of control performance from the original (unencrypted) bilateral control system.

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arXiv2022

Sample Identifying Complexity of Encrypted Control Systems Under Least Squares Identification

A sample identifying complexity has been introduced in the previous study to capture an adversary's estimation error of system identification. The complexity plays a crucial role in defining the security of encrypted control systems and designing a controller and security parameter for the systems. This study proposes a novel sample identifying complexity of encrypted control systems under an adversary who identifies system parameters using a least squares method. The proposed complexity is characterized by a controllability Gramian and ratio of identification input variance to the noise variance. We examine the tightness of the proposed complexity and its changes associated with the Gramian and variance ratio through numerical simulations. The simulation results demonstrate that the proposed complexity captures a behavior of estimation error with a sufficient level. Moreover, it confirmed that the effect of controllability Gramian in the proposed complexity becomes larger as the variance ratio increases.

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arXiv2022

Towards Provably Secure Encrypted Control Using Homomorphic Encryption

Encrypted control is a promising method for the secure outsourcing of controller computation to a public cloud. However, a feasible method for security proofs of control has not yet been developed in the field of encrypted control systems. Additionally, cryptography does not consider certain types of attacks on encrypted control systems; therefore, the security of such a system cannot be guaranteed using a secure cryptosystem. This study proposes a novel security definition for encrypted control under attack for control systems using cryptography. It applies the concept of provable security, which is the security of cryptosystems based on mathematical proofs, to encrypted control systems. Furthermore, this study analyzes the relation between the proposed security and the conventional security of cryptosystems. The results of the analysis demonstrated that the security of an encrypted control system can be enhanced by employing secure homomorphic encryption.

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arXiv2022

Designing Optimal Key Lengths and Control Laws for Encrypted Control Systems based on Sample Identifying Complexity and Deciphering Time

In the state-of-the-art literature on cryptography and control theory, there has been no systematic methodology of constructing cyber-physical systems that can achieve desired control performance while being protected against eavesdropping attacks. In this paper, we tackle this challenging problem. We first propose two novel notions referred to as sample identifying complexity and sample deciphering time in an encrypted-control framework. The former explicitly captures the relation between the dynamical characteristics of control systems and the level of identifiability of the systems while the latter shows the relation between the computation time for the identification and the key length of a cryptosystem. Based on these two tractable new notions, we propose a systematic method for designing the both of an optimal key length to prevent system identification with a given precision within a given life span of systems, and of an optimal controller to maximize both of the control performance and the difficulty of the identification. The efficiency of the proposed method in terms of security level and realtime-ness is investigated through numerical simulations. To the best of our knowledge, this paper first connect the relationship between the security of cryptography and dynamical systems from a control-theoretic perspective.

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arXiv2021

Detailed Dynamic Model of Antagonistic PAM System and its Experimental Validation: Sensor-less Angle and Torque Control with UKF

This study proposes a detailed nonlinear mathematical model of an antagonistic pneumatic artificial muscle (PAM) actuator system for estimating the joint angle and torque using an unscented Kalman filter (UKF). The proposed model is described in a hybrid state-space representation. It includes the contraction force of the PAM, joint dynamics, fluid dynamics of compressed air, mass flows of a valve, and friction models. A part of the friction models is modified to obtain a novel form of Coulomb friction depending on the inner pressure of the PAM. For model validation, offline and online UKF estimations and sensor-less tracking control of the joint angle and torque are conducted to evaluate the estimation accuracy and tracking control performance. The estimation accuracy is less than 7.91 %, and the steady-state tracking control performance is more than 94.75 %. These results confirm that the proposed model is detailed and could be used as the state estimator of an antagonistic PAM system.

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arXiv2021

Sensor-less Angle and Stiffness Control of Antagonistic PAM Actuator Using Reference Set

This paper proposes a simultaneous control method for the angle and stiffness of the joint in an antagonistic pneumatic artificial muscle (PAM) actuator system using only pressure measurements, and clarifies the allowable references for the PAM actuator system. To achieve a sensor-less control, the proposed method estimates the joint angle and contraction forces using an unscented Kalman filter that employs a detailed model of the actuator system. Unlike previous control methods, the proposed method does not require any encoder and force sensor to achieve angle and stiffness control of the PAM actuator system. Experimental validations using three control scenarios confirm that the proposed method can control the joint angle and stiffness simultaneously and independently. Moreover, it is shown that a reference admissible set can be used as an indicator to establish reference values by demonstrating that the reference set covers the experimentally obtained trajectories of the angle and stiffness.

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