Distributed Re-Encryption Using Secret Sharing

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Solution Overview

Problem

Multi-key fully homomorphic encryption systems face the challenge of decryption key leakage during re-encryption, especially when the re-encryption key is compromised or the computation server colludes with the receiver.

Innovation Solution

A distributed re-encryption apparatus and method that employs secret-sharing to store and utilize shares of a re-encryption key, preventing the leakage of user decryption keys by re-encrypting ciphertexts into different encryption keys without decrypting them, using a cryptographic system with distributed re-encryption apparatuses and secure computation in a secret sharing scheme.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If re-encryption key is stored centrally for re-encryption operations, then re-encryption can be performed efficiently, but user decryption key may leak if re-encryption key is compromised or computation server colludes with receiver

Engineering Contradiction:

Improvere-encryption efficiencyVSAvoiddecryption key security

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The re-encryption key is segmented into multiple shares using secret sharing scheme, distributed across multiple apparatuses rather than stored centrally. This allows re-encryption operations to proceed by collecting sufficient shares without any single apparatus holding the complete key, preventing key leakage even if some apparatuses are compromised.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A trusted third party (key generation apparatus) is introduced to generate and distribute the secret shares. This intermediary ensures that no single entity possesses the complete re-encryption key, while still enabling authorized re-encryption operations when sufficient shares are collected, thus mediating between security requirements and operational efficiency.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If re-encryption key is distributed among multiple apparatuses using secret sharing, then decryption key security is improved, but re-encryption operation complexity increases

Engineering Contradiction:

Improvedecryption key securityVSAvoidre-encryption system complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The re-encryption key is divided into multiple shares that are distributed across different apparatuses. Each apparatus stores only a portion (share) of the key, and re-encryption operations require collaboration among multiple apparatuses to collect sufficient shares. This segmentation enhances security while the modular structure manages complexity through clear division of responsibilities.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The key generation apparatus serves multiple functions: generating encryption keys, generating re-encryption keys, and distributing secret shares. This multi-functionality reduces overall system complexity by consolidating key management operations in a single trusted apparatus rather than requiring separate specialized components for each function.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Ease of operation

If computation server performs re-encryption operations, then users can access encrypted data without decryption, but user decryption key may leak if server colludes with receiver

Engineering Contradiction:

Improvedata access convenienceVSAvoidkey leakage risk

Core Design Contradiction:

Ease of operationVSObject-affected harmful factors

Solution Approach 1:

The re-encryption capability is segmented and distributed across multiple apparatuses holding different shares of the re-encryption key. The computation server alone cannot perform re-encryption; it must collaborate with other apparatuses to collect sufficient shares. This segmentation prevents key leakage even if the server colludes with a receiver, as the server cannot obtain the complete re-encryption key from any single source.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The system proactively prevents key leakage by distributing secret shares before any re-encryption operation occurs. The computation server is given only limited access to perform re-encryption when sufficient shares are collected, but never possesses the complete re-encryption key. This preliminary distribution of key material in a segmented form creates inherent protection against collusion attacks.

Inventive Principle:

Principle #9Preliminary anti-action

Data Source

PatentUS20250023717A1Distributed re-encryption apparatus, cryptographic system, distributed re-encryption method, and distributed re-encryption program

Publication Date: 2025.01.16 NEC CORP

AI summary

A distributed re-encryption apparatus comprises: a distributed re-encryption key storage part that stores shares obtained by secret-sharing a re-encryption key that re-encrypts a ciphertext into one encrypted with a different encryption key without decrypting the ciphertext; and a distributed re-encryption part that re-encrypts using the shares of the re-encryption key the ciphertext into one encrypted with a different encryption key without decrypting the ciphertext.