Conversion Key Generation for Homomorphic Ciphertext Reuse
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Solution Overview
Problem
Existing homomorphic encryption techniques require proxy re-encryption before performing operations, limiting analysis by specific analysts and preventing reuse of intermediate data, as all users must convert keys and re-execute operations individually.
Innovation Solution
A conversion key generation device encrypts a decryption key with a public key to generate a conversion key, allowing ciphertexts encrypted with different keys to be decrypted after homomorphic operations, enabling controlled decryption and secure reuse of intermediate data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If proxy re-encryption is performed before homomorphic operations, then key control and decryption security are improved, but operational flexibility and data reuse capability deteriorate
Solution Approach 1:
The patent applies preliminary action by performing homomorphic operations on ciphertexts encrypted with different keys before performing proxy re-encryption. This allows intermediate computation results to be obtained and reused without re-executing operations, while still maintaining the ability to control decryption through conversion keys. The conversion keys are generated after the homomorphic operations, enabling flexible key management without compromising operational flexibility.
2Ease of operation
If all users use a common key for homomorphic operations, then operational simplicity is improved, but security and privacy protection deteriorate
Solution Approach 1:
The patent introduces conversion keys as an intermediary mechanism that enables homomorphic operations on ciphertexts encrypted with different keys without requiring a common key. The conversion keys act as mediators that allow the system to maintain both operational simplicity (by allowing operations on differently encrypted data) and privacy protection (by enabling controlled decryption through key management). This eliminates the need for all users to share a common key while preserving the ability to perform homomorphic operations.
3Reliability
If proxy re-encryption is required before homomorphic operations, then key management control is improved, but communication overhead and processing time increase
Solution Approach 1:
The patent inverts the conventional sequence by performing homomorphic operations first on ciphertexts with different keys, and then performing proxy re-encryption to generate conversion keys. This inversion eliminates the need to determine decryption recipients before performing operations, allowing intermediate results to be reused without re-executing expensive homomorphic operations. The key management control is maintained through the generation of conversion keys after the operations, reducing communication overhead and processing time.
Data Source
AI summary
A key acquisition unit (411) acquires a decryption key ski in a pair of a conversion source and a public key pkj in a pair of a conversion target, out of a plurality of pairs of a decryption key and a public key. A conversion key generation unit (412) encrypts the decryption key ski acquired by the key acquisition unit (411) with the public key pkj, so as to generate a conversion key rki→j for converting a ciphertext encrypted with a public key pki in the pair of the conversion source into a converted ciphertext that can be decrypted with a decryption key skj in the pair of the conversion target. An output unit (413) outputs the conversion key rki→j generated by the conversion key generation unit (412).