Semigroup ElGamal Decryption Service Key Security
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Solution Overview
Problem
Cloud-managed key cryptographic schemes face safety issues as users may acquire decryption capabilities through repeated use of the decryption service, making it difficult to prevent the use of encrypted data even when the decryption service is stopped.
Innovation Solution
A private key conforming to the ElGamal encryption system is held on a semigroup where calculating the order of an element is computationally difficult, ensuring that the decryption service is secure by using ElGamal encryption on a semigroup with a difficult order calculation, thus preventing users from obtaining the private key even with repeated decryption service usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a key is held in a reliable server device and decryption service is provided to authorized persons, then the ability to stop users from using encrypted data is improved, but users may acquire decryption capability through repeated use making it impossible to stop usage even when service is stopped
Solution Approach 1:
The decryption process is segmented into multiple independent steps: receiving ciphertext, receiving decryption request information, computing decryption result information, and outputting the result. Each step processes only necessary data without exposing the private key, preventing users from acquiring full decryption capability through repeated requests.
Solution Approach 2:
Decryption request information acts as an intermediary that enables the decryption process without directly exposing the private key to users. The server uses this intermediate information to compute decryption results while maintaining key security, thereby controlling usage without giving users direct access to cryptographic keys.
2Reliability
If private key is held in decryption service providing device, then decryption service can be controlled and stopped, but repeated use of the service may allow users to acquire decryption capability
Solution Approach 1:
The private key is extracted from the decryption service providing device and held separately in a key holding device. The decryption service only processes ciphertext and decryption requests without accessing the private key directly, eliminating the risk of key exposure while maintaining service control.
Solution Approach 2:
Decryption request information serves as an intermediary between the user's decryption needs and the secured private key. This intermediate mechanism allows the system to provide decryption functionality while preventing direct key exposure, thereby controlling service usage without compromising key security.
3Adaptability or versatility
If cloud-managed key cryptographic scheme is used, then key distribution control is improved, but safety is compromised as users may obtain decryption capability through repeated service use
Solution Approach 1:
The cryptographic scheme is segmented into key management functions (performed by the key holding device) and decryption service functions (performed by the decryption service providing device). This segmentation maintains the flexibility of cloud-managed key schemes while enhancing safety by preventing users from acquiring decryption capability through service repetition.
Solution Approach 2:
Decryption request information acts as an intermediary that enables the cloud-managed key scheme to maintain flexibility while improving safety. The intermediate mechanism allows the system to adapt to different decryption requests without exposing the private key, thereby resolving the contradiction between versatility and safety.
Data Source
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AI summary
A private key is held which conforms to an ElGamal encryption system on a semigroup, calculation of an order of an element of the semigroup being computationally difficult, information corresponding to ciphertext conforming to the ElGamal encryption system is input, a private key s is used to decrypt the information corresponding to the ciphertext in conformance to the ElGamal encryption system, and information corresponding to a result of decrypting the ciphertext is obtained and output. Alternatively, whether it is computationally difficult or easy to calculate the order of the element of the semigroup is determined, and the safety of a decryption service providing device is evaluated based on the determination result.