Re-encryption Key Update for Message Confidentiality
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Solution Overview
Problem
In message transmission systems, updating the session key when users join or leave a group poses a confidentiality risk, as a deleted user cannot read messages and may infer message contents from search queries if the search key is not updated, allowing unauthorized inference of message contents.
Innovation Solution
A message transmission system that includes a server and communication terminals, where the system generates and updates re-encryption keys when the session key changes, re-encrypting search indices to match the new session key, preventing unauthorized inference of message contents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the session key is updated when a user is deleted, then message confidentiality is improved, but the search functionality becomes unavailable for deleted users
Solution Approach 1:
The system segments the encryption keys into two distinct types: session keys for message encryption and re-encryption keys for index maintenance. This segmentation allows the session key to be updated for confidentiality while the re-encryption key maintains search functionality independently.
Solution Approach 2:
The re-encryption key acts as an intermediary that bridges the old session key and the new session key. It enables the server to re-encrypt search indices from the old session key to the new session key without requiring the deleted user's participation, thus maintaining both confidentiality and search functionality.
2Ease of operation
If the search key is not updated when a user is deleted, then search functionality is maintained, but message confidentiality is compromised as the deleted user can infer message contents from search queries
Solution Approach 1:
The system performs preliminary action by generating and storing the re-encryption key before the user is deleted. This pre-prepared key enables the server to update search indices after user deletion, maintaining both search functionality and confidentiality without requiring the deleted user's involvement.
Solution Approach 2:
The system changes the cryptographic parameter by introducing a re-encryption key that operates differently from the session key. This parameter change allows the search index to be transformed from one encrypted under the old session key to another encrypted under the new session key, maintaining functionality while improving security.
3Reliability
If cloud-based message transmission is used to prevent data storage on terminals, then terminal security is improved, but server-side threats become more significant
Solution Approach 1:
The system applies asymmetry by using different encryption mechanisms for different purposes: session keys for message confidentiality and re-encryption keys for index maintenance. This asymmetric approach ensures that even if the server is compromised, the deleted user cannot infer message contents because the search indices are encrypted with re-encryption keys that don't reveal session key information.
Data Source
AI summary
The present invention makes it possible to improve confidentiality. A communication terminal stores a session key shared with other communication terminals (S3). The communication terminal makes a pair of an index generated by using the session key for a character string relating to a message and a message identifier, and transmits it to a server apparatus (S4). The server apparatus stores the pair of the index and the message identifier (S5). The communication terminal generates, when the session key is updated, a re-encryption key with a session key before update and a session key after update (S8). The server apparatus updates, by using the re-encryption key, the stored index to an index generated by using the session key after update (S9). The communication terminal encrypts a search keyword with the session key to generate a search query (S10). The server apparatus extracts a message identifier of which the index matches the search query (S11).