Re-encryption Key Generator for Collusion-Resistant File Sharing

The system segments key management by introducing a group administrator who holds a separate key pair from individual users. Files are encrypted with the group administrator's public key, and decryption rights are delegated through re-encryption keys rather than requiring individual encryption operations for each user. This segmentation resolves the contradiction by maintaining security through unique keys while simplifying key management through centralized group administration.

The group administrator acts as an intermediary between the file uploader and individual users. Instead of direct key pairing between uploader and each user, the group administrator's key pair mediates the encryption and decryption process. Re-encryption keys enable the server to transform ciphertext encrypted for the group administrator into ciphertext decryptable by individual users without exposing private keys, thus simplifying key management while maintaining security.

The system segments the single shared secret into multiple individual private keys, each held by different users. The group administrator holds one private key, and each user holds another. This segmentation ensures that if one private key leaks, only the specific user affected is compromised, not the entire system. The server uses re-encryption keys to transform ciphertext without needing any user's private key, thus simplifying key management while dramatically improving security against key leaks.

The re-encryption key mechanism acts as an intermediary that enables key transformation without exposing any private keys. The server can re-encrypt files from the group administrator's encryption to individual user encryptions using re-encryption keys, eliminating the need for users to share private keys while maintaining simplified key management. This resolves the contradiction by preserving the simplicity of centralized key management while achieving the security of individual keys.

The re-encryption key serves as an intermediary that enables the server to perform key transformation without storing any user's private key or the group administrator's private key. The server only stores public keys and re-encryption keys, which are mathematically designed to enable ciphertext transformation without revealing private key information. This resolves the contradiction by maintaining file sharing capability while preventing collusion attacks, as the server cannot derive private keys from the stored re-encryption keys.

The system implements local quality by giving different cryptographic properties to different parts of the key structure. Re-encryption keys have the specific property of enabling ciphertext transformation without revealing private key information, while private keys maintain their secrecy. The server stores only the public-facing components (public keys and re-encryption keys) that have the local quality of enabling sharing functionality without the harmful quality of exposing private key information, thus preventing collusion attacks while maintaining file sharing capability.