reencryption

Reencryption is a cryptographic operation that transforms a ciphertext produced under one public key into a new ciphertext that can be decrypted under a different public key, without exposing the underlying plaintext. This enables data to be shared or reallocated to new recipients without requiring the data owner to re-encrypt and resend the data.

The most studied form is proxy re-encryption (PRE). In a PRE scheme, there are typically three parties: the data owner (the delegator), the intended new recipient, and a semi-trusted proxy. The owner uses their private key and the new recipient’s public key to generate a re-encryption key. The proxy applies this key to an existing ciphertext, producing a new ciphertext that the recipient can decrypt with their private key. In ideal PRE schemes, the proxy learns nothing about the plaintext during transformation.

Variants of reencryption differ in capabilities and security. Some schemes allow multi-hop re-encryption, where ciphertext can be transformed through a chain of recipients, while others are single-hop. Non-interactive PRE schemes do not require ongoing interaction after the re-encryption key is issued. Security analyses focus on preserving ciphertext indistinguishability (against chosen-plaintext or chosen-ciphertext attacks) and ensuring the proxy cannot recover plaintext or derive private keys.

Applications include secure data sharing in cloud storage, revocable access control, and end-to-end encrypted collaboration. Reencryption supports key rotation and access revocation by enabling re-encryption to new recipients without re-encrypting the entire dataset. Challenges include key management, performance overhead, and ensuring robust revocation and trust assumptions about the proxy.