Proxy Re-Encryption.

A cryptographic technique that lets a semi-trusted proxy transform a ciphertext encrypted under Alice's key into one decryptable by Bob without learning the plaintext. It belongs to the Cryptography category of cybersecurity.

A cryptographic technique that lets a semi-trusted proxy transform a ciphertext encrypted under Alice's key into one decryptable by Bob without learning the plaintext.

Proxy Re-Encryption (PRE), formalised by Blaze, Bleumer, and Strauss in 1998 and later refined by Ateniese et al., allows Alice to delegate decryption of her ciphertexts to Bob by issuing the proxy a re-encryption key rk_{A->B}. The proxy converts each ciphertext under pk_A into a ciphertext under pk_B without ever seeing the plaintext or learning either private key. Schemes can be unidirectional or bidirectional, single-hop or multi-hop, and may be built on ElGamal, BBS98, AFGH, or pairing-based constructions; lattice-based PRE offers post-quantum candidates. PRE is the cryptographic foundation of NuCypher/Threshold, Umbral, secure data sharing in DRM, encrypted email forwarding, and key rotation in encrypted cloud storage without re-encrypting bulk data.

Defences for Proxy Re-Encryption typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: PRE, Atomic proxy re-encryption.