Atomic Proxy Cryptography

This paper introduces atomic proxy cryptography, in which an atomic proxy function, in conjunction with a public proxy key, converts ciphertext (messages in a public key encryption scheme or signatures in a digital signature scheme) for one key (k 1) into ciphertext for another (k 2). Proxy keys, once generated, may be made public and proxy functions applied in untrusted environments. Various kinds of proxy functions might exist; symmetric atomic proxy functions assume that the holder of k 2 unconditionally trusts the holder of k 1 , while asymmetric proxy functions do not. It is not clear whether proxy functions exist for previous public-key cryptosys-tems. Several new public-key cryptosystems with symmetric proxy functions are described: an encryption scheme, which is at least as secure as Diie-Hellman, an identiication scheme, which is at least as secure as the discrete log, and a signature scheme derived from the identiication scheme via a hash function.