Cryptographic method and apparatus

摘要

A cryptographic method and apparatus is provided in which a first party (A) receives and modifies a public key (p, g, y) for which there exists a corresponding private key (x) held by a second party (TA). The public key is modified by exponentiating at least one element (y) of the received public key using as exponent a hash of a string (STR) that comprises information concerning at least one action to be taken by the second party (TA). The string (STR) is made available to the second party (TA) to enable the latter to modify its private key to compliment the modified public key. In a preferred embodiment, the method and apparatus are applied to the use of the ElGamal encryption/decryption scheme, with the second party (TA) acting as a trusted authority that only releases the decrypted message to a third party (B) if the latter satisfies an identity condition specified in the string. For privacy reasons, the third party (B) may blind the encrypted message before passing it to the second party for decryption.