Mediated RSA cryptographic method and system using blinding

摘要

A mediated RSA cryptographic method and system is provided in which a sender (10) encrypts a message (m) using an encryption exponent e and a public modulus n, and a recipient (20) and a trusted authority (50) cooperate with each other to decrypt the encrypted message by using respective components dU, dT of a decryption exponent. In order to prevent the trusted authority (50) from reading the message in the event that it has access to the recipient decryption exponent components dU, the recipient (20) blinds the encrypted message before passing it to the trusted authority (50). This blinding is effected by a modulo-n blinding operation using a factor r where r is a secret random number. The trusted authority (50) then applies its decryption exponent component dT to the message and returns the result to the recipient (20) who cancels the blinding and applies its decryption exponent component dU to recover the message. In a preferred embodiment, the encryption exponent is based on a string (STR) comprising one or more conditions; this string is passed to the trusted authority (50) along with the blinded message and the trusted authority checks that the or each condition is satisfied before it determines and applies the appropriate decryption exponent component dT. In a further embodiment the encryption exponent is based on a string chosen by the sender such that identifier-based mediated RSA is performed.