Method and apparatus for providing short-term private keys in public key cryptographic systems

摘要

A computing entity, 11, has an associated static public/private key pair formed by a static private key comprising a secret (sA), and a static public key (P,R) comprising both a first element (P) and that element combined with the secret (sA). The secret (sA) is stored in higher-security storage provided, for example, by a smartcard, 20. A short-term private key (St) is provided for use by a computing entity 11 in effecting cryptographic operations during an operational period. This short-term private key (St) is generated, independently of any pending cryptographic operations, by mapping a string (str) to a second element (P') and multiplying that element by said secret (sA), the first and second elements (P,P') being such that a computable bilinear map exists for these elements. The short-term private key (St) is stored in lower-security storage, 15, in the computing entity, 11, for a limited period that encompasses the operational period in respect of which the key (St) was generated. A second embodiment relates to a cryptographic system comprising: a first entity arranged to use the private key of an associated static public/private key pair to form a plurality of different short-term private keys each for use during a corresponding limited operational period; a public key infrastructure for providing a certificate associating the first entity with the public key of its static public/private key pair; and a second entity arranged to use a known formula and known data to migrate the static public key of the first entity, whilst retaining the assurance provided by said certificate, to form short-term public keys each for use, during a corresponding said limited operational period, in carrying out cryptographic operations for which there exist complimentary operations requiring use of the corresponding short-term private key.