Identity-based on-line/off-line signcryption

We present an identity-based on-line/off-line signcryption scheme, where most of computations are carried out when the message is not available(i.e., off-line stage) and the on-line part of our scheme does not require any exponent computations and therefore is very efficient. It combines the functionalities of signature and encryption and is provably secure in the random oracle model. We also show that our scheme is indistinguishable against adaptive chosen-ciphertext attacks (IND-IDSC-CCA2) and is existentially unforgeable against adaptive chosen-message attacks (EF-IDSC-ACMA).