Provable Secure Identity Based Signcryption Schemes without Random Oracles

Signcryption is a cryptographic primitive which performs encryption and signature in a single logical step with the cost lower than signature-then-encryption approach. Recently, Li et al. [35] proposed the first provable secure identity based signcryption without random oracles. In their scheme sender signs the ciphertext. However, in [11] Boyen showed that non-repudiation is easily achieved if the sender sign the plaintext rather than ciphertext. In this paper we proposed an identity based signcryption scheme without random oracles, which provides the non-repudiation with respect to plaintext. We also proposed anidentity based public verifiable signcryption scheme with third party verification in the standard model.