Abstract:

We analyze six signcryption schemes and find confidentiality problem in all of them and unforgeability problem in two of them. Then some concrete attacks are presented for these problems. We improve the six schemes using the following methods: binding the sender in the encryption part, binding the receiver in the signature part, verifying equation without plaintext information and binding public key when producing partial private key. These improved schemes are verified in the random oracle model, and security analysis shows that these improved schemes are secure. Finally we point out that some principles must be paid attention to when designing  signcryption schemes.

Key words: signcryption, identitybased signcryption, certificateless signcryption, public key replacement attack, bilinear pairing