We show that the Zhang-Yang-Zhu-Zhang identity-based authenticatable ring signcryption scheme is not secure against chosen plaintext attacks. Furthermore, we propose an improved scheme that remedies the weakness of the Zhang-Yang-Zhu-Zhang scheme. The improved scheme has shorter ciphertext size than the Zhang-Yang-Zhu-Zhang scheme. We then prove that the improved scheme satisfies confidentiality, unforgeability, anonymity and authenticatability. Copyright.
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