International Association
for Cryptologic Research

Confidentiality and authenticity are two fundamental security requirement of Public key Cryptography. These are achieved by encryption scheme and digital signatures respectively. Here we present a provably secure signcryption scheme in random oracle

model by modifying Libert et al\'s scheme [2]. Our scheme is more e±cient and secure than Libert et al\'s scheme. Tan [1] proved that this scheme is not secure against non-adaptive chosen cipher text attacks. It has been also proved that the semantically secure

symmetric encryption scheme proposed in the Libert et al\'s scheme is not su±cient to guarantee to be secure against adaptive chosen ciphertext attacks. Here we proposed a modified version of Libert et al\'s scheme. The security of which is proven using two as-

sumptions, namely the Strong Diffie-Hellman (SDH) and Diffie-Hellman Inversion (DHI)

in the random oracle model.