Security analysis of two signature schemes and their improved schemes

Unforgeabilty is a primitive property of a secure digital signature. As two extensions of digital signature, signcryption and certificateles signature play an important role in the sensitive transmission. In this work, we analyze the security of two signature schemes, one is the certificateless signature scheme[17] which was proposed by Gorantla et al in CIS 2005, the other is an efficient short signcryption scheme[81 which was proposed by Ma et al in Inscrypto 2006. Then, we show that the two schemes were insecure. In Ma et al's scheme, if the recipient is dishonest, then he can produce any forgery on an arbitrary message and convince the trusted third party that the forgeable signcrytion comes from the signer. While, in Gorantla et al's scheme, any one can forge a signature on an arbitrary message in the name of the others. Finally, we give the corresponding improved scheme, respectively.