ABSTRACT

Before a message is sent out, the sender of the message would sign it using a digital signature scheme and then encrypt the message (and the signature) use a private key encryption algorithm under a randomly chosen message encryption key. The random message encryption key would then be encrypted using the recipient's public key. We call this process two-step approach: “signature-then-encryption”. Concept signcryption, first proposed by Zheng, is a cryptography primitive which combines both the functions of digital signature and public key encryption in a logical single step, and with a computational cost and communication overhead are significantly lower than that needed by traditional signature then encryption. In Zheng’s scheme, the signature verification can be done recipient’s private key at receiver end and its security are based on mainly Discrete Logarithm Problem (DLP), reversing one way hash function. In proposed modified signcryption scheme, security is based on the intractability of three hard problems: Discrete Logarithm Problem, reversing one way hash function and to determine prime factors of a composit number. Proposed signcryption scheme has all the benefits of signcryption and is also able to resolve the dispute / problem of non repudiation by independent third party, without compromising with sender and recipient’s private keys.