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Public key cryptosystems are secure only when the authenticity of the public key is assured. Shao proposed a new scheme to overcome the problems of the existing schemes, which suffers from two major drawbacks. The first drawback is the availability of users' passwords in plaintext format in key server which are prone to attacks by ill-minded users. The second one is depending on the key server blindly for certificate generation, without further verification by the user. To overcome these severe drawbacks, we proposed an improved key authentication scheme based on Chinese remainder theorem and discrete logarithms. Our scheme allows the user to generate his/ her certificate without the help of any trusted third party. This scheme is intended for online services, military and defense applications to exchange keys securely. [ABSTRACT FROM AUTHOR]

In public key cryptosystem, the protecting of public keys against intruders is very important. Usually, key authentication schemes are required to preserve public keys. Most of the key authentication schemes require one or more authorities to authenticate public keys. Some other key authentication schemes require no authorities to authenticate public keys. The Zuhua Shao proposed a new scheme to overcome the drawbacks of existing schemes, but it has some vulnerabilities. This scheme relies only on key server for certificate generation. In a distributed network, the server can easily impersonate other legitimate users and listen the messages directed to them. Moreover, in this scheme, users’ passwords are stored in the form of plaintext. In this paper, slight modifications are pointed out to overcome these severe drawbacks. [ABSTRACT FROM AUTHOR]

A strong designated verifier signature (SDVS) scheme only allows a designated verifier to validate signer's signatures for ensuring confidentiality. At the same time, the designated verifier can not transfer the signature to any third party, since he can also generate another computationally indistinguishable SDVS, which is referred to as non-transferability. A proxy signature scheme is a special type of digital signature schemes, which enables an authorized proxy signer to create a valid proxy signature on behalf of the original one. The resulted proxy signature is publicly verifiable by anyone. In this paper, we elaborate on the merits of SDVS schemes and proxy signature schemes to propose an efficient strong designated verifier proxy signature (SDVPS) scheme in which only a designated verifier can be convinced of the proxy signer's identity. The proposed scheme has crucial benefits in organizational operations and electronic commerce. Compared with related schemes, ours has not only shorter signature length, but also lower computational costs. Moreover, the security requirement of unforgeability against existential forgery under adaptive chosen-message attacks (EF-CMA) is proved in the random oracle model. [ABSTRACT FROM AUTHOR]

In cryptography, a 'Three-Pass Protocol' facilitates a secure communication of confidential messages, over insecure channels, without the need of any exchange of keys. Each communicating entity is required to generate a pair of keys, related to each other. One of the keys is used for encryption and the other key is used for decryption. Transmission of information between a sender and the intended recipient requires making of three passes. Each pass involves exchange of an encrypted message between the communicating entities. In pass 1, the sender encrypts the plain text with one of its keys and sends the resulting cipher text to the intended recipient. The intended recipient further encrypts the received cipher text with one of its keys and bounces the doubly-encrypted cipher text back to the sender. This cipher text exchanged in pass 2 has double encryption-one applied by the sender and the other applied by the intended recipient. The sender decrypts the doubly-encrypted cipher text using its second key and removes its part of the encryption. The resulting cipher text now has only one encryption-the one applied by the intended recipient. In pass 3, this singly-encrypted cipher text is sent by the sender to the intended recipient. The intended recipient receives the cipher text and removes the residual encryption using its second key; and successfully recovers the original plain text, meant to be conveyed to the intended recipient in a secure way. In all the three passes, the message is encrypted-having single encryption during passes 1 and 3 and double encryption during pass 2. Thus, the two communicating entities are able to exchange information in a secure way, without any need of exchange of keys. [ABSTRACT FROM AUTHOR]