WSEAS TRANSACTIONS on COMMUNICATIONS Masao Tanabe, Keita Sugiyama, Masaki Aida Construction Methods of Closed Users Group using Multiparty Protocol MASAO TANABE1, KEITA SUGIYAMA2, MASAKI AIDA1 1The Graduate School of System Design Tokyo Metropolitan University 6-6 Asahigaoka, Hino-shi, Tokyo 191-0065 JAPAN [email protected], [email protected] 2Nomura Research Institute, Ltd. Marunouchi Kitaguchi Building, 1-6-5 Marunouchi, Chiyoda-ku, Tokyo 100-0005 JAPAN Abstract: In recent years, the Internet has become the most important infrastructure in the information society. Until now, a client-server system such as e-mail or WWW that is offered by a fixed service provider has played an important role. However, in such client-server systems, it is impossible to continue to provide services when the servers are halted by some failure. For this reason, a pure Peer-to-Peer (P2P) network, in which a user also acts as a provider, has lately attracted considerable attention as a new commercial infrastructure, because it enables its users to provide a service without a dedicated server. However, it is difficult to provide security on a pure P2P network because each terminal on the network has the same privilege. In this paper, we study a construction method of a Closed Users Group (CUG) without any administration methods which can control user access to certain applications on a pure P2P network. In our method, we assume that each member of a CUG has a certification which is issued by a certification authority on a public key infrastructure (PKI) that enables members to communicate safely. We realize this PKI on a P2P network using a multi-party protocol. In addition, we study the reliability issues involved in this method and propose some methods of issuing a certification to a new member. Finally, we discuss strong points and applications of one of these methods. Key-Words: - Closed Users Group, Multi-party protocol, Certification authority, P2P network, Public key infrastructure, I2P Network 1 Introduction providing complicated functions. A pure On the Internet, many applications such as e- P2Pnetwork has been used to exchange and mail or WWW are provided by client-server share files. Since it has a high fault tolerance, it systems that rely on central server management. is expected to be useful for a commercial This client-server system has the advantages of infrastructure. However, in order to utilize a easily constructing systems and managing its pure P2P network as a commercial users and services. On the other hand, it has the infrastructure, it is necessary to provide security disadvantages of having a single point of failure. function. Since it is difficult to manage users All services provided by a server will become and services and provide high grade functions, unavailable if the server stops. For these very few pure P2P networks have been utilized reasons, a pure Peer-to-Peer (P2P) network in as a commercial infrastructure so far. On the which there is no central dedicated server and contrary, we can utilize pure P2P networks as a clients communicate directly with each other commercial infrastructure if we could resolve has attracted considerable attention. A pure P2P security issues of them. network has the advantages of being highly For example, as a study on security issues in scalable and having a high fault tolerance. On a pure P2P network, realization of a public key the other hand, it has the disadvantages of being infrastructure (PKI) on a pure P2P network difficult to manage its users and services and of using a multi-party protocol was proposed in [1]. E-ISSN: 2224-2864 197 Issue 5, Volume 11, May 2012 WSEAS TRANSACTIONS on COMMUNICATIONS Masao Tanabe, Keita Sugiyama, Masaki Aida By developing this study, we study a construction method of a Closed Users Group (CUG) without any administration methods which can be accessed by only admitted users Certification to one application which considers simple Authority ②②② Issue a Certificate ④④④B Confirms User A’s Certification by security function on a pure P2P network in this （（（Encrypted Sentence using CA’s Public Key of User A. by CA Secret Key））） CA paper. We introduce a PKI in order to certify ③③③Get A’s Certification from CA ①①① Request a Certificate （（（Certification: Information A B regarding A’s public key））） users to each other. Moreover, we realize this ⑤⑤⑤Encrypt a Message using A’s PKI on a pure P2P network using a multi-party Public Key and Transmit it protocol. Then we study the reliability issues on this method and we propose some methods of issuing a certification to a new member. Finally, we discuss strong points and applications of one of these methods. Fig. 1 Public Key Infrastructure (PKI) The remainder of this paper is organized as follows. In Section 2, we discuss how to In the case of constructing a certification construct a CUG on a pure P2P network, and authority (CA) on a pure P2P network, it is desirable explain a construction method of a CUG using a to manage it in a distributed manner by its members. multi-party protocol. In Section 3, we study However, it is dangerous that a specific user who is reliability issues on this method and propose assigned an administration role knows the security some methods of issuing a certification to a new information stored in the CA. For this reason, we member. In Section 4, we discuss strong points utilize a multi-party protocol [2] to construct the CA and applications of one of proposed methods. on a pure P2P network instead of assigning an Finally, Section 5 concludes this paper. administration role to a specific user. This multi- party protocol is described in the following: 2 Construction CUG on pure P2P Number of members who attend the network is n . network using multiparty protocol When member i has secret information x , In this section, we discuss how to construct a CUG i on a pure P2P network and then explain a each member calculates the following construction method of a CUG using a multi-party function keeping its secret information protocol. secret: ⋅⋅⋅= xxfy ni ),,( . (1) All members can know y without 2.1 Construction CUG on pure P2P network A Public Key Infrastructure (PKI) is a certification disclosing their secret information at all. system using a cipher (Fig.1). In this figure, a CA is a certification authority and both A and B are users The signature issuing methods using a multi- in this PKI. When User A requests its certification party protocol have already been proposed [3][4]. from the CA, the CA issues it which is a text We construct a PKI on a pure P2P network using encrypted by the CA secret key. Then user A this multi-party protocol by members who compose transmits its certification to user B, whom it would a CUG (Fig.2). As issuing a certification like to communicate with. User B confirms user A's corresponds to the digital signature technically, we certification by using the CA's public key of user A. can construct a CUG on a pure P2P network by If user B would like to communicate with user A, it issuing a certification using our multi-party protocol. encrypts its message using the public key of user A. If a PKI will be constructed on a pure P2P network, it will be possible to construct a CUG by assuming that user who has its certification belongs to the CUG. E-ISSN: 2224-2864 198 Issue 5, Volume 11, May 2012 WSEAS TRANSACTIONS on COMMUNICATIONS Masao Tanabe, Keita Sugiyama, Masaki Aida a b : an integer b can be divided by an integer a Initial Member q : a prime number which satisfies q p −1 Member Zn : a set of integers that are more than zero and less than n Multi-Party Protocol New (Public Key Infrastructure) Certification Member * Zn : a set of integers which are included in Participation Member Request Zn and coprime to n * e : a random number included in Zq and is not zero g : a primitive root of p and is included in * Fig. 2 Simplified P2P Network Certification Model Z p x : a secret key and is included in Z . q 2.2 CUG construction method using multi- party protocol Now we explain a (,)k n threshold method as Next, we explain a signature issuing method by a an example of VSS. Assume the dealer has a secret multi-party protocol. This protocol is basically s∈ Zq and is committed to s through public composed of a protocol called Verifiable Secret information h= gs mod p . This secret can be Sharing (VSS) as illustrated in Fig.3. VSS is a method to divide secret information, and divided distributed to PP1,,⋅⋅⋅ n as follows: secret information can be verified whether they are genuine or not. We use a (,)k n threshold digital PROTOCOL DISTRIBUTE (at the dealer) signature scheme that k out of n members must step1: Choose a random polynomial k −1 cooperate to decrypt a ciphertext [5]. We compose a f() u= f0 + f 1 u + ⋅⋅⋅ + fk −1 u (2) random distribution method of secret information over Z of degree k −1 satisfying f)0( = s . that creates information corresponding to a shared q secret key and a public key in the network using the Compute si = f i)( . VSS scheme. Using this, we can construct the desired multi-party signature protocol that plural step2: Send s secretly to P and broadcast members sign to a specific data. i i fi (g mod p )i=1, ⋅⋅⋅ , k − 1 to all n participants.