102 | PART A. NATURAL AND APPLIED SCIENCES INTERNATIONAL JOURNAL of ACADEMIC RESEARCH Vol. 4. No. 3. May, 2012 ENHANCED PARTIALLY BLIND SIGNATURE WITH MESSAGE RECOVERY USING AGREEMENT FACTORS Dr. Abdulameer Khalf Hussain, Prof. Reyadh Naoum, Mustafa Nouman Murad Middle East University Amman (JORDAN) E-mails: Abdulameer.hussain@yahoo.com, rnaoum@meu.edu.jo, mustafa@orasnet.com ABSTRACT This paper presents a new partially blind signature, dedicated for sensitive applications, and depending on agreement factors. In the traditional partially blind signature schemes, there is an agreement between the sender and the signer, but in this method, the agreement is used to recover blind messages by an individual group of users and to enforce the system’s security.The blind signature used in this method is based on RSA scheme designed by Chaum [1] as an initial step to our system with modifications. The proposed method enhances the important properties of blind signature schemes such as unforgeability, the anonymity, blindness and others. The security of this developed system is analyzed to show the importance of agreement in blind signatures. The most important property of this scheme is to classify messages according to their sensitivity that can be used to recover unblined message by authentication group selected by the singer and to use agreement secure information to get more secure system. Key word: Digital Signatures, Blind digital signature, Agreement, Authentication. 1. INTRODUCTION Chaum in [1, 2] introduced the concept of the blind signature as a mechanism allows a client requests a signature from a signer to sign a blinded-message. One of the important properties of blind signatures is the blindness, so they are widely used in many e-commerce applications [3,4]. It is essential to provide sender's privacy by blind signatures especially in applications such as digital cash and e-voting. In blind signature scheme the parties involved are the signer and the requester in such a way that the signer has no authority over the message sent by a requester. In addition, a third party may involved in blind signature scheme whose responsibility is to verify the signature without any knowledge about the secrets of both parties so the services must be authenticated and secure. Also, blind signature scheme can prevent nonrepudiation in which the services may be denied between the requester and the service providers. [5, 6]. The properties of any blind signature scheme are: correctness, authenticity, ungorgeability, non-reusability, non- repudiation, integrity, blindness and untraceability [7, 8]. Some schemes had been introduced such as which includes braid groups to build a key agreement and a public key encryption scheme [9, 10].To ensure privacy, the anonymity of message owner (in blind signature) must be protected [11]. An extension of the blind signature is a partially blind signature. This type of signature includes commonly agreed information which remains clearly visible despite of the blinding process. Blind signature provides a partial blindness property, so this scheme is considered more efficient than ordinary blind signature schemes especially when this type of signature is used in an electronic cash application, so many partially blind signature schemes had been appeared in recent years [12, 13]. A partially blind signature scheme allows the signer to embed the non-removable common information into the blind signature. The common information is known and pre-agreed by both the signer and the requester before the signing process. In the past, many partially blind signature schemes [14, 15] were also proposed and the objective is to improve the performance on the requester side or enhance the security. Some partially blind signatures are based on RSA such Abe and Fujisaki's scheme [16]. 2. RELATED WORKS In [17], a new verifiably encrypted signature scheme and a partially blind signature scheme was proposed. In this scheme the signatures are used in optimistic contact signing in order to provide a fair exchange and also the signer is allowed to sign a partially blinded message that contains pre-agreed information. Another signature scheme was proposed [18]. This scheme enables the requester to sign a message on behalf of an entire group. This scheme is considered a dynamic group blind signature scheme. A short group signature scheme in [19] was constructed using RSA signature. Security of this group signature is based on the Strong Diffie-Hellman assumption and a new assumption in bilinear groups called the Decision Linear assumption. In [20] a new scheme of dynamic group blind signature based on elliptic curve discrete logarithm problem (ECDLP) was suggested. This scheme the dynamic group blind signature and the knowledge signature to the elliptic curve cyclic group. They enhanced the security and efficiency of group signatures. A proposal of a forward-secure multisignature scheme and a forward-secure blind signature scheme is found in [21]. In [22], the authors proposed a scheme that allows the signer to add information in the signed message. The requester cannot modify either this information or the signed message. This added information contains certain information such as date to stamp the signature.