امضاهاي ديجيتال حلقوي آستانه اي مبتني بر شناسه

چكيده انگليسي :

Identity based Threshold Ring Signatures Fereshteh Shiravi f shiravi@ec iut ac ir Date of Submission 2012 03 06 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Persian Supervisor Mehdi Berenjkoub brnjkb@cc iut ac irAbstract Ring signature formally introduced by Rivest Shamir and Tauman in 2001 is a type of signatureschemes similar to group signature schemes but have two distinct properties spontaneity and anonymity Spontaneity ensures that no participation or collaboration of other ring members is needed for the actualsigner to form a ring or group of members and generate a signature on behalf of this ring The spontaneityof these schemes is desirable for ad hoc groups such as mobile ad hoc networks Anonymity means signer ambiguous in the sense that a signature can be verified as coming from a certain ring without revealing theidentity of the actual signer Anonymity is becoming a major concern in many multi user electroniccommerce applications such as e cash and online games Due to ring signature s distinct and interestingproperties it has been studied extensively and there are many applications for ring signature proposed Inmany applications such as e voting e auction and authentication protocols we need to protect a signer sidentity from being known by eavesdroppers or other parties in a system In threshold ring signatureschemes any group of t entities spontaneously conscript arbitrarily n t entities to generate a publiclyverifiable t out of n signature on behalf of the whole group yet the actual signers remain anonymous Inrecent years many proposals of identity based ring signature schemes have been published In identitybased cryptography systems such as digital signature key exchanging and secure authentication users arenot capable of generating a private key for ownselves For this reason there is a trusted party called theprivate key generator PKG who does the system setup To obtain a private key for his identity a userwould go to the PKG and prove his identity The PKG would then generate the appropriate private key andpass it on to the user Since the PKG is able to compute the private key corresponding to any identity it hasto be completely trusted The PKG is free to engage in malicious activities without any risk of beingconfronted in a court of law The malicious activities could include signing on behalf of user or worse still generating and distributing private keys for any identity The key feature of ring signature is to protectanonymity of the signer According to the key feature of ring signature it is necessary to propose solutionsto completely avoid signature forgery by PKG The main goal of this thesis is to propose the solutions toavoid users signature forgery by PKG In the thesis at first the concept of identity based threshold ringsignature and forging signature by PKG are completely described Then two solutions have been proposedto avoid signature forgery by PKG In the first solution forging signature by PKG is too difficult and in thesecond solution forging signature by PKG is not possible Finally we propose a new identity basedthreshold ring signature whose security is based on the hardness of the RSA problem This is the firstidentity based threshold ring signature scheme which does not employ bilinear pairings Furthermore theresulting scheme satisfies an interesting property the real oeners of a ring signature can disclose theiridentity every time and prove that they are persons who actually signed the message Keywords Ring signature Identity based cryptography Private Key Generator PKG Secretsharing Bilinear pairings Thereshold ring signature

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امضاهاي ديجيتال حلقوي آستانه اي مبتني بر شناسه

مركز توليد كليد خصوصي , تسهيم راز شامير , زوج نگارهاي دوخطي