International Association for Cryptologic Research

Ph.D. Database

The aim of the IACR Ph.D. database is twofold. On the first hand, we want to offer an overview of Ph.D. already completed in the domain of cryptology. Where possible, this should also include a subject classification, an abstract, and access to the full text. On the second hand, it deals with Ph.D. subjects currently under investigation. This way, we provide a timely map of contemporary research in cryptology. All entries or changes need to be approved by an editor. You can contact them via phds (at) iacr.org.

Details

Amit K Awasthi (#317)
Name Amit K Awasthi
Personal Homepage http://www.psit.in/faculty/math/awasthi/
Topic of his/her doctorate. A Study of Some Delegated Digital Signature Schemes
Category cryptographic protocols
Ph.D. Supervisor(s) Sundar Lal
Year of completion 2007
Abstract Mimicking handwritten signatures electronically is the genesis of the concept of the digital signatures. Cryptography provides mathematical tools for generating digital signatures. Delegation of the signing authority is a common practice among the signers. If a leader is on leave, he may delegate his signing capability to his deputy. There are many more situations where an effective delegation of signing power is a must. In this thesis we discuss and propose some new protocols for effective delegation with specific requirements. This thesis is compiled in two parts -- \textbf{First Part} consists of schemes in which delegation is based on finite fields. The \textbf{Second Part} is the study of delegations based on elliptic curves. There are total nine chapters in this thesis-- six in first part and rest three in second part. Below we summarize the contents of various chapters: The \textbf{First Chapter} is devoted to an introduction to cryptographic terms which are necessary to understand the thesis. In this chapter we survey some digital signature schemes which we frequently use in rest of the thesis. In the \textbf{Second Chapter}, we study some existing delegation schemes. We discuss several kind of delegation and also the security requirements for such schemes. At the end of the Chapter, we summarize schemes that are developed to fulfill specific needs of effective delegation of digital signatures. In the \textbf{Third Chapter}, we consider group signature schemes where instead of a single original signer, a group of $n$ signers delegates its signing authority efficiently and transparently to a single signer. In this chapter we propose a protocol to select a delegated signer such that at receiver's site this delegated signature may be verified. The proxy signer may sign any message on behalf of whole group at a time. (This work was accepted at the conference SCI-2003, Orlando, USA.) In some cases, the original signer may delegate its signing capability to a specified proxy group while ensuring individual accountability of each participant signer. The proxy signature scheme that achieves such purpose is called \emph{multi-proxy signature scheme} and the signature generated by the specified proxy group is called multi-proxy signature for the original signer. Recently, such scheme has been discussed by Lin et al. In the \textbf{Fourth Chapter}, we discuss a new multi-proxy signature scheme, which requires less computational overhead than in Lin's scheme and fulfills the requirement of \emph{partial delegation with warrant} simultaneously. In digital signature schemes, the concept of \emph{Blind Signature} ensures anonymity of users. Untracability and unlinkability are two main properties of real coin, which require mimicking electronically. In the \textbf{Fifth Chapter}, we propose a proxy blind signature scheme. In production of coins, the user makes the bank blindly sign a coin using blind signature protocol. Here users are in possession of a valid coin which the bank cannot recognize nor link with the users. Whenever a user goes through a valid branch to withdraw a coin, the branch needs to make proxy blind signature on behalf of the signee bank. (This work in published form is available on \texttt{http://www.eprint.iacr.org/2003/072}) In \textbf{Chapter Six}, we propose a proxy signature scheme in which the \emph{warrant message}, i.e., the message about delegation-relation, can be recovered from the proxy signatures. Normally warrant message is hashed and appended to the signature. However, in our scheme warrant message is conveyed within proxy signature and recovered by the verifier. It saves both communication bandwidth and storage space. (This work in published form is available on \texttt{http://www.eprint.iacr.org/2003/073}) The \textbf{Second Part} is devoted to some protocols defined over elliptic curve with bilinear pairings. This is a new setting and is complementary to PKI based cryptosystems. In recent years the bilinear pairings have found many applications in cryptography and several ID-based cryptosystems using bilinear pairings over elliptic curves or hyper-elliptic curves have been discussed. The \textbf{Second Part} of the thesis begins with \textbf{Chapter Seven} in which we summarize some mathematical concepts which form the base for further chapters of the thesis. In 2001, Rivest et al. firstly introduced the concept of ring signatures. A ring signature is a simplified group signature without any manager. It protects the anonymity of the signer. In 2002, Zhang and Kim proposed a new ID-based ring signature scheme using pairings. A year later Lin and Wu proposed a more efficient ID-based ring signature scheme. However both these schemes have some inconsistency in computational aspect. The \textbf{Eighth Chapter} consists of two new schemes, namely, ID-based ring signature scheme and ID-based proxy ring signature scheme from bilinear pairings. Both these schemes not only take care of the inconsistencies in above two schemes (Zhang's scheme and Lin's scheme) but also computationally more efficient than existing one. The last chapter, \textbf{Chapter Nine}, consists of an ID-based directed signature scheme using bilinear pairings. A directed (or designated receiver) signature scheme is a signature scheme with the property that signatures are verifiable only with the consent of receiver. The performance of the underlying scheme is similar to the performance of the underlying identity based signature scheme.