- C Pandu Rangan

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- PublicationSanitizable signatures with strong transparency in the standard model(21-12-2010)
;Agrawal, Shivank ;Kumar, Swarun ;Shareef, AmjedShow more Sanitizable signatures provide several security features which are useful in many scenarios including military and medical applications. Sanitizable signatures allow a semi-trusted party to update some part of the digitally signed document without interacting with the original signer. Such schemes, where the verifier cannot identify whether the message has been sanitized, are said to possess strong transparency. In this paper, we have described the first efficient and provably secure sanitizable signature scheme having strong transparency under the standard model. © 2010 Springer-Verlag.Show more - PublicationForcing out a confession threshold discernible ring signatures(01-12-2010)
;Kumar, Swarun ;Agrawal, Shivank ;Venkatesan, Ramarathnam ;Lokam, SatyaShow more Ring signature schemes (Rivest et al., 2001) enable a signer to sign a message and remain hidden within an arbitrary group A of n people, called a ring. The signer may choose this ring arbitrarily without any setup procedure or the consent of anyone in A. Among several variations of the notion, step out ring signatures introduced in (Klonowski et al., 2008) address the issue of a ring member proving that she is not the original signer of a message, in case of dispute. First we show that the scheme in (Klonowski et al., 2008) has several flaws and design a correct scheme and prove formally the security of the same. Then we use the basic constructs of our scheme to design a protocol for a new problem, which we refer to as threshold discernible ring signatures. In threshold discernible ring signatures, a group B of t members can co-operate to identify the original signer of a ring signature that involved a group A of n alleged signers, where B C A and n > t. This is the first time that this problem is considered in the literature and we formally prove the security of our novel scheme in the random oracle model.Show more - PublicationA linear algorithm for the all-bidirectional-edges problem on planar graphs(01-03-1993)
;Ramprasad, P. B.Show more The all-bidirectional-edges problem is to find an edge-labeling of an undirected network G=(V, E), with a source and a sink, such that an edge e=uv in E is labeled 〈u, v〉 or 〈u, u〉 (or both) depending on the existence of a (simple) path from the source to the sink traversing e, that visits the vertices u and v in the order u, v or v, u respectively. The best-known algorithm for this problem requires O(|V|·|E|) time [5]. We show that the problem is solvable optimally on a planar graph. © 1993 Springer-Verlag New York Inc.Show more - PublicationCommunication optimal multi-valued asynchronous broadcast protocol(27-08-2010)
;Patra, ArpitaShow more Broadcast (BC) is considered as the most fundamental primitive for fault-tolerant distributed computing and cryptographic protocols. An important and practical variant of BC is Asynchronous BC (known as A-cast). An A-cast protocol enables a specific party called sender (possibly corrupted) to send some message identically to a set of parties despite the presence of an adversary who may corrupt some of the parties in a malicious manner. Though the existing protocol for A-cast is designed for a single bit message, in real life applications typically A-cast is invoked on long message (whose size can be in gigabytes) rather than on single bit. Therefore, it is important to design efficient multi-valued A-cast protocols (i.e protocols with long message) which extract several advantages offered by directly dealing with long messages and are far better than multiple invocations to existing protocols for single bit. In this paper, we design highly efficient, communication optimal, optimally resilient multi-valued A-cast protocol for long messages, based on access to the existing A-cast protocol for short messages. Our protocol also provides better communication complexity than existing protocol for A-cast. © 2010 Springer-Verlag Berlin Heidelberg.Show more - PublicationThe round complexity of verifiable secret sharing revisited(29-10-2009)
;Patra, Arpita ;Choudhary, Ashish ;Rabin, TalShow more The round complexity of interactive protocols is one of their most important complexity measures. In this work we prove that existing lower bounds for the round complexity of VSS can be circumvented by introducing a negligible probability of error in the reconstruction phase. Previous results show matching lower and upper bounds of three rounds for VSS, with n = 3t + 1, where the reconstruction of the secrets always succeeds, i.e. with probability 1. In contrast we show that with a negligible probability of error in the reconstruction phase: 1 There exists an efficient 2-round VSS protocol for n = 3t + 1. If we assume that the adversary is non-rushing then we can achieve a 1-round reconstruction phase. 1 There exists an efficient 1-round VSS for t = 1 and n > 3. 1 We prove that our results are optimal both in resilience and number of sharing rounds by showing: 1 There does not exist a 2-round WSS (and hence VSS) for n ≤ 3t. 1 There does not exist a 1-round VSS protocol for t ≥ 2 and n ≥ 4. © 2009 Springer.Show more - PublicationOptimal Parallel Algorithm for Finding st-Ambitus of a Planar Biconnected Graph(01-01-1996)
;Easwarakumar, K. S. ;Krishnan, S. V.; Seshadri, S.Show more A cycle C passing through two specific vertices s and t of a biconnected graph is said to be an st-ambitus if its bridges do not interlace in some special way. We present algorithms for st-ambitus for planar biconnected graphs, which are much simpler than the one known for general graphs [MT]. Our algorithm runs in O(n) time on a sequential machine and (log n) parallel time using O(n/log n) processors on an EREW PRAM.Show more - PublicationEfficient compilers for after-the-fact leakage: From cpa to cca-2 secure pke to ake(01-01-2017)
;Chakraborty, Suvradip ;Paul, GoutamShow more The goal of leakage-resilient cryptography is to construct cryptographic algorithms that are secure even if the adversary obtains side-channel information from the real world implementation of these algorithms. Most of the prior works on leakage-resilient cryptography consider leakage models where the adversary has access to the leakage oracle before the challenge-ciphertext is generated (before-the-fact leakage). In this model, there are generic compilers that transform any leakage-resilient CPA-secure public key encryption (PKE) scheme to its CCA-2 variant using Naor-Yung type of transformations. In this work, we give an efficient generic compiler for transforming a leakage-resilient CPA-secure PKE to leakage-resilient CCA-2 secure PKE in presence of after-the-fact split-state (bounded) memory leakage model, where the adversary has access to the leakage oracle even after the challenge phase. The salient feature of our transformation is that the leakage rate (defined as the ratio of the amount of leakage to the size of secret key) of the transformed after-the-fact CCA-2 secure PKE is same as the leakage rate of the underlying after-the-fact CPA-secure PKE, which is 1 − o(1). We then present another generic compiler for transforming an after-the-fact leakage-resilient CCA-2 secure PKE to a leakage-resilient authenticated key exchange (AKE) protocol in the bounded after-the-fact leakage-resilient eCK (BAFL-eCK) model proposed by Alawatugoda et al. (ASIACCS’14). To the best of our knowledge, this gives the first compiler that transform any leakage-resilient CCA-2 secure PKE to an AKE protocol in the leakage variant of the eCK model.Show more - PublicationOptimal parallel algorithms for path problems on planar graphs(10-07-1995)
;Srikrishna, G.Show more Given a biconnected planar graph G and a pair of vertices s and t, the two disjoint problem asks to find a pair of internally disjoint paths from s to t. We present a simple and efficient parallel algorithm for the same. Our algorithm uses the notion of bridges in a novel way and this results in a more elegant and simple algorithm than the existing one. The all-bidirectional-edges (ABE) problem is to find an edge labeling such that an edge (u, v) in E is labeled 〈u, v〉 or 〈v, u〉 or both depending on the existence of a simple path from s to t that visits the vertices in the order u, v or v, u or both, respectively. We present an optimal parallel algorithm for the same. © 1995.Show more - PublicationAn identity based ring signcryption scheme with public verifiability(01-12-2010)
;Selvi, S. Sharmila Deva ;Vivek, S. Sree ;Anand, Sakhi S.Show more Signcryption is a cryptographic primitive which offers authentication and confidentiality simultaneously with a cost lower than signing and encrypting the message independently. Ring signcryption enables a user to anonymously signcrypt a message on behalf of a set of users including himself. Thus a ring signcrypted message has anonymity in addition to authentication and confidentiality. Ring signcryption schemes have no centralized coordination: any user can choose a ring of users, that includes himself and signcrypt any message without any assistance from the other group members. Ring Signcryption is useful for leaking trustworthy secrets in an anonymous, authenticated and confidential way. To the best of our knowledge, ten identity based ring signcryption schemes are reported in the literature. Three of them were proved to be insecure in (Li et al., 2008a), (Zhang et al., 2009a) and (Vivek et al., 2009). Four of them were proved to be insecure in (Selvi et al., 2009). In this paper, we show that one among the remaining three schemes, (Zhang et al., 2009b) is not secure against confidentiality, existential unforgeability and anonymity attacks. We propose a new anonymous ring signcryption scheme which is an extension to (Selvi et al., 2009) and give formal security proofs for our system in the random oracle model. Our scheme is publicly verifiable which none of the existing unbroken schemes can achieve.Show more - PublicationA new linear algorithm for the two path problem on chordal graphs(01-01-1988)
;Krishnan, S. V.; Seshadri, S.Show more Let G= (V,E) be a finite undirected graph with four distinguished vertices s, t, u, v. The two path problem (TPP) is to determine whether there exist two vertex disjoint paths connecting s with t and u with v and to find such paths if they exist. In this paper, a simple and efficient algorithm for TPP restricted to 2-connected chordal graphs is given. The reduction of TPP for a general chordal graph to the TPP for a 2-connected chordal graph is outlined.Show more