## CryptoDB

### Sanjit Chatterjee

#### Publications

Year
Venue
Title
2015
ASIACRYPT
2015
ASIACRYPT
2014
EPRINT
2012
PKC
2010
EPRINT
We focus on the implementation and security aspects of cryptographic protocols that use Type 1 and Type 4 pairings. On the implementation front, we report improved timings for Type 1 pairings derived from supersingular elliptic curves in characteristic 2 and 3 and the first timings for supersingular genus-2 curves in characteristic 2 at the 128-bit security level. In the case of Type 4 pairings, our main contribution is a new method for hashing into ${\mathbb G}_2$ which makes the Type 4 setting almost as efficient as Type 3. On the security front, for some well-known protocols we discuss to what extent the security arguments are tenable when one moves to genus-2 curves in the Type 1 case. In Type 4, we observe that the Boneh-Shacham group signature scheme, the very first protocol for which the Type 4 setting was introduced in the literature, is trivially insecure, and we describe a small modification that appears to restore its security.
2009
EPRINT
In 2003, Boneh, Gentry, Lynn and Shacham (BGLS) devised the first provably-secure aggregate signature scheme. Their scheme uses bilinear pairings and their security proof is in the random oracle model. The first pairing-based aggregate signature scheme which has a security proof that does not make the random oracle assumption was proposed in 2006 by Lu, Ostrovsky, Sahai, Shacham and Waters (LOSSW). In this paper, we compare the security and efficiency of the BGLS and LOSSW schemes when asymmetric pairings derived from Barreto-Naehrig (BN) elliptic curves are employed.
2007
EPRINT
At Eurocrypt 2005, Boneh, Boyen and Goh presented a constant size ciphertext hierarchical identity based encryption (HIBE) protocol. Our main contribution is to present a variant of the BBG-HIBE. The new HIBE is proved to be secure (without any degradation) in an extension of the sID model (denoted the s$^+$-ID model) and the components of the identities are from $\bbbz_p$, where $p$ is a suitable large prime. The BBG-HIBE is proved to be secure in the selective-ID (sID) security model and the components of the identities are from $\bbbz_p^*$. In the s$^+$-ID model the adversary is allowed to vary the length of the challenge identity whereas this is not allowed in the sID model. The new HIBE shares all the good features of the BBG-HIBE. The drawback is that the public parameters and the private key are longer than that of the BBG-HIBE. We also provide two more extensions of the basic constant size ciphertext HIBE. The first is a constant size ciphertext HIBE secure in the generalised selective-ID model $\clsM_2$. The second one is a product construction composed of two HIBEs and a trade-off is possible between the private key size and the ciphertext size.
2006
ASIACRYPT
2006
PKC
2006
EPRINT
We generalize the selective-ID security model for HIBE by introducing two new security models. Broadly speaking, both these models allow the adversary to commit to a set of identities and in the challenge phase choose any one of the previously committed identities. Two constructions of HIBE are presented which are secure in the two models. Further, we show that the HIBEs can be modified to obtain a multiple receiver IBE which is secure in the selective-ID model without the random oracle assumption.
2006
EPRINT
At Eurocrypt 2005, Waters proposed an efficient identity based encryption (IBE) scheme. One drawback of this scheme is that the size of the public parameter is rather large. Our first contribution is a generalization of Waters scheme. In particular, we show that there is an interesting trade-off between the tightness of the security reduction and smallness of the public parameter size. For a given security level, this implies that if one reduces the public parameter size there is a corresponding increase in the computational cost. This introduces a flexibility in choosing the public parameter size without compromising in security. In concrete terms, to achieve $80$-bit security for 160-bit identities we show that compared to Waters protocol the public parameter size can be reduced by almost $90 \%$ while increasing the computation cost by $30\%$. Our second contribution is to extend the IBE protocol to a hierarchical IBE (HIBE) protocol which can be shown to be secure in the full model without the use of random oracle. A previous construction of a HIBE in the same setting is due to Waters. Our construction improves upon Waters' suggestion by significantly reducing the number of public parameters.
2006
EPRINT
The current work considers the problem of obtaining a hierarchical identity-based encryption (HIBE) protocol which is secure against adaptive key extraction and decryption queries. Such a protocol is obtained by modifying an earlier protocol by Chatterjee and Sarkar (which, in turn, is based on a protocol due to Waters) which is secure only against adaptive key extraction queries. The setting is quite general in the sense that random oracles are not used and security is based on the hardness of the decisional bilinear Diffie-Hellman (DBDH) problem. In this setting, the new construction provides the most efficient (H)IBE protocol known till date. The technique for answering decryption queries in the proof is based on earlier work by Boyen, Mei and Waters. Ciphertext validity testing is done indirectly through a symmetric authentication algorithm in a manner similar to the Kurosawa-Desmedt public key encryption protocol. Additionally, we perform symmetric encryption and authentication by a single authenticated encryption algorithm.

PKC 2019
Asiacrypt 2013