Abstract. This paper provides either security proofs or attacks for a large number of identity-based identification and signature schemes defined either explicitly or implicitly in existing literature. Underlying these are a framework that on the one hand helps explain how these schemes are derived, and on the other hand enables modular security analyses, thereby helping to understand, simplify and unify previous work. 1

Email phishing attacks are one of today’s most common and costly forms of digital identity theft, where an adversary tricks a user into revealing their personal information by impersonating an established company. Such attacks could be mitigated with digitally-signed emails, if these signatures did not: (1) destroy the traditional repudiability of email, and (2) require the unrealistic, widespread adoption of a Public-Key Infrastructure (PKI). In order to overcome these obstacles, we introduce, define, and implement separable (a.k.a. crossdomain) identity-based ring signatures (SIBR, pronounced “cyber, ” signatures). The ring structure of these signatures provides repudiability. With identity-based public keys, a full PKI is no longer required. Separability allows ring constructions across different identity-based master key domains. Together, these properties make SIBR signatures a practical solution to the email spoofing problem. Our construction yields a number of interesting components. First, we present several novel proofs of knowledge of bilinear map pre-images. We then present new identity-based identification (IBI) and signature (IBS) schemes based on these proofs. We note how our constructions share system parameters with the existing identity-based encryption schemes of Boneh-Franklin and Waters, thereby forming complete identity-based cryptosystems. We finally construct the first SIBR signature schemes by transforming our new signature schemes and certain other signature schemes.

A leakage-resilient cryptosystem remains secure even if arbitrary, but bounded, information about the secret key (or possibly other internal state information) is leaked to an adversary. Denote the length of the secret key by n. We show a signature scheme tolerating (optimal) leakage of up to n − nǫ bits of information about the secret key, and a more efficient one-time signature scheme that tolerates leakage of ( 1 4 −ǫ) ·n bits of information about the signer’s entire state. The latter construction extends to give a leakage-resilient t-time signature scheme. All these constructions are in the standard model under general assumptions. 1

ID-based cryptosystem has been, for a few years, the most active area of research and currently is of great interest to the cryptographic society. In this work we survey three fundamental ID-based cryptographic primitives Digital Signature, Encryption and Key Agreement, which are based on the mathematical concepts Integer Factorization, Quadratic Residues and Bilinear Pairings. We review several schemes along with their efficiency and security considerations. The survey helps in understanding the research work carried out in the area of ID-based cryptosystems from the year 1984 to 2004.

Abstract. In this paper, we present some security flaws of the keyexposure free chameleon hash scheme based on factoring [9]. Besides, we propose an improved chameleon hash scheme without key exposure based on factoring which enjoys all the desired security notions of chameleon hashing.