Anonymity and privacy in public key cryptography
Andrew M. Odlyzko, Vishal Saraswat · 2012
The Internet has become an essential part of our daily lives. However, various agents collect information about us giving rise to a plethora of privacy concerns. This thesis aims to guard the privacy of users by providing them anonymity in their online communication. In the first part of this thesis, Chapter 2 based on our paper [17], we work on anonymity in identity based public-key cryptosystems (IBE). We provide the first ever instance and to date the only known non-trivial instance of anonymizing a non-anonymous IBE. We do so by constructing a public-key encryption with keyword search (PEKS) which is equivalent to anonymous identity-based encryption (aIBE) as proved in [1]. Our PEKS is the first ever scheme not based on bilinear forms, and is based on the hardness of the quadratic residuosity problem. We construct our PEKS scheme using a non-trivial transformation of the Cocks IBE scheme. PEKS in itself is a very useful tool. It enables delegation of searching capabilities on encrypted data to a third party, who does not hold the entire secret key, but only an appropriate token which allows searching operations while preserving data privacy. In the second part of this thesis, Chapter 3 based on our paper [28], we work on anonymity in digital signature schemes. We provide means of providing anonymity to the signer of a message by generically transforming any given digital signature scheme to an anonymous signature scheme. We present a new formalism of anonymous signature, where instead of the message as in previously presented schemes [33, 18], a part of the signature is withheld to maintain anonymity. We introduce the notion of unpretendability to guarantee infeasibility for someone other than the correct signer to pretend authorship of the message and signature. Our definition retains applicability for all previous applications of the anonymous signature, provides stronger security, and is conceptually simpler. *Please refer to dissertation for citation references.