(a) We give a simple modification of Waters signatures, where messages are encoded such that each two encoded messages have a suitably large Hamming distance. Somewhat surprisingly, this simple modification suffices to prove security under the CDH assumption with a reduction loss of O(q).
(b) We also show that any black-box security proof for a signature scheme with re-randomizable signatures must have a reduction loss of at least \Omega(q), or the underlying hardness assumption is false. Since both Waters signatures and our variant from (a) are re-randomizable, this proves our reduction from (a) optimal up to a constant factor.
Understanding and optimizing the security loss of a cryptosystem is important to derive concrete parameters, such as the size of the underlying group. We provide a complete picture for Waters-like signatures: there is an inherent lower bound for the security loss, and we show how to achieve it.Category / Keywords: public-key cryptography / Digital signatures, Waters signatures, provable security, black-box reductions Publication Info: PKC 2012 Date: received 23 Dec 2011, last revised 18 Sep 2012 Contact author: tibor jager at kit edu Available formats: PDF | BibTeX Citation Note: Typos fixed Version: 20120918:141600 (All versions of this report) Discussion forum: Show discussion | Start new discussion