Weak property of malleability in NTRUSign

Abstract

A new type of signature scheme, called NTRUSign, based on solving the approximately closest vector problem in a NTRU lattice was proposed at CT-RSA'03. However no security proof against chosen messages attack has been made for this scheme. In this paper, we show that NTRUSign signature scheme contains the weakness of malleability. From this, one can derive new valid signatures from any previous message-signature pair which means that NTRUSign is not secure against strongly existential forgery. Finally, we propose a simple technique to avoid this flaw in NTRUSign scheme.