Cryptanalysis of countermeasures against multiple transmission attacks on NTRU

The original Number Theory Research Unit (NTRU) public key cryptosystem is vulnerable to multiple transmission attacks, and the designers of NTRU presented two countermeasures to prevent such attacks. In this study, the authors show that the first countermeasure is still not secure, the plaintext can be revealed by a linearisation attack technique. Moreover, they demonstrate that the first countermeasure is even not secure for broadcast attacks, a class of more general attacks than multiple transmission attacks. For the second countermeasure, they show that one special case of its padding function for the plaintext is also insecure and the original plaintext can be obtained by lattice methods.