Analysis and Improvements of NTRU Encryption Paddings

NTRU is an efficient patented public-key cryptosystem proposed in 1996 by Hoffstein, Pipher and Silverman. Although no devastating weakness of NTRU has been found, Jaulmes and Joux presented at Crypto ’00 a simple chosen-ciphertext attack against NTRU as originally described. This led Hoffstein and Silverman to propose three encryption padding schemes more or less based on previous work by Fujisaki and Okamoto on strengthening encryption schemes. It was claimed that these three padding schemes made NTRU secure against adaptive chosen-ciphertext attacks (INDCCA2) in the random oracle model. In this paper, we analyze and compare the three NTRU schemes obtained. It turns out that the first one is not even semantically secure (IND-CPA). The second and third ones can be proven IND-CCA2–secure in the random oracle model, under however rather unusual assumptions. They indeed require a partialdomain one-wayness of the NTRU one-way function which is likely to be a stronger assumption than the one-wayness of the NTRU one-way function. We propose several modifications to achieve IND-CCA2–security in the random oracle model under the original NTRU inversion assumption.