Making NTRUEncrypt and NTRUSign as Secure as Standard Worst-Case Problems over Ideal Lattices.

Damien Stehlé, Ron Steinfeld

Joel Gärtner

Danilo Ciaffi

In 1996 Hoffstein, Pipher ad Silverman presented NTRUEncrypt, which is to date the fastest known lattice-based encryption scheme. Its moderate key-sizes, excellent asymptotic performance and conjectured resistance to quantum attacks make it a perfect candidate to succeed where factorization and discrete log fail. Unfortunately, no security proof has been produced for NTRUEncrypt nor for its signature counterpart NTRUSign. In 2013 Stehle and Steinfield proposed to apply some mild modification to the encryption and signature scheme to make them provably secure, under the assumed quantum hardness of standard worst-case lattice problems, restricted to a family of lattices related to some cyclotomic fields. In particular they showed that if the secret key polynomials of the encryption scheme are chosen from discrete Gaussians, then the public key, i.e their ratio, is statistically indistinguishable from uniform. The security will then follow from the hardness of the R-LWE problem.The aim of this thesis is to present Stehle's and Steinfield's work in a slightly more accessi-ble form, providing some more background and details in some points.

Léo Ducas, Vadim Lyubashevsky, Thomas Prest

Reza Ebrahimi Atani, Shahabaddin Ebrahimi Atani, Amir Hassani Karbasi

Jarvis and Nevins presented ETRU in 2013 which has applausive performance with moderate key-sizes and conjectured resistance to quantum computers. ETRU, as an efficient NTRUEncrypt-like cryptosystem, is over the ring of Eisenstein integers that is faster with smaller keys for the same or better level of security than does NTRUEncrypt which is a desirable alternative to public-key cryptosystems based on factorisation and discrete logarithm problem. However, because of its construction, doubts have regularly arisen on its security. In this paper, we propose how to modify ETRU to make it provably secure, under our modified assumption of quantum hardness of standard worst-case lattice problems, restricted to extended ideal lattices related to some extensions of cyclotomic fields structures. We describe the structure of all generated polynomial rings of quotient over direct product of Dedekind domains Z and Z[ζ3], where ζ3 is complex cube root of unity. We give a detailed description to show that if the private key polynomials of the ETRU are selected from direct product of some Dedekind domains using discrete Gaussians, then the public key, which is their ratio, is statistically indistinguishable from uniform over its range. The security then proves for our main system from the already proven hardness of the R-SIS and R-LWE problems by their extensions.

Wu Yan Fang, Zheng Huang, Wang Li Cheng +1 more

Since the presentation of NTRU public-key cryptosystem by Hoffstein, Pipher and Silverman, its favorable properties, such as easily created keys, high speed, excellent performance and conjectured resistance to quantum computers, have made it to be of great use. This paper proposes an enhanced scheme based on the hard learning with error over ring (R-LWE) problem to improve the security of the modified NTRUEncrypt presented by Stehle and Steinfled. We used part of the padding ideas of Fujisaki and Okamoto to obtain this scheme. It is semantically secure in strong sense of indistinguishability against adaptive chosen-ciphertext attacks in the random oracle model assuming the quantum hardness of standard worst-case problem over ideal lattices. It is also possible to arbitrarily decrease the error probability, and even to eliminate it completely. We gave the detailed analysis using the known results from classic works. Furthermore, this scheme owns many advantages such as the uniformity of public key, usual assumptions and the freedom for coding messages.