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NFLlib: NTT-Based Fast Lattice Library

Part of the Lecture Notes in Computer Science book series (LNSC,volume 9610)

Abstract

Recent years have witnessed an increased interest in lattice cryptography. Besides its strong security guarantees, its simplicity and versatility make this powerful theoretical tool a promising competitive alternative to classical cryptographic schemes.

In this paper, we introduce NFLlib, an efficient and open-source C++ library dedicated to ideal lattice cryptography in the widely-spread polynomial ring \(\mathbb Z_{p}[x]/(x^n+1)\) for n a power of 2. The library combines algorithmic optimizations (Chinese Remainder Theorem, optimized Number Theoretic Transform) together with programming optimization techniques (SSE and AVX2 specializations, C++ expression templates, etc.), and will be fully available under an open source license.

The library compares very favorably to other libraries used in ideal lattice cryptography implementations (namely the generic number theory libraries NTL and flint implementing polynomial arithmetic, and the optimized library for lattice homomorphic encryption HElib): restricting the library to the aforementioned polynomial ring allows to gain several orders of magnitude in efficiency.

Keywords

  • C++ library
  • Implementation
  • Ideal lattice cryptography
  • Number theoretic transform
  • Chinese remainder theorem
  • SEE specializations

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Notes

  1. 1.

    The open-source IPsec-based VPN solution strongSwan [31] includes the BLISS lattice signature [9] as an IKEv2 public key authentication method starting from version 5.2.2.

  2. 2.

    This is also hinted at in the HElib library [16, 17] which modifies the internal routines of NTL to achieve better performances — although for any cyclotomic polynomial ring \(\mathbb Z_p[x]/(\Phi )\).

  3. 3.

    Even though architecture-optimized implementations will always outperform generic libraries, this paper tackles the issue of designing an efficient library that can be used on a large range of architecture. Also, NFLlib includes state-of-the-art SSE and AVX2 optimizations for the NTT and the modular multiplication operation.

  4. 4.

    At the heart of many kinds of ideal-lattice schemes (ranging from classical encryption to fully homomorphic encryption and multilinear maps) is the decision-Ring-Learning-With-Errors (dRLWE) assumption. Working with cyclotomic polynomials \(\varPhi (x)=x^n+1\) implies that we have provable worst-case hardness for dRLWE with essentially any large enough p — splitting, inert, or anywhere in between [6]. In NFLlib, we therefore chose a p that splits completely for efficiency reasons.

  5. 5.

    NFLlib has been designed to work with a wide range of parameters: polynomial degrees \(2\leqslant n\leqslant 2^{20}\) and moduli \(2^{13}<p<2^{1000\cdot 62}\). However, the users of NFLlib are responsible for selecting parameters (np) that ensure \(\kappa \) bits of security for the specific application they are developing. We refer to [2, 3, 23] for selecting concrete security parameters of lattice encryption schemes.

  6. 6.

    In HElib, the instantiation of a FHEContext — storing the modulus decomposition — is needed to use DoubleCRT objects. Now, this constructor try to produced primes of a size close to 44 bits and this size is hard-coded in the value FHE_p2Size (maybe to fit largely the long primitive type and be able to do specific homomorphic operations?).

    For the sake of comparison, we kept this hardcoded value. Therefore the benchmarks of HElib are with a 44-bit prime for parameters (1) and (2), with two 44-bit primes for parameters (3) and 141 44-bit primes for parameters (4).

  7. 7.

    TurboBoost and Hyperthreading were disabled during the benchmarks. We chose an AWS machine as a typical cloud environment which allows reproductibility of the results.

  8. 8.

    We neglected the cost of the (linear) negative wrapped convolution computation in NTL to mitigate the impact of a non highly-optimized hand-made implementation; one would therefore have to expect slightly worse timings when working over \(R_p\).

  9. 9.

    We choose two parameter sets from [23], a 14-bit modulus with polynomials of degree 256, and the same modulus with polynomials of degree 512. These two parameter sets correspond roughly to 128 and 256 bits of security. Note that if these estimates are too low it is possible to choose parameters such as (14, 1024) and the performance presented in Table 4 is just divided by two.

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Acknowledgements

This work has been supported in part by the European Union’s H2020 Programme under grant agreement number ICT-644209 and by French’s FUI project CRYPTOCOMP.

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Aguilar-Melchor, C., Barrier, J., Guelton, S., Guinet, A., Killijian, MO., Lepoint, T. (2016). NFLlib: NTT-Based Fast Lattice Library. In: Sako, K. (eds) Topics in Cryptology - CT-RSA 2016. CT-RSA 2016. Lecture Notes in Computer Science(), vol 9610. Springer, Cham. https://doi.org/10.1007/978-3-319-29485-8_20

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