# Tag Info

14

The TL;DR: From a theoretic point of view, Gaussians are the better choice, both for the easiness of the security proof and its optimality in terms of tightness; In practice, most of the time you can replace Gaussians by other distributions without too much trouble. Theory First, let me elaborate on a few reasons why Gaussians are better in theory: When ...

8

From Status Report on the Second Round of the NIST Post-Quantum Cryptography Standardization Process 3.12 NewHope NewHope is a KEM based on the presumed hardness of the RLWE problem. At its core is Regev’s original idea for public-key encryption from plain LWE but specialized to a power-of-2 cyclotomic ring structure, enabling smaller ciphertext and key ...

6

We don’t always use power-of-two cyclotomics for RLWE. Many cryptosystems use other cyclotomics, or subfields thereof, or even other fields altogether. For example, many FHE schemes use non-two-power cyclotomics for “packing” and SIMD operations on plaintexts. However, it is simplest to properly define and use RLWE over two-power cyclotomics, in large part ...

6

There is no known reduction from LWE to MLWE (or to RLWE). That is, it could be that both MLWE and RLWE are broken, yet LWE is secure. However, this seems highly unlikely. To support the security of LWE, we have reductions showing that breaking the average-case hardness of LWE requires breaking the worst-case hardness of some lattice problems - which would ...

5

I'm assuming $n$ is a power of $2$ and that $q$ is an odd prime larger than $n$. I'm discarding the trivial case $s_1 = s_2$. If you consider everything $\mod q$, then it is most likely over the choice of $a$ that there exists $s_1 \neq s_2$ such that $\|a s_1 - a s_2\| = \sqrt{n}$. Indeed, $a$ is invertible in $R_q$ with probability about $1 - n/q$. Take $... 5 One main difference is that in Ring-LWE, the ring$R$is the full ring of integers$\mathcal{O}_K$of a number field$K$, whereas in Poly-LWE it is of the form$R=\mathbb{Z}[x]/f(x)$for some irreducible$f(x)$; this ring is (isomorphic to) an order of the number field$K=\mathbb{Q}(x)/f(x)$, but may not be the full ring of integers. Another important ... 4 I think you are mixing up some concepts... There are two things here: the embedding and the norm. Those schemes are defined over polynomial rings, but for several reasons (e.g. to relate them with lattice problems, to have some geometry) we want to analyze them in vector spaces. So, representing those ring elements as vectors is what we call embedding. ... 4 I believe it is also used in other lattice based schemes that use standard LWE. For example, the Frodo paper. They used a$seed_A$and a Gen($\cdot$) function to compute$A$. Then Alice sends$seed_A$instead of$A$for the actually exchange. Gen($\cdot$) is a prior-agreed pseudorandom function that extracts and extends the seed. 4 δ_0: the root Hermite factor required β: the BKZ block size d: the dimension of the lattice being reduced m: the number of LWE samples used 4 For the second question, it depends on whether we can trust Bob to generate$a$uniformly at random and without any “hidden structure.” If we can trust him, then this is secure assuming RLWE is hard; what Bob gets is merely one properly distributed RLWE sample. However, if Bob may choose$a$in a malicious or “sneaky” way, then there can be attacks. For ... 3 Yes, there are. The following table is taken from this paper of Ducas and van Woerden, although the results are not derived there (in the below,$p$is an odd prime, and$n, mare coprime). \begin{align*} \mathbb{Z}[\zeta_{2^k}]&\cong \mathbb{Z}^{2^{k-1}}\\ \mathbb{Z}[\zeta_p] &\cong A_{p-1}^*\\ \mathbb{Z}[\zeta_{p^k}]&\cong \bigoplus_{i = 1}^{p^... 3 The decomposition helps with the noise growth of your scheme. You see, the decryption only works if you "error" (here the terms you denote bye$) is small enough. If$t*e$grows beyong the modulus (all the computations are done modulo some number, let's say$q$), the mod$t$operation won't yield the message$m$, but$m$+$t*e$mod$q$. Now when you're ... 3 By looking the encryption procedure, you will see that we use a different sum of the vectors$\vec a_i$'s at each encryption. Thus, every ciphertext has the form $$(\vec a, ~\vec a\cdot \vec s + e + \frac{p}{2} \cdot m)$$ with different terms$\vec a \in \mathbb{Z}_p^n$and$e \in \mathbb{Z}$. Notice that the probability that two ciphertexts have the same ... 3 There is a condition that is not considered, that is, the value after modulo 257 should be in$\mathbb Z_q$. When$q = 257, \mathbb Z_q = \{ -128, ... , 128 \}$, so,$(4+128)\mod 257$should be$-125$rather than$132$. And$-125 \mod 2 = 1$. Thus,$sk_a \neq sk_b$and the output of oracle$\mathcal B$is$0$. 3 (The full version of the paper is at https://eprint.iacr.org/2012/230, and my answer below refers to it.) The answer to your question is that a different part of the reduction ensures that the oracle has advantage very close to 1 over the random samples$(a_i, a_i s + e_i + r_i)$, i.e., the oracle outputs the correct answer for almost all choices of$s, a_i,...

3

Everything you write looks correct. However, you may be expecting the distributed decryption protocol to have a security property that it does not (and was not intended to, and really cannot in your example) have. Specifically, the Mukherjee-Wichs paper you linked defines security to say (roughly) that, given the evaluated ciphertext, its underlying ...

3

The ciphertexts contains a certain amount of noise for security reasons. The downside is that if this noise is too big, the decryption will fail. When using homomorphic operations, the noise contained in the output ciphertext will be bigger than the one in the input ciphertexts. Now when adding ciphertexts, the new noise is just the sum of previous noises, ...

3

Whether the interaction will reveal Alice's secret key? I'm reading this like, Bob chose the base point $G$ and sends it to Alice then Alice sends back to Bob $[s]G$. The reason is simple, the elliptic curves use scalar multiplication, not exponentiation. This notation is used for multiplicative groups. This commitment now turns into a discrete logarithm ...

2

Instead of plain matrix, a matrix similar this (but bigger): +a -h -g -f -e -d -c -b +b +a -h -g -f -e -d -c +c +b +a -h -g -f -e -d +d +c +b +a -h -g -f -e +e +d +c +b +a -h -g -f +f +e +d +c +b +a -h -g +g +f +e +d +c +b +a -h +h +g +f +e +d +c +b +a which one can deduce it's equivalant in addition and multiplication to a polynomial reduced by $X^8+1$. ...

2

The security proofs on the papers about (R)LWE use several samples (usually denoted by $m$) because then the results (and the security guarantees) are stronger. And, anyway, they usually give upper bounds to $m$ (as being at most polynomially big in $n$), but not lower bounds. For both, the decisional and the search version of the problem, giving less ...

2

Sage itself has an internal negacyclic convolution, which is what is necessary here. To avoid type errors, we convert the polynomials to lists of coefficients, and work with those instead: from sage.rings.polynomial.convolution import _negaconvolution_fft n = 10 # degree 1024 Rq = GF(40961) R.<X> = PolynomialRing(Rq) S.<x> = R.quotient_ring(X^(...

2

Why use $f(x) = x^{2^n} + 1$? This is a very special polynomial called a "cyclotomic" polynomial. Cyclotomic polynomials are very interesting, and I recommend reading up on them separately (wikipedia). One very useful property is that the $m^{th}$ cyclotomic polynomial has roots that are all the $m^{th}$ primitive roots of unity. For any $n \geq 1$, the ...

2

You should visit the Homomorphic Encryption Standardization web page. There you can find Homomorphic Encryption Standardization. Also, there is a workshop The Second Homomorphic Encryption Standardization Workshop there you can find this document Homomorphic Encryption Standard see section 2.0.3 I hope, all this will help in your research.

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The "guesses" are part of the enumeration of all possible values of $s\bmod \mathfrak{q}_iR^\vee$. The reduction has two parts essentially: A way of telling if the guess was correct (the $\mathsf{WDLWE}_{q, \Psi}^i$ oracle) A small space of values to try (as values $s\bmod \mathfrak{q}_i R^\vee$ are within $R_q^\vee/\mathfrak{q}_iR_q^\vee$, which ...

1

There are to my knowledge two articles that develop further on generalized member test (or even function) extraction, I hope this may help you out: https://link.springer.com/chapter/10.1007/978-3-319-22174-8_7 and https://eprint.iacr.org/2017/996.pdf A small difference there is that the ring dimension is a prime rather than a power of 2, and this ...

1

Let's first think about the ordinary LWE. There are two ways of choosing the secret, one is that there is no restriction on the secret; Another way is to choose a short secret. Applebaum, Cash, Peikert, Sahai prove that the latter is still sound. In other words, the secret can be chosen uniformly from the polynomial ring or from its bounded distribution, ...

1

You're right that essentially all schemes require that the secrets are small in a concrete sense. While I'm not terribly familiar with the standard, I believe the tables in question were generated by the LWE Estimator of Albrecht et al. This page of its documentation states various secret distributions they support. I expect "uniform" corresponds to "bounded ...

1

convert $v-u \cdot s$ to centered representation as described here seems to work

1

I guess the latest real production grade homomorphic library is Microsoft SEAL, which implements the BFV and the CKKS encryption schemes. I'm not a big MS fan. There are other options to explore: HELib implements the BGV scheme with GHS optimizations. NuFHE implements a GPU reference of fully homomorphic encryption on torus Also checkout the open group ...

1

This is necessary for security. Consider if $q$ was a multiple of $t$, so $q = v\cdot t$. Take your ciphertext $(a, \langle a, s \rangle + t\cdot e + m)$ and multiply through by $v$. You now have $$(v\cdot a, \langle v\cdot a, s \rangle + v\cdot t\cdot e + v \cdot m) = (v\cdot a, \ \langle v \cdot a, s \rangle + v\cdot m) \mod q$$ Suppose you're playing ...

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