In this post, we discuss an important class of algebraic structures known as $q$-ary lattices that are central to lattice-based cryptographic primitives.

Hardness of problems

Computational hardness usually revolves around problems with worst-case hardness guarantees since we want to design algorithms that run efficiently even on the worst possible input.

On the other hand, cryptographic schemes require security guarantees for random keys. Therefore, cryptographic applications require problems with average-case hardness guarantees.

However, it is not immediately apparent how to create hard instances of problems with worst-case hardness guarantees. In other words, how to design the most secure keys for cryptographic applications is unclear.

Any random instance of a problem with average-case hardness guarantee is hard with a positive probability. Suppose one could show that such a random instance is as hard as the hardest instance (a reduction from average-case hardness to worst-case hardness). In that case, such a reduction immediately lends itself to applications in cryptography. This reduction is precisely the central thesis of the seminal work Ajtai'96 ¹.

Lattice-based Cryptography

What is a Lattice?

A Lattice $\Lambda$ or $\mathcal{L}$ is defined as the set of all integer linear combinations of $n$ linearly independent $m$-dimensional vectors $B=\{\mathrm{b_1},\mathrm{b_2},\ldots,\mathrm{b_n}\}$. Formally, we denote lattices as $$\mathcal{L}(B)=\{Bx, x\in\mathbb{Z}^{m}\}=\left\{\sum_{i=1}^{n}x_i\mathrm{b_i} : x_i\in\mathbb{Z}\right\}.$$ The set $B\in\mathbb{R}^{n\times m}$ is said to be the basis of the lattice $\Lambda$. We note here that **any lattice $\Lambda$ is characterized by its basis**. Shortly, *we will generalize the definition of lattices* by showing a different characterization of lattices which will be useful in cryptographic construction schemes.

Why Lattice-based Cryptography?

Ajtai'96 ¹ showed reductions from lattice based problems with average-case hardness guarantees to lattice problems with worst-case guarantees. Intuitively, this shows that any randomly sampled instance of a lattice is as hard as the hardest instance.

Later in this post, we will discuss a few lattice problems with known worst-case hardness guarantees. This immediately makes lattices an extremely important tool in cryptography.

In fact, lattice based cryptographic constructions are invaluable for their many potential advantages as follows:

1. Lattice-based schemes usually only require linear operations on integers which leads to asymptotic efficiency.
2. Lattice-based schemes have been shown to be resistant to cryptanalysis by quantum algorithms unlike current classical cryptographic schemes which are based on factoring or discrete log (Shor'95 ²). This makes lattice-based cryptography the cornerstone of post-quantum cryptography.
3. As noted earlier, random instances of lattice based constructions are “as hard as possible”, which lends itself to conceptual simplicity while designing cryptographic schemes.

Hard Lattice problems w/ known worst-case hardness guarantees.

In this section, we discuss some important worst-case hard problems associated with lattices. As stated earlier, we would like to show a reduction from randomly generated lattices to instances of one of these problems.

Shortest Vector Problem $\left(\mathrm{SVP}_\gamma\right)$

In the $\gamma$-approximate Shortest Vector Problem, we are asked find the length of the shortest non-zero vector (denoted by $\lambda_1$) in an $n$-dimensional lattice, approximately, up to a polynomial factor $\gamma$.

• It is known that the approximate (and also decision) SVP is NP-hard under a randomized reduction.³

Shortest Independent Vector Problem $\left(\mathrm{SIVP}_\gamma\right)$

The goal of the $\gamma$-approximate Shortest Independent Vector problem is to output a set of $n$ linearly independent lattice vectors in an $n$-dimensional lattice, approximately of length$\leq\gamma\lambda_n$.

• SIVP is NP-hard to approximate for any constant approximation factor.⁴

Shortest Basis Problem $\left(\mathrm{SBP}_\gamma\right)$

The $\gamma$-approximate Shortest Basis Problem asks us to find a basis $B={\mathrm{b_1},\mathrm{b_2},\ldots,\mathrm{b_n}}$ for an $n$-dimensional lattice $\mathcal{L}(B)$ such that $\mathrm{max}_i \lVert\mathrm{b_i}\rVert$ is the smallest possible upto a factor of $\gamma$.

Later in this post we will see the connection between the above problems.

Constructing random instances of a Hard Lattice problem

We now state some important results from Ajtai'96¹ and Ajtai'99⁵ which deal with the construction of random instances of hard lattice problems. The following lemma shows that hardness of SBP by reduction to SVP.

Lemma 1 (Ajtai'96)

If $\mathrm{SBP}_\gamma$ has no polynomial time solution for $\gamma=\mathrm{poly}(n)$, then we can generate a random lattice $\Lambda$ (over some distribution $\mathcal{D}$) together with a “short” vector $x\in\Lambda$ in polynomial time s.t. there is no algorithm which can find a vector shorter than $\sqrt{n}$ in $\Lambda$ over $\mathcal{D}$ w.p. greater than $n^{-c}$, for sufficiently large $n$ and any $c>0$.

Note: Here “short'' refers to $|x|\leq\sqrt{n}$. Later Ajtai'99 ⁵ extended Lemma 1 to the following theorem:

Theorem 2 (Main Theorem, Ajtai'99)

If $\mathrm{SBP}_\gamma$ has no polynomial time solution for $\gamma=\mathrm{poly}(n)$, then we can generate a random lattice $\Lambda$ (from the same distribution $\mathcal{D}$ as in Lemma 1 together with a “short” basis $B$ of $\Lambda$ in polynomial time s.t. there is no algorithm which can find a vector shorter than $\sqrt{n}$ in $\Lambda$ over $\mathcal{D}$ w.p. greater than $n^{-c}$, for sufficiently large $n$ and any $c>0$.

As we see, Theorem 2 clearly gives us a way to construct a random instance of a hard problem ($\mathrm{SBP}_\gamma$). A short basis means $\lVert \mathrm{b_i} \rVert\leq n\sqrt{n},i\in[n]$, where $B=\{\mathrm{b_1},\mathrm{b_2},\ldots,\mathrm{b_n}\}$. In the next, we concretely define the class of “random” lattices required by the above results.

The SIS problem

In this section, we take a deeper dive into the special class of random lattices given by Ajtai'96¹, where it was shown how to

• Construct a family of one-way functions (based on the earlier family of random lattices as seen in Lemma 1), s.t.,
• If we invert any function from this family of functions with non-negligible probability, it implies that we can solve any instance of the $\mathrm{SVP}_\gamma$ for $\gamma=O({n^c}),c>1$. Therefore, by the previous discussion, we see that the above construction implies an average-case to worst-case reduction. GGH11⁶ later extended the analysis of Ajtai96 to show that the family of one-way functions is also collision-resistant, which is a stronger and more useful property for cryptographic applications.

We now define the SIS problem and state a lemma (without proof) of its hardness.

Short Integer Solution ($\mathrm{SIS}_{n,m,q,\beta}$)

Let $A$ be a uniformly random matrix , i.e. , $A=\{a_0| a_1 | \ldots | a_{m-1}\}$ consists of $m$ uniformly random vectors $a_i\in\mathbb{Z}^n_q,i\in[m]$. Find a nontrivial $x\in\mathbb{Z}^m$, s.t. $|x|\leq\beta$ and $Ax\equiv 0\mod{q}$.

Here $A\sim\mathbb{Z}_{q}^{n\times m}$ where $\mathbb{Z}^n_q$ is the set of all $n$-dimensional integer vectors modulo $q$, i.e., $\mathbb{Z}^n_q={\left\langle x_1,\ldots, x_n \right\rangle}$, where $x_i\in{0,1,\ldots,q-1}$ for all $i\in[n]$.

We also note here that without the constraint $|x|\leq\beta$, the $\mathrm{SIS}$ is easy to solve using Gaussian elimination.

Hardness of $\mathrm{SIS}$

For any $m = \mathrm{poly}(n)$, any $\beta > 0$, and any sufficiently large $q \geq \beta n^c$ (for any constant $c >0$), solving $\mathrm{SIS}({n,m,q,\beta})$ with nonnegligible probability is at least as hard as solving $GapSVP_\gamma$ and ${SIVP}_\gamma$ for some $\gamma = \beta \cdot O(\sqrt{n})$ with a high probability in the worst-case scenario.

Now we turn our attention to the construction of GGH11⁶ based on the hardness of $\mathrm{SIS}$.

A specific CRHF construction (GGH11)

We define a Collision-Resistant Hash function (CRHF) as follows:

• Set $m>n\frac{\log{q}}{\log{d}}$.
• Key: A matrix $A$ chosen uniformly at random from $\mathbb{Z}^{m\times n}_q$.
• Hash function: Define a Shrinking function (shrinking since the domain is greater than the range): $f_A:{0,\ldots, d-1}^m\xrightarrow{}\mathbb{Z}^n_q$ s.t. $f_A(x)=Ax\mod q$.
• Goal: Find $x,x^{\prime}\in{0,1}^m$ s.t. $f_A(x)=f_A(x^{\prime})$.

Note that finding a solution to the collision problem yields a solution $z=x-x^\prime$ to the $\mathrm{SIS}$ problem as $0=f_A(x)-f_A(x^{\prime})=Ax-Ax^\prime=Az\equiv 0\mod{q}$.

We now define a class of random lattices known as $q$-ary lattices and connect it to the $\mathrm{SIS}$ problem.

$q$-ary Lattices at last!

Given $A\sim \mathbb{Z}^{n\times m}_q$, $q$-ary lattices $\Lambda_q^\perp(A)$ are defined as $$\Lambda_q^\perp(A)=\{z\in\mathbb{Z}^m:Az=0\mod{q}\}$$ We note that the randomness of $\Lambda_q^\perp(A)$ stems from the randomness of $A$ itself. $\Lambda_q^\perp(A)$ contains all vectors that are orthogonal modulo $q$ to the rows of $A$, i.e. $\Lambda_q^\perp(A)$ corresponds to the code whose parity check matrix is $A$.

Here, we see that we have successfully characterized a lattice in terms of a parity check matrix $A$ instead of a basis. The class of $q$-ary lattices is the class of random lattices required in Lemma 1 and Theorem 2. We end this article with another lemma by Ajtai96 and some properties of $q$-ary lattices which connects the average-case hardness guarantees with worst-case guarantees.

Lemma 2 (Ajtai96)

Finding short $z\in\Lambda_q^\perp(A)$ s.t. $\left|{z}\right|\leq \beta < q$, implies solving $\mathrm{GapSVP}_{\beta\sqrt{n}}$ on any n-dimensional lattice.

Properties of $q$-ary Lattices

• A $q$-ary lattice $L$ satisfies $q\mathbb{Z}^n \subseteq L \subseteq \mathbb{Z}^n$ for $q\in\mathbb{Z}$.
• Any integer lattice $\mathcal{L}\subseteq\mathbb{Z}^n$ is a $q$-ary lattice for some $q$, e.g., whenever q is an integer multiple of the determinant $\mathrm{det}(\mathcal{L})$. We are only interested when $q$ is much smaller than $\left|{\mathrm{det}(L)}\right|$.
• The following two problems are equivalent:
  • Finding a short vector in $\Lambda_q^\perp(A)$ s.t. $A\sim \mathbb{Z}^{n\times m}_q$.
  • Finding a short solution to a set of $n$ random equations modulo $q$ in $m$ variables.
• The dual of any $q$-ary lattice is also a $q$-ary lattice. We represent the dual of a $q$-ary lattice $\Lambda_q^\perp(A)$ using the notation $\Lambda_q(A)$, which is defined as $$\Lambda_q(A)=\{z\in\mathbb{Z}^m:z=A^Ts\mod{q},,s\in\mathbb{Z}^n\}$$ Here $\Lambda_q(A)$ is the code generated by the rows of $A$, i.e., $A$ itself is the generator matrix of the code. Contrast this with $\Lambda_q^\perp(A)$ where $A$ is the parity check matrix.

References