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Let $q$ be a prime number and let $\mathbb{Z}_q = \left\{1,\dots,q-1\right\}$; I need to prove that the family $\mathcal{H} = \left\{h_s \colon \mathbb{Z}_q \rightarrow \mathbb{Z}_q\right\}_{s \in \mathbb{Z}_q^3}$ is 3-wise independent, where $h_s$ is defined as:

$$h_s(x):=h_{s_0,s_1,s_2}(x):=s_0 + s_1 x + s_2 x^2 \bmod q$$

How could I do it? My intuition would be proving that it is 1-wise independent and use the property $$x_1,\dots,x_t \in \mathcal{X}, \quad y_1,\dots,y_t \in \mathcal{Y}, \quad \Pr[h_s(x_1)=y_1 \wedge \dots\wedge h_s(x_t)=y_t \mid s \leftarrow _\$ \mathcal{S}]=\frac{1}{|\mathcal{Y}|^t},$$ but I'm not sure how to do that.

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  • $\begingroup$ Try to use the definition of 3-wise independence. Using the definition is always a good idea. $\endgroup$ – Yuval Filmus Nov 11 '18 at 23:29
  • $\begingroup$ The notation $\mathbb{Z}_q$ that you use is highly nonstandard; usually $\mathbb{Z}_q$ is the cyclic group of $q$ elements. $\endgroup$ – Yuval Filmus Nov 11 '18 at 23:31
  • $\begingroup$ Indeed, there is no guarantee that the image of your function actually lies in $\mathbb{Z}_q$. You probably meant the field $\mathbb{F}_q$ with $q$ elements. $\endgroup$ – Yuval Filmus Nov 11 '18 at 23:32
  • $\begingroup$ Note that being 3-wise independent is stronger than being 1-wise independent. For example, you family is 3-wise but not 4-wise independent. $\endgroup$ – Yuval Filmus Nov 11 '18 at 23:34
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Let's assume that $\mathbb{Z}_q$ is the field of size $q$. In order for your family to be 3-wise independent, we need the following condition to hold: for every $x_1,x_2,x_3,y_1,y_2,y_3 \in \mathbb{Z}_q$ where the $x_i$ are all different, $$ \Pr_s[h_s(x_1) = y_1 \land h_s(x_2) = y_2 \land h_s(x_3) = y_3] = \frac{1}{q^3}. $$ Since your family consists of $q^3$ functions, this is the same as the following property:

If $x_1,x_2,x_3 \in \mathbb{Z}_q$ are all different, then for every $y_1,y_2,y_3 \in \mathbb{Z}_q$ there exist unique $s_0,s_1,s_2 \in \mathbb{Z}_q$ such that $$ s_0 + s_1 x_i + s_2 x_i^2 = y_i, \quad i \in \{1,2,3\}. $$

This is a property that you can prove using linear algebra.

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