Basics of Polynomials for Cryptography

A polynomial $\phi$ of degree $d$ is a vector of $d+1$ coefficients:

\begin{align} \phi &= [\phi_0, \phi_1, \phi_2, \dots, \phi_d] \end{align}

For example, $\phi = [1, 10, 9]$ is a degree 2 polynomial. Also, $\phi’ = [1, 10, 9, 0, 0, 0]$ is also a degree 2 polynomial, since the zero coefficients at the end do not count. But $\phi’’ = [1, 10, 9, 0, 0, 0, 1]$ is a degree 6 polynomial, since the last non-zero coefficient is $\phi_6 = 3$.

“A list of numbers? That makes no sense!” Don’t panic! You are probably more familiar to polynomials expressed as function of a variable $X$: \begin{align} \phi(X) &= \phi_0 + \phi_1\cdot X + \phi_2\cdot X^2 + \cdots + \phi_d\cdot X^d\\
&= \sum_{i=0}^{d+1} \phi_i X^i \end{align}

For example, $\phi = [1, 10, 9]$ and $\phi(X) = 9X^2 + 10X + 1$ are one and the same thing.

Note: The degree is defined as the index $i$ of the last non-zero coefficient: $\deg(\phi)=i$ s.t. $\forall j > i, \phi_j = 0$.

The basics of polynomials

Roots of polynomials

We say $z$ is a root of $\phi(X)$ if $\phi(z) = 0$. In this case, $\exists q(X)$ such that $\phi(X) = q(X)(X-z)$.

But what if $z$ is also a root $q(X)$? We can capture this notion as follows: we say $z$ has a multiplicity $k$ if $\exists q’(X)$ such that $\phi(X) = q’(X) (X-z)^k$.

The polynomial remainder theorem

This theorem says that:

\begin{align} \phi(a) = y\Leftrightarrow \exists q(X), \phi(X) &= q(X)(X-a) + \phi(a) \end{align}

This property is leveraged by certain cryptosystems¹.

Dividing polynomials

Division of polynomials conceptually resembles division of integers.

Specifically, dividing a polynomial $a(X)$ by $b(X)$ gives a quotient $q(X)$ and a remainder $r(X)$ such that:

\[a(X) = q(X) b(X) + r(X)\]

Importantly, $\deg{r} < \deg{b}$ and, if $\deg{a} \ge \deg{b}$, then $\deg{q} = \deg{a} - \deg{b}$. Otherwise, $q(X) = 0$.