# How to use the Extended Euclidean algorithm to invert a finite field element?

I was doing some practice on cryptography (I'm new to this topic) and was wondering what the following question even means or what it is asking me to find. I do know how to do Extended Euclidean algorithm, but I have no idea how to apply it to this problem. I do not need the answer to it but I would appreciate someone explaining to me what the question is asking for.

Calculate by hand, using the Extended Euclidean algorithm: $(\mathtt{0x35})^{-1}$ in $\operatorname{GF}(2^8)$ with the irreducible polynomial $m(x) = (\mathtt{0x11B})$. (2 marks)

(Note: this is the finite field used by AES, ‘$\mathtt{0x}$’ denotes the hexadecimal representation.)

## 2 Answers

To answer this question, we need to understand what is meant by:

the field $\operatorname{GF}(2^8)$ generated by the irreducible polynomial $m(x) = (\mathtt{0x11B})$.

and I'll try a gentle, progressive introduction to that.

One simple way to see this field is that as the set of the 256 octets $\{\mathtt{0x00},\mathtt{0x01},\dots,\mathtt{0xFF}\}$; with additive law bitwise XOR $\oplus\;$; and multiplicative law the commutative operation $*$ defined in either one of the following ways:

• By having $\mathtt{0x01}$ as the neutral element, $\mathtt{0x02}*\mathtt{0x80}=\mathtt{0x1B}\;$ (that's the given $\mathtt{0x11B}$ truncated to its low-order 8 bits), and the more intuitive $\mathtt{0x02}*\mathtt{0x02}=\mathtt{0x04}\;$, $\mathtt{0x02}*\mathtt{0x04}=\mathtt{0x08}\;$, $\mathtt{0x02}*\mathtt{0x08}=\mathtt{0x10}\;$, $\mathtt{0x02}*\mathtt{0x10}=\mathtt{0x20}\;$, $\mathtt{0x02}*\mathtt{0x20}=\mathtt{0x40}\;$, $\mathtt{0x02}*\mathtt{0x40}=\mathtt{0x80}\;$. These and the properties of $*$ (commutativity, associativity, distributivity with respect to $\oplus\;$) are enough to fully define $*$. The constant $\mathtt{0x1B}$ is such that every element except $\mathtt{0x00}$ (the neutral element of $\oplus\;$) has a multiplicative inverse, which can be found by inspection of the multiplication table (building this table, and deriving an algorithm which computes the product of two arbitrary elements without this table and at most 8 iterations, is left as a recommended exercise to the reader). This gives the result asked in the question, but not by the method thought, and is impractical by hand.
• Alternatively, we can define a commutative ring $(\mathbb N,\oplus,\otimes)$ where $\oplus$ is bitwise XOR, and $\otimes$ (sometime called carry-less multiplication) is the same as common multiplication when either argument is a power of two, and the associativity of $\otimes$ with respect to $\oplus$ is used to define the other cases. For example $\mathtt{0xDB}\otimes\mathtt{0x42}$ $=\mathtt{0xDB}\otimes(\mathtt{0x40}\oplus\mathtt{0x02})$ $=(\mathtt{0xDB}\otimes\mathtt{0x40})\oplus(\mathtt{0xDB}\otimes\mathtt{0x02})$ $=\mathtt{0x36C0}\oplus\mathtt{0x1B6}$ $=\mathtt{0x3776}$. We can define an analog to Euclidean division in $(\mathbb N,\oplus,\otimes)$, giving quotient $q$ and rest $r$ from dividend $d$ and non-zero divisor $n$, such that $d=(q\otimes n)\oplus r$, and the bit size of $r$ is less than the bit size of $n$. That allows us to define an analog in $(\mathbb N,\oplus,\otimes)$ of modular reduction in the ring $(\mathbb Z,+,\times)$. Our operation $*$ in $\operatorname{GF}(2^8)$ is $\otimes$, followed by reduction modulo $m=\mathtt{0x11B}$. That constant is such that $(\operatorname{GF}(2^8),\oplus,*)$ is a field, much like $(\mathbb Z_n,+,\times)$ is a field when $n$ is prime. Now we can apply the Extended Euclidean algorithm and answer the question by the method asked. We do as for computing an inverse modulo a positive integer, but use $\oplus$ instead of addition and subtraction, $\otimes$ instead of multiplication, and the analog of Euclidean division in $(\mathbb N,\oplus,\otimes)$.

An equivalent way to see the field $\operatorname{GF}(2^8)$ is as the set of the 256 polynomials with one variable, of degree less than 8, having bit coefficients per Boolean algebra; additive law the addition of polynomials; and multiplicative law the multiplication of polynomials followed by modular polynomial reduction of the product by the polynomial $m(x)=x^8+x^4+x^3+x+1$ (corresponding to bits in $\mathtt{0x11B}\;$). A necessary and sufficient condition for every element except zero being invertible, is that $m(x)$ is irreducible, which holds. Now we can apply the Extended Euclidean algorithm and answer the question by the method asked. We do as for computing an inverse modulo a positive integer, but use $m(x)$ as the modulus, and division of polynomials with bit coefficients instead of Euclidean division.

The Extended Euclidean Algorithm produces the greatest common divisor and the Bézout coefficients for two integers $a$ and $b$. If $gcd(a,b) = 1$ and $a<b$, then the Bézout coefficients also give you the multiplicative inverse of $a$ in $\mathbb{Z}/b\mathbb{Z}$.

Because the operations used in the Extended Euclidean Algorithm are defined on polynomials, the process can be applied to elements in $GF(p^n)$. The question is simply asking you to apply the algorithm and determine the multiplicative inverse of a particular element.

Apparently the level of academic maturity of students who are assigned this problem does not require them to be familiar with hexadecimal notation.