### Finite Fields

And so on. We can use this as the definition of division to help us. Note that like in normal math, you cannot divide by 0.

## Discrete Mathematics/Finite fields - Wikibooks, open books for an open world

Yet that is one of the remarkable things about finite fields: finite fields are closed under division. That is, dividing any two numbers where the denominator is not 0 will result in another finite field element. Essentially, the theorem says:. We can reduce the division problem to a multiplication problem as long as we can figure out what b —1 is.

We know:. So in other words, we can calculate the inverse using the exponentiation operator. This is a relatively expensive calculation as exponentiating grows very fast.

Division is the most expensive operation for that reason. To lessen the expense, we can use the pow function in Python, which does exponentiation.

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The pow function, however, has an optional third argument that makes our calculation more efficient. Specifically, pow will modulo by the third argument. The first does normal division and the second does integer division. For example, a —3 needs to be a finite field element, but the current code does not take care of this case. We want, for example, something like this to work:.

Thankfully, we can use some math we already know to solve this. This fact means that we can multiply by a p —1 as many times as we want. So, for a —3 , we can do:. Thankfully, we can do even better. This will make the pow function not work as hard:.

## Finite Fields and Their Applications

In this chapter we learned about finite fields and how to implement them in Python. We turn next to the other mathematical component that we need for elliptic curve cryptography: elliptic curves. Stay ahead with the world's most comprehensive technology and business learning platform.

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### Wolfram|Alpha Pages

Chapter 1. Finite Fields One of the most difficult things about learning how to program Bitcoin is knowing where to start. Learning Higher-Level Math Learning about new mathematical structures can be a bit intimidating, and in this chapter, I hope to dispel the myth that high-level math is difficult. Defining Finite Sets If the order or size of the set is p , we can call the elements of the set, 0, 1, 2, … p — 1.

Modulo Arithmetic One of the tools we can use to make a finite field closed under addition, subtraction, multiplication, and division is something called modulo arithmetic. Figure Long division example 1. Long division example 2. Clock going forward 47 hours. Finite Field Addition and Subtraction Remember that we need to define finite field addition such that we ensure the result is still in the set. Addition in a finite field is defined with the modulo operator, as explained earlier.

Why Prime Fields are Useful The answer to Exercise 5 is why we choose to use finite fields with a prime number of elements. We can code it this way: class FieldElement :. While this representation is very fast it is limited to finite fields of small cardinality.

### 18.2 Comparisons of Finite Field Elements

If the characteristic of such a field is 2 then NTL is used internally to represent the field sage. However, this distinction is internal only and the user usually does not have to worry about it because consistency across all implementations is aimed for. In all extension field implementations the user may either specify a minimal polynomial or leave the choice to Sage.

Sage contains a database of Conway polynomials which also can be queried independently of finite field construction. A pseudo-Conway polynomial satisfies all of the conditions required of a Conway polynomial except the condition that it is lexicographically first. They are therefore not unique. If no variable name is specified for an extension field, Sage will fit the finite field into a compatible lattice of field extensions defined by pseudo-Conway polynomials. This lattice is stored in an AlgebraicClosureFiniteField object; different algebraic closure objects can be created by using a different prefix keyword to the finite field constructor.

Note that the computation of pseudo-Conway polynomials is expensive when the degree is large and highly composite. If a variable name is specified then a random polynomial is used instead, which will be much faster to find. While Sage supports basic arithmetic in finite fields some more advanced features for computing with finite fields are still not implemented.