In mathematics, a normal basis in field theory is a special kind of basis for Galois extensions of finite degree, characterised as forming a single orbit for the Galois group. The normal basis theorem states that any finite Galois extension of fields has a normal basis. In algebraic number theory the study of the more refined question of the existence of a normal integral basis is part of Galois module theory.
Contents
In the case of finite fields, this means that each of the basis elements is related to any one of them by applying the Frobenius p-th power mapping repeatedly, where p is the characteristic of the field. Let GF(pm) be a field with pm elements, and β an element of it such that the m elements
are linearly independent. Then this set forms a normal basis for GF(pm) over GF(p).
Usage
This basis is frequently used in cryptographic applications that are based on the discrete logarithm problem such as elliptic curve cryptography. Hardware implementations of normal basis arithmetic typically have far less power consumption than other bases.
When representing elements as a binary string (e.g. in GF(23) the most significant bit represents β22=β4, the middle bit represents β21=β2, and the least significant bit represents β20=β), we can square elements by doing a left circular shift (left shifting β4 would give β8, but since we are working in GF(23) this wraps around to β). This makes the normal basis especially attractive for cryptosystems that utilize frequent squaring.
Primitive normal basis
A primitive normal basis of an extension of finite fields E/F is a normal basis for E/F which is generated by a primitive element of E. Lenstra and Schoof (1987) proved that every finite field extension possesses a primitive normal basis, the case when F is a prime field having been settled by Harold Davenport.
Free elements
If E/F is a Galois extension with group G and x in E generates a normal basis then x is free in E/F. If x has the property that for every subgroup H of G, with fixed field H°, x is free for E/H°, then x is said to be completely free in E/F. Every Galois extension has a completely free element.