sumo/galois-field

Finite field and algebraic extension field arithmetic

Galois Field

An efficient implementation of Galois fields used in cryptography research.

Technical background

A Galois field , for prime and positive , is a field (, +, , 0, 1) of finite order. Explicitly,

• (, +, 0) is an abelian group,
• (, , 1) is an abelian group,
• is distributive over +, and
• is finite.

Prime fields

Any Galois field has a unique characteristic , the minimum positive such that , and is prime. The smallest Galois field of characteristic is a prime field, and any Galois field of characteristic is a finite-dimensional vector space over its prime subfield.

For example, is a Galois field of characteristic 2 that is a two-dimensional vector space over the prime subfield .

Extension fields

Any Galois field has order a prime power for prime and positive , and there is a Galois field of any prime power order that is unique up to non-unique isomorphism. Any Galois field can be constructed as an extension field over a smaller Galois subfield , through the identification for an irreducible monic polynomial of degree in the polynomial ring .

For example, has order and can be constructed as an extension field where is an irreducible monic quadratic polynomial in .

Binary fields

A Galois field of the form for big positive is a sum of for a non-empty set of . For computational efficiency in cryptography, an element of a binary field can be represented by an integer that represents a bit string. It should always be used when the field characteristic is 2.

For example, can be represented as the integer 283 that represents the bit string 100011011.

Example usage

Include the following required language extensions.

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE PatternSynonyms #-}

Import the following functions at minimum.

import Data.Field.Galois (Prime, Extension, IrreducibleMonic(poly), Binary,
                          pattern X, pattern X2, pattern X3, pattern Y)

Prime fields

The following type declaration creates a prime field of a given characteristic.

type Fq = Prime 21888242871839275222246405745257275088696311157297823662689037894645226208583

Note that the characteristic given must be prime.

Galois field arithmetic can then be performed in this prime field.

fq :: Fq
fq = 5216004179354450092383934373463611881445186046129513844852096383579774061693

fq' :: Fq
fq' = 10757805228921058098980668000791497318123219899766237205512608761387909753942

arithmeticFq :: (Fq, Fq, Fq, Fq)
arithmeticFq = (fq + fq', fq - fq', fq * fq', fq / fq')

Extension fields

The following data type declaration creates a polynomial given an irreducible monic polynomial.

data P2
instance IrreducibleMonic P2 Fq where
  poly _ = X2 + 1

The following type declaration then creates an extension field with this polynomial.

type Fq2 = Extension P2 Fq

Note that the polynomial given must be irreducible and monic in the prime field.

Similarly, further extension fields can be constructed iteratively as follows.

data P6
instance IrreducibleMonic P6 Fq2 where
  poly _ = X3 - (9 + Y X)

type Fq6 = Extension P6 Fq2

data P12
instance IrreducibleMonic P12 Fq6 where
  poly _ = X2 - Y X

type Fq12 = Extension P12 Fq6

Note that X, X2, X3 accesses the current indeterminate variables and Y descends the tower of indeterminate variables.

Galois field arithmetic can then be performed in this extension field.

fq12 :: Fq12
fq12 =
  [ [ [ 4025484419428246835913352650763180341703148406593523188761836807196412398582
      , 5087667423921547416057913184603782240965080921431854177822601074227980319916
      ]
    , [ 8868355606921194740459469119392835913522089996670570126495590065213716724895
      , 12102922015173003259571598121107256676524158824223867520503152166796819430680
      ]
    , [ 92336131326695228787620679552727214674825150151172467042221065081506740785
      , 5482141053831906120660063289735740072497978400199436576451083698548025220729
      ]
    ]
  , [ [ 7642691434343136168639899684817459509291669149586986497725240920715691142493
      , 1211355239100959901694672926661748059183573115580181831221700974591509515378
      ]
    , [ 20725578899076721876257429467489710434807801418821512117896292558010284413176
      , 17642016461759614884877567642064231230128683506116557502360384546280794322728
      ]
    , [ 17449282511578147452934743657918270744212677919657988500433959352763226500950
      , 1205855382909824928004884982625565310515751070464736233368671939944606335817
      ]
    ]
  ]

fq12' :: Fq12
fq12' =
  [ [ [ 495492586688946756331205475947141303903957329539236899715542920513774223311
      , 9283314577619389303419433707421707208215462819919253486023883680690371740600
      ]
    , [ 11142072730721162663710262820927009044232748085260948776285443777221023820448
      , 1275691922864139043351956162286567343365697673070760209966772441869205291758
      ]
    , [ 20007029371545157738471875537558122753684185825574273033359718514421878893242
      , 9839139739201376418106411333971304469387172772449235880774992683057627654905
      ]
    ]
  , [ [ 9503058454919356208294350412959497499007919434690988218543143506584310390240
      , 19236630380322614936323642336645412102299542253751028194541390082750834966816
      ]
    , [ 18019769232924676175188431592335242333439728011993142930089933693043738917983
      , 11549213142100201239212924317641009159759841794532519457441596987622070613872
      ]
    , [ 9656683724785441232932664175488314398614795173462019188529258009817332577664
      , 20666848762667934776817320505559846916719041700736383328805334359135638079015
      ]
    ]
  ]

arithmeticFq12 :: (Fq12, Fq12, Fq12, Fq12)
arithmeticFq12 = (fq12 + fq12', fq12 - fq12', fq12 * fq12', fq12 / fq12')

Note that

where , , is a tower of indeterminate variables, is constructed by

[ [ [a, b], [c, d], [e, f] ]
, [ [g, h], [i, j], [k, l] ] ] :: Fq12

Binary fields

The following type declaration creates a binary field modulo a given irreducible binary polynomial.

type F2m = Binary 0x80000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000425

Note that the polynomial given must be irreducible in .

Galois field arithmetic can then be performed in this binary field.

f2m :: F2m
f2m = 0x303001d34b856296c16c0d40d3cd7750a93d1d2955fa80aa5f40fc8db7b2abdbde53950f4c0d293cdd711a35b67fb1499ae60038614f1394abfa3b4c850d927e1e7769c8eec2d19

f2m' :: F2m
f2m' = 0x37bf27342da639b6dccfffeb73d69d78c6c27a6009cbbca1980f8533921e8a684423e43bab08a576291af8f461bb2a8b3531d2f0485c19b16e2f1516e23dd3c1a4827af1b8ac15b

arithmeticF2m :: (F2m, F2m, F2m, F2m)
arithmeticF2m = (f2m + f2m', f2m - f2m', f2m * f2m', f2m / f2m')

Disclaimer

This is experimental code meant for research-grade projects only. Please do not use this code in production until it has matured significantly.

License

Copyright (c) 2019-2020 Adjoint Inc.

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