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On the construction of the asymmetric Chudnovsky
multiplication algorithm in finite fields without
derivated evaluation
Alexis Bonnecaze, Stéphane Ballet, Nicolas Baudru, Mila Tukumuli
To cite this version:
Alexis Bonnecaze, Stéphane Ballet, Nicolas Baudru, Mila Tukumuli. On the construction of the
asym-metric Chudnovsky multiplication algorithm in finite fields without derivated evaluation. Comptes
Rendus. Mathématique, Centre Mersenne (2020-..) ; Elsevier Masson (2002-2019), 2017, 355 (7),
pp.729 - 733. �10.1016/j.crma.2017.06.002�. �hal-01705865�
Contents lists available atScienceDirect
C. R.
Acad.
Sci.
Paris,
Ser. I
www.sciencedirect.com
Number theory/Computer science
On
the
construction
of
the
asymmetric
Chudnovsky
multiplication
algorithm
in
finite
fields
without
derivated
evaluation
Construction
effective
de
l’algorithme
asymétrique
de
multiplication
de
Chudnovsky
dans
les
corps
finis
Stéphane Ballet
a,
Nicolas Baudru
b,
Alexis Bonnecaze
a,
Mila Tukumuli
a aAixMarseilleUniv,CNRS,CentraleMarseille,I2M,Marseille,FrancebAixMarseilleUniv,CNRS,CentraleMarseille,LIF,Marseille,France
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Articlehistory:
Received21November2016 Acceptedafterrevision2June2017 Availableonline16June2017 PresentedbytheEditorialBoard
TheChudnovskyalgorithmforthemultiplicationinextensionsoffinitefieldsprovidesa bilinearcomplexityuniformlylinearwithrespecttothedegreeoftheextension.Recently, Randriambololona hasgeneralized the method,allowingasymmetry intheinterpolation procedureand leadingtonewupperboundsonthebilinearcomplexity.Inthisnote,we describethe construction ofthisasymmetricmethodwithoutderived evaluation. To do this,wetranslatethisgeneralizationintothelanguageofalgebraicfunctionfieldsandwe giveastrategyofconstructionandimplementation.
©2017Académiedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
r
é
s
u
m
é
L’algorithme de multiplication dans les corps finis de Chudnovsky a une complexité
bilinéaireuniformémentlinéaireenledegrédel’extension.Randriambololonaarécemment généralisé cette méthode en introduisant l’asymétrie dans la procédure d’interpolation et en obtenant ainsi de nouvelles bornes surla complexité bilinéaire. Dans cette note, nous décrivons la construction de cette méthode asymétrique sans évaluation dérivée. Pourcefaire,noustraduisonscettegénéralisationdanslelangagedescorpsdefonctions algébriques,etnousdonnonsunestratégiedeconstructionetd’implantation.
©2017Académiedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
E-mailaddresses:stephane.BALLET@univ-amu.fr(S. Ballet),nicolas.BAUDRU@univ-amu.fr(N. Baudru),alexis.BONNECAZE@univ-amu.fr(A. Bonnecaze),
tukumulimila@gmail.com(M. Tukumuli).
http://dx.doi.org/10.1016/j.crma.2017.06.002
1631-073X/©2017Académiedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
730 S. Ballet et al. / C. R. Acad. Sci. Paris, Ser. I 355 (2017) 729–733
1. Introduction
Letq beaprimepower,
F
q thefinitefieldwithq elementsandF
qn thedegreen extensionofF
q.Amongallalgorithmsofmultiplicationsin
F
qn,thosebasedontheChudnovsky–Chudnovsky[6]methodareknowntoprovidethelowestbilinearcomplexity.Thismethodisbasedoninterpolationonalgebraiccurvesdefinedoverafinitefieldandprovidesabilinear com-plexity,whichislinearinn.Theoriginalalgorithmusesonlypointsofdegree1,withmultiplicity1.BalletandRolland[4,5]
andArnaud[1]improvedthealgorithm,introducinginterpolationatpointsofhigherdegreeorhighermultiplicity.The sym-metryoftheoriginalconstructioninvolves2-torsionpointsthatrepresentanobstacletotheimprovementofupperbilinear complexity bounds. Toeliminate thisdifficulty, Randriambololona[8]allowed asymmetry inthe interpolation procedure, andthenPieltantandRandriambololona[7]derivednewbounds,uniforminq,ofthebilinearcomplexity.Unlikesymmetric constructions,noeffectiveimplementationofthisasymmetricconstructionhasbeendoneyet.When g
=
1,itisknown[3]thatanasymmetricalgorithmcanalwaysbesymmetrized.However,forgreatervaluesofg,itmaynotbethecase.Thus,it isofinteresttoknowan effectiveconstructionofthisasymmetricalgorithm.Sofar,noeffectiveimplementationhasbeen proposedforsuchanalgorithm.
1.1. Multiplicationalgorithmandtensorrank
Themultiplicationoftwoelementsof
F
qn isanF
q-bilinearapplicationfromF
qn×F
qn ontoF
qn.Thenitcanbeconsideredas an
F
q-linearapplication fromthe tensorproductF
qn⊗
FqF
qn ontoF
qn. Consequently, itcan also be considered asanelement Tmof
F
qn⊗
F qF
qn⊗
Fq
F
qn wheredenotesthedual.When Tmiswritten
Tm
=
ri=1
xi
⊗
yi⊗
ci,
(1)wherether elements xi aswell asthe r elements yi areinthedual
F
qn ofF
qn whilether elementsci areinF
qn,thefollowingholdsforanyx
,
y∈ F
qn:x·
y=
ri=1xi
(
x)
yi(
y)
ci.Thedecomposition(1)isnotunique.Definition1.1.Everyexpressionx
·
y=
ri=1xi(
x)
yi(
y)
cidefinesabilinearmultiplicationalgorithmU
ofbilinearcomplexityμ
(U)
=
r.Suchanalgorithmissaidsymmetricifxi=
yiforalli.Definition1.2. Theminimalnumberofsummands ina decompositionofthetensor Tm ofthemultiplicationiscalledthe
bilinearcomplexity(resp.symmetricbilinearcomplexity)ofthemultiplicationandisdenotedby
μ
q(
n)
(resp.μ
symq(
n)
):μ
q(
n)
=
minU
μ
(
U
)
where
U
isrunningoverallbilinearmultiplicationalgorithms(resp.allbilinearsymmetricmultiplicationalgorithms)inF
qnover
F
q.1.2. Organisationofthenote
InSection2,wegiveanexplicittranslationofthegeneralizationoftheChudnovskyalgorithmgivenbyRandriambololona
[8,Theorem3.5].TheninSection3,bydefininganewdesignofthisalgorithm,wegiveastrategyofconstructionand imple-mentation.Inparticular,thankstoasuitablerepresentationoftheRiemann–Rochspaces,wepresentthefirstconstructionof asymmetriceffectivealgorithmsofmultiplicationinfinitefields.Thesealgorithmsaretailoredtohardwareimplementation andthey allow computationstobe parallelized whilemaintaininga lownumberof bilinearmultiplications. InSection 4, wegiveananalysisofthenotasymptoticalcomplexityofthisalgorithm.
2. MultiplicationalgorithmsoftypeChudnovsky:generalizationofRandriambololona
In this section, we present a generalization of Chudnovsky-type algorithms, introduced in [8, Theorem 3.5] by Ran-driambololona, which is possibly asymmetric. Since our aim is to describe explicitly the effective construction of this asymmetric algorithm, we transformthe representation ofthis algorithm,initially made inthe abstract geometrical lan-guage,inthemoreexplicitlanguageofalgebraicfunctionfields.
Let F
/
F
q be an algebraic functionfield overthefinitefieldF
q ofgenus g(
F)
.Wedenote by N1(
F/
F
q)
thenumberofplaces ofdegreeone ofF over
F
q.IfD isadivisor,L(
D)
denotes theRiemann–Rochspaceassociatedwith D.Wedenoteby
O
Q the valuationring oftheplace Q andby FQ its residueclassfieldO
Q/
Q ,whichisisomorphic toF
qdeg Q,wheredeg Q isthedegreeoftheplace Q .
Intheframeworkofalgebraicfunction fields,theresult[8,Theorem3.5]ofRandriambololonacanbestatedasin The-orem 2.1.Notethat wedo nottake intoaccountderived evaluations,sincewe arenot interestedinasymptoticresults.It means thatwedescribe thisasymmetric algorithmwiththedivisor G
=
P1+ · · · +
PN wherethe Pi arepairwisedistinctLet usdefine the following Hadamardproduct in
F
ql1× F
ql2× · · · × F
qlN, where the li’s denote positive integers, by(
u1,
. . . ,
uN)
(
v1,
. . . ,
vN)
= (
u1v1,
. . . ,
uNvN)
.Theorem2.1.LetF
/
F
qbeanalgebraicfunctionfieldofgenusg overF
q.Supposethatthereexistsaplace Q ofdegreen.LetP =
{
P1,
. . . ,
PN}
beasetofN placesofarbitrarydegreenotcontainingtheplaceQ .Supposethatthere existtwoeffectivedivisorsD1,
D2ofF
/
F
qsuchthat:(i) theplaceQ andtheplacesof
P
arenotinthesupportofthedivisorsD1andD2;(ii) thenaturalevaluationmapsEifori
=
1,
2 definedasfollowsaresurjectiveEi
:
L
(
Di)
−→ F
qnFQf
−→
f(
Q)
(iii) thenaturalevaluationmapdefinedasfollowsisinjective
T
:
L
(
D1+
D2)
−→ F
qdeg P1× F
qdeg P2× · · · × F
qdeg P N f−→
(
f(
P1),
f(
P2), . . . ,
f(
PN))
Thenforanytwoelementsx
,
y inF
qn,wehave:xy
=
EQ◦
T|−Im T1T
◦
E−11(
x)
T◦
E−21(
y)
,
whereEQ denotesthecanonicalprojectionfromthevaluationring
O
Q oftheplace Q initsresidueclassfieldFQ,◦
thestandardcompositionmap,T|−Im T1 therestrictionoftheinversemapofT ontheimageofT ,E−i1theinversemapoftherestrictionofthemapEion
thequotientgroup
L(
Di)/
ker EiandtheHadamardproductinF
qdeg P1× F
qdeg P2× · · · × F
qdeg P N;andμ
q(
n)
≤
Ni=1
μ
q(
deg Pi)
.3. Effectivealgorithm
3.1. Methodandstrategyofimplementation
Theconstructionofthealgorithmisbasedonthechoiceoftheplace Q ,theeffectivedivisorsD1 andD2,thebasesof
spaces
L(
D1)
,L(
D2)
andL(
D1+
D2)
andthebasisoftheresidueclassfield FQ.Inpractice, followingtheideas of [2],divisors D1 and D2 arechosen asplaces ofdegree n
+
g−
1.Furthermore,werequiresomeadditionalproperties,whicharedescribedbelow.
3.2. FindinggoodplacesD1,D2,andQ
Inordertoobtainthegoodplaces,weproceedasfollows:
– wedraw atrandom anirreducible polynomial
Q(
x)
ofdegreen inF
q[
X]
andcheck thatthispolynomialis primitiveandtotallydecomposedinthealgebraicfunctionfield F
/
F
q;– wechooseaplace Q ofdegreen abovethepolynomial
Q(
x)
;– wechooseaplace Q ofdegreen amongtheplacesofF
/
F
q lyingabovethepolynomialQ(
x)
;– wedrawatrandomaplace D1 ofdegreen
+
g−
1 andcheckthatD1−
Q isanon-specialdivisorofdegreeg−
1,i.e.dim
L(
D1−
Q)
=
0;– wedrawatrandomaplace D2 ofdegreen
+
g−
1 andcheckthatD2−
Q isanon-specialdivisorofdegreeg−
1,i.e.dim
(
D2−
Q)
=
0.3.3. Choosinggoodbasesofthespaces TheresiduefieldFQ.
Wechoosethecanonicalbasis
B
Q generatedbyarootα
ofthepolynomialQ(
x)
,namelyB
Q= (
1,
α
,
α
2,
...,
α
n−1)
.Fromnowon,weidentify
F
qn to FQ,astheresidueclassfield FQ oftheplace Q isisomorphictothefinitefieldF
qn. TheRiemann–RochspacesL(
D1)
andL(
D2)
.Wechooseasbasisof
L(
Di)
thereciprocalimageB
Di ofthebasisB
Q= (φ
1,
. . . ,
φ
n)
of FQ bytheevaluationmap Ei,namely
B
Di= (
E−1
i
(φ
i),
. . . ,
E−i1(φ
n))
.Letusdenote
B
Di= (
fi,1,
...,
fi,n)
with fi,1=
1 fori=
1,
2. TheRiemann–RochspaceL(
D1+
D2)
.Notethatsince D1 andD2areeffectivedivisors,wehave
L(
D1)
⊂
L(
D1+
D2)
andL(
D2)
⊂
L(
D1+
D2)
.732 S. Ballet et al. / C. R. Acad. Sci. Paris, Ser. I 355 (2017) 729–733
Proposition3.1.LetD1,D2andQ beplaceshavingthepropertiesdescribedin(3.2).Considerthemap
:
L(
D1+
D2)
→
FQ suchthat
(
f)
=
f(
Q)
forf∈
L(
D1+
D2)
.ThereexistsavectorspaceM
⊆
kerofdimensiong suchthat
L
(
D1+
D2)
=
L
(
D1)
⊕
L
r(
D2)
⊕
M
,
where
L
r(
D2)
issuchthatL(
D2)
= F
q⊕
L
r(
D2)
and⊕
denotesthedirectsum.Inparticular,ifg=
0,thenM
=
K erisequal
to
{
0}
.We chooseasbasisof
L(
D1+
D2)
thebasisB
D1+D2 definedbyB
D1+D2= (
f1,
. . . ,
fn,
fn+1,
. . . ,
f2n+g−1)
whereB
D1=
(
f1,
. . . ,
fn)
isthebasisofL(
D1)
,(
fn+1,
. . . ,
f2n−1)
isabasisofL
r(
D2)
such that fn+j=
f2,j+1∈
B
D2 withB
D1 andB
D2definedinSection3.3and
B
M= (
f2n,
. . . ,
f2n+g−1)
isabasisofM
.3.4. Productoftwoelementsin
F
qnLetx
= (
x1,
. . . ,
xn)
andy= (
y1,
. . . ,
yn)
betwoelementsofF
qn givenbytheircomponentsoverF
qrelativetothechosenbasis
B
Q.Accordingtothepreviousnotation,wecanconsiderthatx andy areidentifiedasrespectively fx=
ni=1xif1,i
∈
L(
D1)
and fy=
ni=1yif2,i∈
L(
D2)
.Theproduct fxfy ofthetwoelements fx and fyistheirproductinthevaluationring
O
Q.ThisproductliesinL(
D1+
D2
)
,since D1 andD2 areeffectivedivisors.Weconsiderthat x and y arerespectivelytheelements fx and fy embeddedintheRieman–Roch space
L(
D1+
D2)
,viarespectivelytheembeddings Ii:
L(
Di)
−→
L(
D1+
D2)
,definedby I1(
fx)
andI2
(
fy)
asfollows.If, fx and fyhaverespectivelycoordinates fxi and fyi inB
D1+D2,wherei∈ {
1,
. . . ,
2n+
g−
1}
,wehave:I1
(
fx)
= (
fx1:=
x1,
. . . ,
fxn:=
xn,
0,
. . . ,
0)
andI2(
fy)
= (
fx1:=
y1,
0,
. . . ,
0,
fyn+1:=
y2,
. . . ,
fy2n−1:=
yn,
0,
. . .
0)
.Now itisclearthatknowingx (resp. y)or fx (resp. fy)bytheircoordinatesisthesamething.
Theorem3.2.LetPMsbetheprojectionof
L(
D1+
D2)
ontoM
s=
L(
D1)
⊕
L
r(
D2)
,andletbethemapdefinedasin
Proposi-tion 3.1.Then,foranyelementsx
,
y∈ F
qn,theproductofx byy issuchthatxy
= ◦
PMsT|−Im T1
T◦
I1◦
E−11(
x)
T◦
I2◦
E−21(
y)
,
where
◦
denotesthestandardcompositionmap,T−|Im T1 therestrictionoftheinversemapofT ontheimageofT ,andtheHadamard productasinTheorem 2.1.Wecannowpresentthesetupalgorithm(Algorithm 1),whichisdoneonlyonce.
Algorithm1Setupalgorithm.
INPUT: F/Fq, Q,D1,D2,P1,. . . ,PN.
OUTPUT: T and T−1.
(i) TherepresentationofthefinitefieldFq=<a>,wherea isafixedprimitiveelement.
(ii) ThefunctionfieldF/Fq,theplaceQ ,thedivisorsD1andD2andthepointsP1,. . . ,PNaresuchthatConditions(ii)and(iii)inTheorem 2.1are
satisfied.Inaddition,werequirethat1≤i≤Ndeg Pi=2n+g−1.
(iii) RepresentFqn inthecanonicalbasisBQ= {1,α,α2,...,αn−1},whereFqn=<α>withαaprimitiveelementasinSection3.3.
(iv) Constructabasis(f1,. . . ,fn,fn+1,. . . ,f2n+g−1)ofL(D1+D2),where(f1,. . . ,fn)isthebasisofL(D1),(f1,fn+1,. . . ,f2n−1)thebasisofL(D2)
and(f2n,. . . ,f2n+g−1)thebasisofM,definedinSection3.3.
(v) ComputethematricesT andT−1.
(vi) Computethematrix.
Themultiplicationalgorithm(Algorithm 2)ispresentedhereafter.
4. Complexityanalysis
Intermsofnumberofmultiplicationsin
F
q,thecomplexity ofthismultiplicationalgorithmisasfollows:calculationofz andt needs2
(
2n2+
ng−
n)
multiplications,calculationofu needs(
2n+
2g−
2+
r)
sup1≤i≤rμq(i)
i bilinearmultiplications
andcalculation of2n
−
1 firstcomponents of w needs(
2n+
g−
1)(
2n−
1)
multiplications (remark that,inAlgorithm 2, we justhaveto computethe 2n−
1 first componentsof w). Thecalculation ofxy needs n+
g multiplications.The total complexityintermsofmultiplicationsisboundedby8n2+
n(
4g−
5)
+ (
2n+
2g−
2+
r)
sup1≤i≤r
μq(i)
i .
The generalconstructionoftheset-upalgorithminvolvessome randomchoiceofdivisorshavingprescribedproperties overanexponentiallylargesetofdivisors.Togetapolynomiallyconstructiblealgorithmwithlinearcomplexity,oneneeds toconstructexplicitly(i.e.polynomially)pointsofcorrespondingdegreesn oncurvesofarbitrarygenuswithmanyrational points. Unfortunately,sofaritisunknown howtoproducesuch points(cf.[9,Section 4,Remark5] and[8,Remark 6.6]). Hence,theasymptoticcomplexityofsuchaconstructionisanopenproblem.
Algorithm2Multiplicationalgorithm. INPUT: x= (x1,. . . ,xn)and y= (y1,. . . ,yn). OUTPUT: xy. (i) Compute ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ z1,d1 . . . zn,dn zn+1,dn+1 . . . zN,dN ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ = ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ z1 . . . zn zn+1 . . . z2n+g−1 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ =T ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ x1 . . . xn 0 . . . 0 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ and ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ t1,d1 . . . tn,dn tn+1,dn+1 . . . tN,dN ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ = ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ t1 . . . tn tn+1 . . . t2n+g−1 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ =T ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ y1 0 y2 . . . yn 0 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ ,
whereNi=1di=2n+g−1,(zi,j,ti,j)∈ (Fqd j)2,(zi,ti)∈ (Fq)2and0 standsforthenullvector.
(ii) ComputetheHadamardproductu= (u1,d1,. . . ,uN,dN)= (u1,. . . ,u2n+g−1),whereui,di=zi,diti,di,inFqd1× Fqd2× · · · × FqdN asinTheorem 2.1.
(iii) Computew= (w1,. . . ,w2n+g−1)=T−1(u).
(iv) Extractw= (w1,. . . ,w2n−1)(instep(iii),justthe2n−1 firstcomponentshavetobecomputed).
(v) Returnxy= (w).
References
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[2]S.Ballet,CurveswithmanypointsandmultiplicationcomplexityinanyextensionofFq,FiniteFieldsAppl.5 (4)(1999)364–377.
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