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Hessian form of an elliptic curve : ウィキペディア英語版
Hessian form of an elliptic curve
In geometry, the Hessian curve is a plane curve similar to folium of Descartes. It is named after the German mathematician Otto Hesse.
This curve was suggested for application in elliptic curve cryptography, because arithmetic in this curve representation is faster and needs less memory than arithmetic in standard Weierstrass form.〔Cauchy-Desbove's Formulae: ''Hessian-elliptic Curves and Side-Channel Attacks'', Marc Joye and Jean-Jacques Quisquarter〕
== Definition ==

Let K be a field and consider an elliptic curve E in the
following special case of Weierstrass form over K :
: Y^2+a_1 XY+a_3 Y=X^3
where the curve has discriminant
\Delta = (a_3^3(a_1^3 - 27a_3)) = a_3^3 \delta.
Then the point P=(0,0) has order 3.
To prove that P=(0,0) has order 3, note that the tangent to E
at P is the line Y=0 which intersects
E with multiplicity 3 at P.
Conversely,
given a point P of order 3 on an elliptic curve E
both defined over a field K one can put the curve into Weierstrass
form with P=(0,0) so that the tangent at P
is the line Y=0. Then the equation of the curve is
Y^2+a_1 XY+a_3 Y=X^3 with a_1,a_3\in K.
Now, to obtain the Hessian curve, it is necessary to do the following transformation:
First let \mu denote a root of the polynomial
: T^3-\delta T^2+T+ a_3\delta^2=0.
Then
: \mu=.
Note that if K has a finite field of order
q\equiv 2 (mod 3), then every element of K has a unique cube root; in general,
\mu lies in an extension field of ''K''.
Now by defining the following value D=\frac another curve, C, is obtained, that is birationally equivalent to E:
: C : x^3 + y^3 + z^3= Dxyz
which is called ''cubic Hessian form'' (in projective coordinates)
: C : x^3 + y^3 + 1= Dxy
in the ''affine plane'' ( satisfying x=\frac and y=\frac ).
Furthermore, D^3\ne1 (otherwise, the curve would be singular).
Starting from the Hessian curve, a birationally equivalent Weierstrass equation is given by
: v^2 = u^3 - 27D(D^3 + 8)u + 54(D^6 - 20 D^3 - 8), \,
under the transformations:
: (x,y) = (\eta (u + 9D^2), - 1 + \eta(3D^3 - Dx -12)) \,
and
: (u,v) = (-9D^2 + \varepsilon x, 3\varepsilon(y - 1)) \,
where:
:\eta = \frac
and
:\varepsilon = \frac

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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