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<!-- Deleted image removed: [[Image:Edwards curve.jpg|300px|right|thumb|An Edward curve of equation <math>x^2+y^2=1-300x^2y^2</math>]] -->
In [[mathematics]], an '''Edwards curve''' is a new representation of [[elliptic curve]]s, discovered by [[Harold Edwards (mathematician)|Harold Edwards]] in 2007. The concept of elliptic curves over [[finite fields]] is widely used in [[elliptic curve cryptography]]. Applications of Edwards curves to [[cryptography]] were developed by [[Daniel J. Bernstein|Bernstein]] and Lange: they pointed out several advantages of the Edwards form in comparison to the more well known [[elliptic curve|Weierstrass form]].
 
==Definition==
 
[[File:Edward-curves.svg|right|thumb|Edwards curves of equation ''x''<sup>2</sup>&nbsp;+&nbsp;''y''<sup>2</sup>&nbsp;=&nbsp;1&nbsp;−&nbsp;''d''&nbsp;·''x''<sup>2</sup>·''y''<sup>2</sup> over the real numbers for ''d''&nbsp;=&nbsp;300 (red), ''d''&nbsp;=&nbsp;√8 (yellow) and ''d''&nbsp;=&nbsp;−0.9 (blue)]]
The equation of an Edwards curve over a [[field (mathematics)|field]] ''K'' which does not
have characteristic 2 is:
 
: <math> x^2 + y^2 = 1 + d x^2 y^2 \, </math>
 
for some [[scalar field|scalar]] <math>d\in K\setminus\{0,1\}</math>.
Also the following form with parameters ''c'' and ''d'' is called an Edwards curve:
 
: <math> x^2 + y^2 = c^2(1 + dx^2 y^2) \, </math>
 
where ''c'',&nbsp;''d''&nbsp;∈&nbsp;''K'' with ''cd''(1&nbsp;−&nbsp;''c''<sup>4</sup>·''d'')&nbsp;≠&nbsp;0.
 
Every Edwards curve is [[birationally equivalent]] to an elliptic curve in [[elliptic curve|Weierstrass form]]. If ''K'' is finite, then a sizeable fraction of all elliptic curves over ''K'' can be written as Edwards curves.
Often elliptic curves in Edwards form are defined having c=1, without loss of generality. In the following sections, it is assumed that c=1.
 
==The group law==
(See also [[elliptic curve#The group law|Weierstrass curve group law]])
 
It is possible to do some operations on the points on any elliptic curve, such as adding two or more points and doubling or tripling one. Usually, given two points ''P'' and ''Q'' on an elliptic curve, the point ''P''&nbsp;+&nbsp;''Q'' is directly related to third point of intersection between the curve and the line that passes trough ''P'' and ''Q''; but in the case of Edwards curve this is not true: indeed the curve expressed in Edwards form has degree 4, so drawing a line one gets not 3 but 4 intersection points. For this case a geometric explanation of the addition law is given in
<ref>{{Cite web|url=http://eprint.iacr.org/2009/155|title=Faster Computation of the Tate Pairing|author=Christophe Arene|coauthors=and Tanja Lange and Michael Naehrig and Christophe Ritzenthaler|year=2009|accessdate=28 February 2010}}</ref>
 
===Edwards addition law===
[[File:Edwards Curve,Clock group diagram.jpg|300px|right|thumb|Clock group]]
It is possible to add points on an elliptic curve, and, in this way, obtain another point that belongs to the curve as well.
 
To understand better the concept of "addition" between points on a curve, a nice example is given by the circle:
 
take the circle of radius 1
 
:<math>{\displaystyle}x^2+y^2=1</math>
 
and consider two points P<sub>1</sub>=(x<sub>1</sub>,y<sub>1</sub>), P<sub>2</sub>=(x<sub>2</sub>,y<sub>2</sub>) on it. Let α<sub>1</sub> and α<sub>2</sub> be the angles such that:
 
:<math>{\displaystyle}P_{1}=(x_{1},y_{1})=(\sin{\alpha_{1}},\cos{\alpha_{1}})</math>
:<math>{\displaystyle}P_{2}=(x_{2},y_{2})=(\sin{\alpha_{2}},\cos{\alpha_{2}})</math>
 
The sum of P<sub>1</sub> and P<sub>2</sub> is, thus, given by the sum of "their angles". That is, the point P<sub>3</sub>=P<sub>1</sub>+P<sub>2</sub> is a point on the circle with coordinates (x<sub>3</sub>,y<sub>3</sub>), where:
 
:<math>{\displaystyle}x_{3}=\sin({\alpha}_{1}+{\alpha}_{2})=\sin{\alpha}_{1}\cos{\alpha}_{2}+\sin{\alpha}_{2}\cos{\alpha}_{1}=x_{1}y_{2}+x_{2}y_{1}</math>
:<math>{\displaystyle}y_{3}=\cos({\alpha}_{1}+{\alpha}_{2})=\cos{\alpha}_{1}\cos{\alpha}_{2}-\sin{\alpha}_{1}\sin{\alpha}_{2}=y_{1}y_{2}-x_{1}x_{2}.</math>
 
In this way, the addition formula for points on the circle of radius 1 is:
 
:<math>{\displaystyle}(x_1,y_1)+(x_2,y_2) = (x_1y_2+x_2y_1,y_1y_2-x_1x_2)</math>.
 
When two points (''x''<sub>1</sub>,&nbsp;''y''<sub>1</sub>) and (''x''<sub>2</sub>,&nbsp;''y''<sub>2</sub>) on an Edwards curve are added, the result is another point which has coordinates:
 
: <math> (x_1,y_1) + (x_2,y_2) = \left( \frac{x_1 y_2 + y_1 x_2}{1 + dx_1 x_2 y_1 y_2}, \frac{y_1 y_2 - x_1 x_2}{1 - dx_1 x_2 y_1 y_2} \right) \, </math>
 
The [[identity element|neutral element]] of this addition is&nbsp;(0,&nbsp;1). The inverse of any point (''x''<sub>1</sub>,&nbsp;''y''<sub>1</sub>) is (−''x''<sub>1</sub>,&nbsp;''y''<sub>1</sub>). The point (0,&nbsp;−1) has order&nbsp;2: this means that the sum of this point to itself gives the "zero element" that is the [[neutral element]] of the group law, i.e. 2(0,&nbsp;−1)&nbsp;=&nbsp;(0,&nbsp;1).
 
If ''d is not a square'' in ''K'', then there are no exceptional points: the denominators 1&nbsp;+&nbsp;''dx''<sub>1</sub>''x''<sub>2</sub>''y''<sub>1</sub>''y''<sub>2</sub> and 1&nbsp;−&nbsp;''dx''<sub>1</sub>''x''<sub>2</sub>''y''<sub>1</sub>''y''<sub>2</sub> are always nonzero. Therefore, the Edwards addition law is complete when ''d'' is not a square in ''K''. This means that the formulas work for all pairs of input points on the edward curve with no exceptions for doubling, no exception for the neutral element, no exception for negatives, etc.<ref name = "Daniel. J Bernstein , Tanja Lange">Daniel. J. Bernstein , Tanja Lange, pag. 3, '' Faster addition and doubling on elliptic curves''</ref> In other words, it is defined for all pairs of input points on the Edwards curve over ''K'' and the result gives the sum of the input points.
 
If ''d is a square'' in ''K'', then the same operation can have exceptional points, i.e. there can be pairs (''x''<sub>1</sub>,&nbsp;''y''<sub>1</sub>) and (''x''<sub>2</sub>,&nbsp;''y''<sub>2</sub>) where 1&nbsp;+&nbsp;''dx''<sub>1</sub>''x''<sub>2</sub>''y''<sub>1</sub>''y''<sub>2</sub>&nbsp;=&nbsp;0 or 1&nbsp;−&nbsp;''dx''<sub>1</sub>''x''<sub>2</sub>''y''<sub>1</sub>''y''<sub>2</sub>&nbsp;=&nbsp;0.
 
One of the attractive feature of the Edwards Addition law is that it is strongly ''unified'' i.e. it can also be used to double a point, simplifying protection against [[side-channel attack]]. The addition formula above is faster than other unified formulas and has the strong property of completeness<ref name = "Daniel. J Bernstein , Tanja Lange"/>
 
'''Example of addition law ''':
 
Let's consider the elliptic curve in the Edwards form with ''d''=2
 
:<math>{\displaystyle}x^2 + y^2 = 1 + 2 x^2 y^2</math>
 
and the point <math>P_1=(1,0)</math> on it. It is possible to prove that the sum of ''P''<sub>1</sub> with the neutral element (0,1) gives again P<sub>1</sub>. Indeed, using the formula given above, the coordinates of the point given by this sum are:
 
:<math>x_3 = \frac{x_1 y_2 + y_1 x_2}{1 + dx_1 x_2 y_1 y_2} = 1</math>
 
:<math>y_3 = \frac{y_1 y_2 - x_1 x_2}{1 - dx_1 x_2 y_1 y_2} = 0</math>
 
==Projective homogeneous coordinates==
In the context of cryptography, [[homogeneous coordinates]] are used to prevent [[elliptic curve cryptography|field inversions]] that appear in the affine formula. To avoid inversions in the original Edwards addition formulas, the curve equation can be written in [[projective space|projective coordinates]] as:
 
<math>(X^2+Y^2)Z^2=Z^4+dX^2Y^2</math>.
 
A projective point (''X''<sub>1</sub>&nbsp;:&nbsp;''Y''<sub>1</sub>&nbsp;:&nbsp;''Z''<sub>1</sub>) corresponds to the [[Affine space|affine point]] (''X''<sub>1</sub>/''Z''<sub>1</sub>,&nbsp;''Y''<sub>1</sub>/''Z''<sub>1</sub>) on the Edwards curve.
 
The identity element is represented by (0&nbsp;:&nbsp;1&nbsp;:&nbsp;1). The inverse of (''X''<sub>1</sub>&nbsp;:&nbsp;''Y''<sub>1</sub>&nbsp;:&nbsp;''Z''<sub>1</sub>) is (−''X''<sub>1</sub>&nbsp;:&nbsp;''Y''<sub>1</sub>&nbsp;:&nbsp;''Z''<sub>1</sub>).
 
An addition formula in projective homogeneous coordinates is given by:
 
: (''X''<sub>3</sub>&nbsp;:&nbsp;''Y''<sub>3</sub>&nbsp;:&nbsp;''Z''<sub>3</sub>) = (''X''<sub>1</sub>&nbsp;:&nbsp;''Y''<sub>1</sub>&nbsp;:&nbsp;''Z''<sub>1</sub>) + (''X''<sub>2</sub>&nbsp;:&nbsp;''Y''<sub>2</sub>&nbsp;:&nbsp;''Z''<sub>2</sub>)
 
where
 
: ''X''<sub>3</sub> = ''Z''<sub>1</sub>''Z''<sub>2</sub>(''X''<sub>1</sub>''Y''<sub>1</sub> − ''Y''<sub>1</sub>''X''<sub>2</sub>)(''X''<sub>1</sub>''Y''<sub>1</sub>''Z''<sub>2</sub><sup>2</sup></sup>2</sub> + ''Z''<sub>1</sub><sup>2</sup>''X''<sub>2</sub>''Y''<sub>2</sub>)
 
: Y<sub>3</sub> = Z<sub>1</sub>Z<sub>2</sub>(X<sub>1</sub>X<sub>2</sub> + Y<sub>1</sub>Y<sub>2</sub>)(X<sub>1</sub>Y<sub>1</sub>Z<sub>2</sub><sup>2</sup> − Z<sub>1</sub><sup>2</sup>X<sub>2</sub>Y<sub>2</sub>)
 
: Z<sub>3</sub> = kZ<sub>1</sub><sup>2</sup>Z<sub>2</sub><sup>2</sup>(X<sub>1</sub>X<sub>2</sub> + Y<sub>1</sub>Y<sub>2</sub>)(X<sub>1</sub>Y<sub>2</sub> − Y<sub>1</sub>X<sub>2</sub>) with ''k''&nbsp;=&nbsp;1/''c''.
 
===Algorithm===
Using the following algorithm, X<sub>3</sub>, Y<sub>3</sub>, Z<sub>3</sub> can be written as:
X<sub>3</sub>→ GJ , Y<sub>3</sub>→ HK, Z<sub>3</sub>→ kJK.d
 
where:
A→ X<sub>1</sub>Z<sub>2</sub>,
 
B→ Y<sub>1</sub>Z<sub>2</sub>,
 
C→ Z<sub>1</sub>X<sub>2</sub>,
 
D→ Z<sub>1</sub>Y<sub>2</sub>,
 
E→ AB,
 
F→ CD,
 
G→ E+F,
 
H→ E-F,
 
J→ (A-C)(B+D)-H,
 
K→ (A+D)(B+C)-G
 
==Doubling==
''Doubling'' can be performed with exactly the same formula as addition. Doubling refers to the case in which the inputs (''x''<sub>1</sub>,&nbsp;''y''<sub>1</sub>) and (''x''<sub>2</sub>,&nbsp;''y''<sub>2</sub>) are known to be equal. Since (''x''<sub>1</sub>,&nbsp;''y''<sub>1</sub>) is on the Edwards curve, one can substitute the coefficient by (''x''<sub>1</sub><sup>2</sup>&nbsp;+&nbsp;''y''<sub>1</sub><sup>2</sup>&nbsp;−&nbsp;1)/''x''<sub>1</sub><sup>2</sup>''y''<sub>1</sub><sup>2</sup> as follows:
 
: <math>
\begin{align}
2(x_1,y_1) & = (x_1,y_1)+(x_1,y_1) \\[6pt]
2(x_1,y_1) & = \left( \frac{2x_1y_1}{1+dx_1^2y_1^2}, \frac{y_1^2-x_1^2}{1-dx_1^2y_1^2} \right) \\[6pt]
& = \left( \frac{2x_1y_1}{x_1^2+y_1^2}, \frac{y_1^2-x_1^2}{2-x_1^2-y_1^2} \right)
\end{align}
</math>
 
This reduces the degree of the denominator from 4 to 2 which is reflected in faster doublings.
A general addition in Edwards coordinates takes 10'''M'''&nbsp;+&nbsp;1'''S'''&nbsp;+&nbsp;1'''C'''&nbsp;+&nbsp;1'''D'''&nbsp;+&nbsp;7'''a''' and doubling costs 3'''M'''&nbsp;+&nbsp;4'''S'''&nbsp;+&nbsp;3'''C'''&nbsp;+&nbsp;6'''a''' where '''M''' is field multiplications, '''S''' is field squarings, '''D''' is the cost of multiplying by a selectable curve parameter and '''a''' is field addition.
 
;Example of doubling
 
As in the previous example for the addition law, let's consider the Edwards curve with d=2:
 
<math>{\displaystyle}x^2 + y^2 = 1 + 2 x^2 y^2</math>
 
and the point P<sub>1</sub>=(1,0). The coordinates of the point 2P<sub>1</sub> are:
 
<math>x_3 = \frac{2x_1y_1}{1+dx_1^2y_1^2} = 0</math>
 
<math>y_3 = \frac{y_1^2-x_1^2}{1-dx_1^2y_1^2} = -1</math>
 
The point obtained from doubling P<sub>1</sub> is thus P<sub>3</sub>=(0,-1).
 
==Mixed Addition==
Mixed addition is the case when Z<sub>2</sub> is known to be 1. In such a case A=Z<sub>1</sub>.Z<sub>2</sub> can be eliminated and the total cost reduces to 9'''M'''+1'''S'''+1'''C'''+1'''D'''+7'''a'''
 
===Algorithm===
A= Z<sub>1</sub>.Z<sub>2</sub>
 
B= Z<sub>I</sub><sup>2</sup>
 
C=X<sub>1</sub>.X<sub>2</sub>
 
E=d.C.D
 
F=B-E
 
G=B+E
 
X<sub>3</sub>= Z<sub>1</sub>.F((X<sub>I</sub>+Y<sub>1</sub>).(X<sub>2</sub>+Y<sub>2</sub>)-C-D)
 
Y<sub>3</sub>= Z<sub>1</sub>.G.(D-C)
 
Z<sub>3</sub>=C.F.G
 
==Tripling==
''Tripling'' can be done by first doubling the point and then adding the result to itself. By applying the curve equation as in doubling, we obtain
 
: <math> 3(x_1,y_1) = \left( \frac{(x_1^2+ y_1^2) - (2 y_1)^2}{4(x_1^2-1)x_1^2 - (x_1^2-y_1^2)^2}x_1, \frac{(x_1^2+ y_1^2) - 2(x_1 )^2}{-4 (y_1^2-1)y_1^2+(x_1^2-y_1^2)^2}y_1 \right). \, </math>
 
There are two sets of formulas for this operation in standard Edwards coordinates. The first one costs 9'''M'''&nbsp;+&nbsp;4'''S''' while the second needs 7'''M'''&nbsp;+&nbsp;7'''S'''. If the '''S/M''' ratio is very small, specifically below 2/3, then the second set is better while for larger ratios the first one is to be preferred.<ref>Bernstein et al., Optimizing Double-Base Elliptic curve single-scalar Multiplication</ref>
Using the addition and doubling formulas (as mentioned above) the point (''X''<sub>1</sub>&nbsp;:&nbsp;''Y''<sub>1</sub>&nbsp;:&nbsp;''Z''<sub>1</sub>) is symbolically computed as 3(''X''<sub>1</sub>&nbsp;:&nbsp;''Y''<sub>1</sub>&nbsp;:&nbsp;''Z''<sub>1</sub>) and compared with (''X''<sub>3</sub>&nbsp;:&nbsp;''Y''<sub>3</sub>&nbsp;:&nbsp;''Z''<sub>3</sub>)
 
;Example of tripling
 
Giving the Edwards curve with d=2, and the point P<sub>1</sub>=(1,0), the point 3P<sub>1</sub> has coordinates:
 
<math>x_3 = \frac{(x_1^2+ y_1^2) - (2 y_1)^2}{4(x_1^2-1)x_1^2 - (x_1^2-y_1^2)^2}x_1 = -1</math>
 
<math>y_3 = \frac{(x_1^2+ y_1^2 - 2(x_1 )^2}{-4 (y_1^2-1)y_1^2+(x_1^2-y_1^2)^2}y_1 = 0</math>
 
So, 3P<sub>1</sub>=(-1,0)=P-<sub>1</sub>. This result can also be found considering the doubling example: 2P<sub>1</sub>=(0,1), so 3P<sub>1</sub> = 2P<sub>1</sub> + P<sub>1</sub> = (0,-1) + P<sub>1</sub> = -P<sub>1</sub>.
 
;Algorithm
 
A=X<sub>1</sub><sup>2</sup>
 
B=Y<sub>1</sub><sup>2</sup>
 
C=(2Z<sub>1</sub>)<sup>2</sup>
 
D=A+B
 
E=D<sup>2</sup>
 
F=2D.(A-B)
 
G=E-B.C
 
H=E-A.C
 
I=F+H
 
J=F-G
 
X<sub>3</sub>=G.J.X1
 
Y<sub>3</sub>=H.I.Y1
 
Z<sub>3</sub>=I.J.Z1
 
This formula costs 9'''M'''&nbsp;+&nbsp;4'''S'''
 
==Inverted Edwards coordinates==
Bernstein and Lange introduced an even faster coordinate system for elliptic curves called the ''Inverted Edward coordinates''<ref>Daniel J.Bernstein . Tanja Lange , pag.2,'' Inverted Edward coordinates''</ref> in which the coordinates (''X''&nbsp;:&nbsp;''Y''&nbsp;:&nbsp;''Z'') satisfy the curve (''X''<sup>2</sup>&nbsp;+&nbsp;''Y''<sup>2</sup>)''Z''<sup>2</sup>&nbsp;=&nbsp;(''dZ''<sup>4</sup>&nbsp;+&nbsp;''X''<sup>2</sup>''Y''<sup>2</sup>) and corresponds
to the affine point (''Z''/''X'',&nbsp;''Z''/''Y'') on the Edwards curve ''x''<sup>2</sup>&nbsp;+&nbsp;''y''<sup>2</sup>&nbsp;=&nbsp;1&nbsp;+&nbsp;''dx''<sup>2</sup>''y''<sup>2</sup> with XYZ&nbsp;≠&nbsp;0.
 
'''Inverted Edwards coordinates''', unlike standard Edwards coordinates, do not have complete addition formulas: some points, such as the neutral element, must be handled separately. But the addition formulas still have the advantage of strong unification: they can be used without change to double a point.
 
For more information about operations with these coordinates see http://hyperelliptic.org/EFD/g1p/auto-edwards-inverted.html
 
==Extended Coordinates for Edward Curves==
There is another coordinates system with which an Edwards curve can be represented; these new coordinates are called '''extended coordinates'''<ref>H. Hisil, K. K. Wong, G. Carter, E. Dawson ''Faster group operations on elliptic curves''</ref> and are even faster than inverted coordinates.  For more information about the time-cost required in the operations with these coordinates see:
http://hyperelliptic.org/EFD/g1p/auto-edwards.html
 
==See also==
*[[Twisted Edwards curve]]
For more information about the running-time required in a specific case, see [[Table of costs of operations in elliptic curves]].
 
==Notes==
{{Reflist}}
 
==References==
* {{Citation
|author=Daniel Bernstein, T.Lange
|year=2007
|title=''Faster Addition and Doubling on Elliptic curves''
|url=http://cr.yp.to/newelliptic/newelliptic-20070906.pdf
}}
* {{Citation | last1=Edwards | first1=Harold M. | title=A normal form for elliptic curves | url=http://www.ams.org/bull/2007-44-03/S0273-0979-07-01153-6/home.html | accessdate=2010-01-29 | publisher=[[American Mathematical Society]] | location=Providence, R.I. | date=4/9/2007 | journal=[[Bulletin of the American Mathematical Society]] | issn=0002-9904 | volume=44 | pages=393–422}}
* ''Faster Group Operations on Elliptic curves'', H. Hisil, K. K. Wong, G. Carter, E. Dawson
* {{Citation
|author=D.J.Bernstein, P.Birkner. T. Lange, C.Peters
|title=''Optimizing Double-Base Elliptic-Curve Single-Scalar Multiplication''
|url=http://cr.yp.to/antiforgery/doublebase-20071028.pdf
}}
* {{Citation | last1=Washington | first1=Lawrence C. | title=Elliptic Curves: Number Theory and Cryptography | publisher=Chapman & Hall/CRC | edition=2nd | series=Discrete Mathematics and its Applications | isbn=978-1-4200-7146-7 | year=2008}}
*{{Citation
|author= Daniel J. Bernstein, T. Lange
|title= Inverted Edwards coordinates
|url=http://cr.yp.to/newelliptic/inverted-20071009.pdf
}}
 
==External links==
* http://hyperelliptic.org/EFD/g1p/index.html
* http://hyperelliptic.org/EFD/g1p/auto-edwards.html
 
{{DEFAULTSORT:Edwards Curve}}
[[Category:Elliptic curves]]
[[Category:Elliptic curve cryptography]]

Revision as of 17:42, 25 January 2014

30 year-old Entertainer or Range Artist Wesley from Drumheller, really loves vehicle, property developers properties for sale in singapore singapore and horse racing. Finds inspiration by traveling to Works of Antoni Gaudí. In mathematics, an Edwards curve is a new representation of elliptic curves, discovered by Harold Edwards in 2007. The concept of elliptic curves over finite fields is widely used in elliptic curve cryptography. Applications of Edwards curves to cryptography were developed by Bernstein and Lange: they pointed out several advantages of the Edwards form in comparison to the more well known Weierstrass form.

Definition

Edwards curves of equation x2 + y2 = 1 − d ·x2·y2 over the real numbers for d = 300 (red), d = √8 (yellow) and d = −0.9 (blue)

The equation of an Edwards curve over a field K which does not have characteristic 2 is:

for some scalar . Also the following form with parameters c and d is called an Edwards curve:

where cd ∈ K with cd(1 − c4·d) ≠ 0.

Every Edwards curve is birationally equivalent to an elliptic curve in Weierstrass form. If K is finite, then a sizeable fraction of all elliptic curves over K can be written as Edwards curves. Often elliptic curves in Edwards form are defined having c=1, without loss of generality. In the following sections, it is assumed that c=1.

The group law

(See also Weierstrass curve group law)

It is possible to do some operations on the points on any elliptic curve, such as adding two or more points and doubling or tripling one. Usually, given two points P and Q on an elliptic curve, the point P + Q is directly related to third point of intersection between the curve and the line that passes trough P and Q; but in the case of Edwards curve this is not true: indeed the curve expressed in Edwards form has degree 4, so drawing a line one gets not 3 but 4 intersection points. For this case a geometric explanation of the addition law is given in [1]

Edwards addition law

File:Edwards Curve,Clock group diagram.jpg
Clock group

It is possible to add points on an elliptic curve, and, in this way, obtain another point that belongs to the curve as well.

To understand better the concept of "addition" between points on a curve, a nice example is given by the circle:

take the circle of radius 1

and consider two points P1=(x1,y1), P2=(x2,y2) on it. Let α1 and α2 be the angles such that:

The sum of P1 and P2 is, thus, given by the sum of "their angles". That is, the point P3=P1+P2 is a point on the circle with coordinates (x3,y3), where:

In this way, the addition formula for points on the circle of radius 1 is:

.

When two points (x1y1) and (x2y2) on an Edwards curve are added, the result is another point which has coordinates:

The neutral element of this addition is (0, 1). The inverse of any point (x1y1) is (−x1y1). The point (0, −1) has order 2: this means that the sum of this point to itself gives the "zero element" that is the neutral element of the group law, i.e. 2(0, −1) = (0, 1).

If d is not a square in K, then there are no exceptional points: the denominators 1 + dx1x2y1y2 and 1 − dx1x2y1y2 are always nonzero. Therefore, the Edwards addition law is complete when d is not a square in K. This means that the formulas work for all pairs of input points on the edward curve with no exceptions for doubling, no exception for the neutral element, no exception for negatives, etc.[2] In other words, it is defined for all pairs of input points on the Edwards curve over K and the result gives the sum of the input points.

If d is a square in K, then the same operation can have exceptional points, i.e. there can be pairs (x1y1) and (x2y2) where 1 + dx1x2y1y2 = 0 or 1 − dx1x2y1y2 = 0.

One of the attractive feature of the Edwards Addition law is that it is strongly unified i.e. it can also be used to double a point, simplifying protection against side-channel attack. The addition formula above is faster than other unified formulas and has the strong property of completeness[2]

Example of addition law :

Let's consider the elliptic curve in the Edwards form with d=2

and the point on it. It is possible to prove that the sum of P1 with the neutral element (0,1) gives again P1. Indeed, using the formula given above, the coordinates of the point given by this sum are:

Projective homogeneous coordinates

In the context of cryptography, homogeneous coordinates are used to prevent field inversions that appear in the affine formula. To avoid inversions in the original Edwards addition formulas, the curve equation can be written in projective coordinates as:

.

A projective point (X1 : Y1 : Z1) corresponds to the affine point (X1/Z1Y1/Z1) on the Edwards curve.

The identity element is represented by (0 : 1 : 1). The inverse of (X1 : Y1 : Z1) is (−X1 : Y1 : Z1).

An addition formula in projective homogeneous coordinates is given by:

(X3 : Y3 : Z3) = (X1 : Y1 : Z1) + (X2 : Y2 : Z2)

where

X3 = Z1Z2(X1Y1Y1X2)(X1Y1Z222 + Z12X2Y2)
Y3 = Z1Z2(X1X2 + Y1Y2)(X1Y1Z22 − Z12X2Y2)
Z3 = kZ12Z22(X1X2 + Y1Y2)(X1Y2 − Y1X2) with k = 1/c.

Algorithm

Using the following algorithm, X3, Y3, Z3 can be written as: X3→ GJ , Y3→ HK, Z3→ kJK.d

where: A→ X1Z2,

B→ Y1Z2,

C→ Z1X2,

D→ Z1Y2,

E→ AB,

F→ CD,

G→ E+F,

H→ E-F,

J→ (A-C)(B+D)-H,

K→ (A+D)(B+C)-G

Doubling

Doubling can be performed with exactly the same formula as addition. Doubling refers to the case in which the inputs (x1y1) and (x2y2) are known to be equal. Since (x1y1) is on the Edwards curve, one can substitute the coefficient by (x12 + y12 − 1)/x12y12 as follows:

This reduces the degree of the denominator from 4 to 2 which is reflected in faster doublings. A general addition in Edwards coordinates takes 10M + 1S + 1C + 1D + 7a and doubling costs 3M + 4S + 3C + 6a where M is field multiplications, S is field squarings, D is the cost of multiplying by a selectable curve parameter and a is field addition.

Example of doubling

As in the previous example for the addition law, let's consider the Edwards curve with d=2:

and the point P1=(1,0). The coordinates of the point 2P1 are:

The point obtained from doubling P1 is thus P3=(0,-1).

Mixed Addition

Mixed addition is the case when Z2 is known to be 1. In such a case A=Z1.Z2 can be eliminated and the total cost reduces to 9M+1S+1C+1D+7a

Algorithm

A= Z1.Z2

B= ZI2

C=X1.X2

E=d.C.D

F=B-E

G=B+E

X3= Z1.F((XI+Y1).(X2+Y2)-C-D)

Y3= Z1.G.(D-C)

Z3=C.F.G

Tripling

Tripling can be done by first doubling the point and then adding the result to itself. By applying the curve equation as in doubling, we obtain

There are two sets of formulas for this operation in standard Edwards coordinates. The first one costs 9M + 4S while the second needs 7M + 7S. If the S/M ratio is very small, specifically below 2/3, then the second set is better while for larger ratios the first one is to be preferred.[3] Using the addition and doubling formulas (as mentioned above) the point (X1 : Y1 : Z1) is symbolically computed as 3(X1 : Y1 : Z1) and compared with (X3 : Y3 : Z3)

Example of tripling

Giving the Edwards curve with d=2, and the point P1=(1,0), the point 3P1 has coordinates:

So, 3P1=(-1,0)=P-1. This result can also be found considering the doubling example: 2P1=(0,1), so 3P1 = 2P1 + P1 = (0,-1) + P1 = -P1.

Algorithm

A=X12

B=Y12

C=(2Z1)2

D=A+B

E=D2

F=2D.(A-B)

G=E-B.C

H=E-A.C

I=F+H

J=F-G

X3=G.J.X1

Y3=H.I.Y1

Z3=I.J.Z1

This formula costs 9M + 4S

Inverted Edwards coordinates

Bernstein and Lange introduced an even faster coordinate system for elliptic curves called the Inverted Edward coordinates[4] in which the coordinates (X : Y : Z) satisfy the curve (X2 + Y2)Z2 = (dZ4 + X2Y2) and corresponds to the affine point (Z/XZ/Y) on the Edwards curve x2 + y2 = 1 + dx2y2 with XYZ ≠ 0.

Inverted Edwards coordinates, unlike standard Edwards coordinates, do not have complete addition formulas: some points, such as the neutral element, must be handled separately. But the addition formulas still have the advantage of strong unification: they can be used without change to double a point.

For more information about operations with these coordinates see http://hyperelliptic.org/EFD/g1p/auto-edwards-inverted.html

Extended Coordinates for Edward Curves

There is another coordinates system with which an Edwards curve can be represented; these new coordinates are called extended coordinates[5] and are even faster than inverted coordinates. For more information about the time-cost required in the operations with these coordinates see: http://hyperelliptic.org/EFD/g1p/auto-edwards.html

See also

For more information about the running-time required in a specific case, see Table of costs of operations in elliptic curves.

Notes

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

References

  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

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    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Faster Group Operations on Elliptic curves, H. Hisil, K. K. Wong, G. Carter, E. Dawson
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010

External links

  1. Template:Cite web
  2. 2.0 2.1 Daniel. J. Bernstein , Tanja Lange, pag. 3, Faster addition and doubling on elliptic curves
  3. Bernstein et al., Optimizing Double-Base Elliptic curve single-scalar Multiplication
  4. Daniel J.Bernstein . Tanja Lange , pag.2, Inverted Edward coordinates
  5. H. Hisil, K. K. Wong, G. Carter, E. Dawson Faster group operations on elliptic curves