#Introduction #Elliptic #Curve #Cryptography

So, welcome to this today’s lecture on elliptic curves cryptography. So, in continuing with public key ciphers, today we shall discuss about a special type of cryptographic technique, this is actually is employed based upon, I mean geometrical interpretations. So, it is a classic combination of algebraic properties with geometric techniques. So,

We shall discuss about two fundamental elliptic curve arithmetic operations, namely point addition and point doubling, and then discuss about some of the forms of elliptic curve equations, which are used in cryptography. And then, discuss about projective coordinates, which are often useful for implementing elliptic curves.

To start with, let us start with a puzzle, that is let us consider this question, that is what is the number of balls that may be piled up as a square pyramid and also rearranged into a square array. So, this problem is a kind of deceivingly simple problem, which

Can be solved easily in this fashion. That is, let us assume that x be the height of the pyramid, so if you assume that x is the height of the pyramid then, so we can we can essentially try to compute the total number of balls in this fashion.

So, for example, if you consider that your pyramid is arranged in various squares, and you start on building up the pyramid, then you can count the number of balls in this fashion, right. So, you can count, you can imagine that the pyramid is made of squares of balls, and suppose

This is the height of the pyramid, and the height of the pyramid is denoted in this case by x, ok. So, what is the total number of balls in the first layer? It is one by one,

So it is 1 square, so there are 1 square numbers of balls. In the second, there it is 2 square number of balls, so if I continue like this, the last layer has got x square number of balls. So, therefore, total number of balls can be computed in this fashion as 1 square

Plus 2 square and so on till x square. Now, these balls by the question can also be rearranged in to square array, so we can actually rearranged these balls and rearrange them in the form of the square. So, the square also has a dimension of y by y and therefore

The total number of balls can also be written to be equal to y square. So, if you solve these two equations, then, therefore we know that the left hand side computes to x into x plus 1 into 2 x plus 1 by 6 and that is equal to y square. Now,

This form of equations is referred to as the elliptic curves, ok. So, it may be remembered at this point, that it has, it its name actually derived from the word, from the terms elliptic integrals and it has nothing to do with ellipses. So,

Therefore, if we solve or rather write or rather draw this kind of this graph, then the graph would look like this. So, therefore, we can actually obtain certain, we can observe certain properties. So, for example, it is symmetric over the

X-axis and there are two distinct loads here. So, this is a kind of form, and this is kind of curves, which are commonly known as elliptic curves. So, these are cubic curves in the variable x, and they are also symmetric above the y-axis, this is a kind of distinct property.

And purely we see that it has got more similarity with ellipses, so therefore, it should not be mistaken that elliptic curves have got any similarity with ellipses. So, now, there was famous method, which is known as the method of diophantus, which is

Which is often used to, it uses a set of known points to actually produce new points. For example, from this graph, immediately we know that there are two common, that there are two trivial solutions, like for example, 0, 0 satisfies this equation, similarly 1, 1 also satisfies this equation.

It can be easily seen like if i substitute 0 and 0 on both sides, will be satisfied, if we substitute 1, 1, then the left hand side computes into 2 1 into 2 into 3, that is 6 divided by 6, and the right hand side also 1 square, which satisfies this equation.

Now, starting from these two trivial roots, like 0, 0 and 1, 1, diophantus methods gives us new nontrivial values, which will satisfy this curve equation. So, how does it work? It is is like this, so the technique is like this, that is we take a line through these

Points, like we take 0, 0 and 1, 1, and we draw a line which connects 0, 0 and 1, 1. So, you can be, it is easily understood that the equation of the line through these points is y equal to x. So, now, let us take this y equal to x and

Find out where does this line intercept the elliptic curve, because since it is a cubic curve curve in x, it can be easily seen that if I take two points on the curve and draw a line, then it should definitely meet the curve on a third point. So, therefore, let

Us try to find out that equation or whether that coordinates of the third point, and it can be found out by substituting y equal to x into the curve, and the curve therefore

Takes the form of x cube minus 3 by 2 x square plus x by 2, it can be checked and that is equal to 0. So, now, we would like to find out the roots for this equation to obtain the x-axis of the third point, third intersecting point.

So, what we have done is essentially like this, we have taken, suppose this is the curve equation, we start with this point 0, 0, we start with the point that the other point, that is 1, 1 and we draw a line through these two points, and it is believed. Therefore,

Since it is a cubic curve in x, this should definitely intersect the curve again at another point, and we are trying to find out x coordinate of that point ok. So, if we do so, that is if we substitute this, then we see that we get this equation

X cube minus 3 by 2 x square plus x by 2 is equal to 0. So, therefore, what we have done is that we have taken this y equal to x equation of the line, and substituted it into the equation

Of the curve, which is given by this. So, basically we are trying to solve simultaneously y equal to x and the equation of the curve. Now, we know from the theory of the equations that since we know that this equation has got three roots, out of them two of them has

Got a value of 0 and 1. So, that means, 0 plus 1 plus the third point which we are trying to find out, let us call it x, this should add up to the value of 3 by 2, right.

Because that is that follows from the theory of equation, that is if we sum up the all the roots, then it is the negative of the first first first coefficient, which is here in this case minus 3 by 2 and therefore, this is equal to plus 3 by 2. So, therefore, from

This we get that x is nothing but equal to half, ok. So, therefore, what is the value of y, since it lies also on the x equal to y line or y equal to x line, therefore y is also equal to half, right. So, therefore, you obtain

Another point, which is 1/2, 1/2 right. So, that means, now this is a third point on the curve, and you just see its is it is a more kind of, we can say it is a more nontrivial root, then the roots that which with which we started with ok.

Now, immediately we know that if 1/2, 1/2 is a point on the curve, then by the symmetrycity of the curve 1/2, minus 1/2 is also a point on the curve right, so that means, that we

Know that if 1/2 and 1/2 are points on the curve, then by the symmetry city of the curve, 1/2, minus 1/2 is also a point in the curve. So, now, we shall try to find out, that is, what is the line which joins 1/2, minus 1/2

And 1, 1? We will again similarly continue this approach, and we will obtain further and further points. So, we our objective is to obtain further and further points, and ensure that the value of x is an integral value, because if we obtain a value of x,

Which is an integral value, then that gives us a nontrivial solution to the puzzle that which we started with, right. So, therefore, what we do is that we take this 1/2 and minus 1/2, and we draw a line through 1/2, minus 1/2 and 1, 1. So, we already

Knew that 1, 1 is your point, we have got a 1/2, minus 1/2 point and we want further points. So, what we do is that we take these two points, and we draw a line, and if we

Can easily check that the line which joins 1, 1 and 1/2, minus 1/2, we will have the equation of y equal to 3 x minus 2, ok. So, this is quite trivial, I am not going through this, but we can obtain the equation of the straight line in this fashion. Now,

We will intercept this with the curve again, that is we will substitute y equal to 3 x minus 2 into the original curve equation, and we obtain an equation, in that in that in that case which has the form of this. So, we are not actually bothered about the higher

Terms, because what we need is the is the value of this, right, because we need to find out the value of x. So, we know that one value is 1; the other value is 1/2, so that means 1 plus 1/2 plus x should give us 51 by 2. So, this means that

The value of x in this case comes out to be 24, and if we substitute this value of x into the equation, we obtain the value of y as 70. So, that means, that we remember that in our case this is, this gives the height of the square pyramid, and this gives the

Dimension of the square arrangement of the ball. So, this what is the number of balls? Therefore, the total number of balls is therefore, in this case 70 square or that is 4900, right. Anyway this is not so important in the perspective of cryptography, but what is important is

That using this technique of diophantus you can produce more and more number of points on the elliptic curve, right. And this technique is actually employed in computing arithmetic or performing arithmetic on the elliptic curve points, that is point addition and point doubling,

Which we will soon we will see. And is the same technique this is still being adopted for elliptic curve operations. So, therefore, now we come into the application of elliptic curve or rather talk about elliptic curves in the context of cryptography. So, this was essentially first proposed or first

Found in the literature and in the on cryptography, in around 1985, developed independently by neal koblitz and victor miller. And the fundamental problem of on which elliptic curve cryptography is based is commonly known as the discrete logarithm problem, like what we have seen

Is discrete logarithm problems on finite fields in the last class. Similarly, we can actually develop elliptic or discrete logarithmic problem on elliptic curve groups, and it is believed to be more difficult than the corresponding problem in the underlined finite fields, and that actually leads to a significant, I mean significant

Important in cryptography, because this reduces the sizes of the operations that which that, which you are doing. And therefore, elliptic curve cryptography is probably the most efficient public key ciphers, at least one of the most efficient public key ciphers that we have seen, and therefore it is quite important.

So, first of all let us try to see some elliptic curves, or let us try to conceptualize how elliptic curve looks like. So, consider this feel of modulo 5, that is if you consider the x field to lie between modulo 5 x and y values, to lie between modulo 5, that means

It can take the value of 0, 1, 2, 3, 4, and that’s it. So, that means, now we are going to start with some points like x equal to 0, so immediately if I substitute y square equal to 3, you see that in modulo 5 there is no solution, that

Is if I take the value of say 0, 1, 2, 3, 4, then if I square them, none of them will be equal to 3, so that means there are no solutions for this. Consider x equal to 1, if you substitute this, you will get x square equal to 6 and therefore

Modulo 5 this is 1, immediately you know that both 1 and 4 are solutions, because you know that 1 squared is 1, and 4 squared is 16, modulo 5 is again one, so therefore these are solution. Similarly, you can obtain the other solutions,

And what you find is that therefore the points on elliptic curves are actually (1, 1), (1, 4), (2, 0), (3, 1), (3, 4) and (4, 0), so let us now not consider this point the infinity, which I will define later on, but actually we can say these are some discrete points

On the elliptic curve. Now, the reason why we are actually considering this example is to understand that although we are actually drawing continuous elliptic curves, they are actually not continuous, but they are discrete curves. That is the curves are actually defined on certain finite elements, that is they are not actually continuous

Curves, but they are actually discrete collection of points. So, using the finite field, we can actually form a elliptic curve groups, where we have got an elliptic curve discrete lock problems, this is often commonly called as the elliptic

Curve discrete logarithm problem, but I will define that later on. But first of all let us at least try to understand that elliptic curves on a finite set of integers are actually a cluster of discrete points and not continuous points.

So, a definition as elliptic curve would look like this, that is an elliptic curve over is defined generally over a field k, like what we have seen in the previous example, it was defined over the points on, it was it was defined over the points on the modulo

5 field, right. So, similarly it is always defined about a field k, and it is a non singular cubic curve, it is non singular, I will define what is mean by non singular, but it is it is non singular cubic curve, it is cubic curve in

X. And in this, in generally in two variables f x,y is equal to 0 with a rational point, so there is an additional point which is called as a point at infinity. Now, what is the purpose or what is the definition of the point in infinity? You will see soon.

The field k is usually taken to be the complex numbers, real numbers, rationals, algebraic extensions of rationals, p-adic numbers, or a generally a finite field, so it can be various kind of, it can be a various kind of algebra with, I mean, based upon the field k can be

Defined. So, elliptic curve groups for cryptography are examined generally with underlying fields on F p, so it is F p, means it is a prime field, where P is greater than 3. And it is a prime or it is a binary extension, where

There are 2 power of m elements, that is F 2 power of m fields. So, generally when whenever elliptic curves are applied for cryptography, then the underlying fields are either in F P or in F 2 power of m.

So, consider this is the very common general form of the elliptic curve equation, which is like this, that is y squared equal to x cube plus a x plus b. So, here this is a plane

Curve, which is defined by an equation of the form y square equal to x cube plus a x plus b. Now, note that x here is again not a discrete, I mean it is not a continuous point. Now, it is chosen from a particular field,

So it is either chosen from may be g f p, where it is all prime numbers like 0 to P minus 1, for a large prime number, or it is g f 2 power of m, that is where the numbers

Or the elements can be expressed in certain discrete points again. So, that is chosen from what is known as the binary fields. So, similar to that, we can actually like, what we have seen previously, we can actually imagine these curves, and these are some common

Ways of denoting this. Although they note that we have drawn continuous curve equations or try to draw continuous curve diagrams, they are actually not continuous again, they are again done discrete points, which are being joined. So, for example, consider this line y squared equal to x cube minus 1, y squared equal to

X cube plus 1, and there are some several points which have been, several graphs which have been drawn. So, what is important here is that this graph is a cubic graph is terms of x, and also this graph or curve needs to be non singular, which means that we have

To ensure that the del f of del x, and the del f of del y are not the same and equal to 0, which means that the graph should generally does not, I mean does not, should not have double roots in it. So, we will see more of that in our future discussions.

Now, common or more generalized form of this elliptic curve equation is what is referred to as the weierstrass equation. Now, it is a two variable equation f x, y equal to 0, which forms a curve in the plane, and the generalized weierstrass equation of elliptic

Curves looks like this, that is y square plus a 1 x y plus a 3 y equal to x cube plus a 2 x square plus a 4 x plus a 6. So, here, you know, we may note that again

That the the cubic order is again again maintained, that is x is again having an order a term of x cube. And the term y has got a power of y square, so this is very important and fundamental to the definition of elliptic curves. So, here x and y and constants all

Belong to a field again of say rational numbers, complex numbers, finite fields F P or galois field g F 2 power of n. So, cryptography, generally we choose them again from the finite fields or from g F 2 power of n. Now, this weierstrass equation depending upon

The characteristic of the field, actually takes various forms. So, the characteristic of a field is actually like, the if I take an element say one, and if I add them say lambda number of times, if I get 0, and that lambda is a minimum number of times when you

Have added, that is commonly referred to as the characteristic of the field. For g F 2 fields, the characteristic is two, like if I take 1 and I add 1, I get 0; that

Is the minimum number of times I need to repeatedly add 1 to get a 0 in the binary fields. So, depending upon the field characteristic, its curve equation varies. For example, here what we have done is that, we have taken the original weierstrass equation, and we are tried to

Kind of manipulate them using the fact that the characteristic field is actually not 2. So, what we have done is that we have started with this original weierstrass equation, which

Was like y square plus a 1 x y plus a 3 y equal to x cube plus a 2 x square plus a 4 x plus a 6. So, what we are now doing is that, we are using the fact that the characteristic

Of the field is not two. And using that we are trying to manipulate this, and write it in this form, like y plus a 1 x by 2 plus a 3 by 2 whole square equal to x cube plus

A 2 plus a 1 square by 4 x square plus a 4 x plus a 3 square by 4 plus a 6. So, do we noted that what we have done is that, we are actually expressed this in this

Form. So, if I simplify this, again this original equation will be obtained. Now, you note that since the characteristic of the field is assumed not to be 2, therefore we can actually divide with the with the number 2, that is important. So, therefore, if I do so the then we can

See that this particular term here, that is y plus a 1 x by 2 plus a 3 by 2 can now I replaced by a single variable. For example, I can replace this by a variable say Y, and

I can call it like Y square equal to X cube plus, I call it like A2 dash X square plus A 4 dash X plus A 6 dash, ok. So, therefore, this is the same weierstrass equation, but written in a more simpler way, assuming the fact that a characteristic field

Is, characteristic of the field is not two. A 1 x by 2. Yeah. Ah y plus a 1 x by 2. y plus a 1 x by 2. So, we yeah. So, we have actually substitute the entire thing by Y,

So we can basically… the entire thing is being substitute by y, here yeah, that is why when you take this two then you have got a one x y term here, right, so therefore, the entire thing is now to being replaced by y here.

Now, similarly you can actually continue simplification in further way, so what you can do is that you can assume now that the characteristic is neither two nor three. So, if we assume that, that you can make further substitutions, like you can, since if you assume that, in

This case, we assume that a characteristic is not equal to 2, we make further assumption like the characteristic is not 3, and therefore we can substitute like x 1 is X plus A2 or rather A2 dash by 3. So, if you do that, then this further reduces

To the form of Y square equal to X, I am just little bit… I am using the notations, but the basic form that it will be reduced to is this. And this is a very common form of the cubic curves, that is we know that it is neither characteristic two field, or neither

A characteristic three field, in which you can actually, I mean, under which assumptions you can derive the curve equation in this simple form. So, therefore, we we can actually start with the original generalized weierstrass equation, and we can actually derive them depending upon the characteristic of the field, we can

Actually make it more simple and you can write them, right. So, therefore, now what are the points on the elliptic curve can actually be written in a more formal way, you can say that the points on the elliptic curves, say call it

An elliptic curve e, which is defined over a field which is l, to be the collection or the union of the of a point at infinity, and the or the other pairs of x, y, which belongs to l cross l, because x also belongs l and y also belongs to l.

So, therefore, it is a subset of the l cross l, the cartesian of l and l right, and all of them, I mean both x and y satisfies the original curve equation, where there are definitely

These two things like y square on the x cube terms, the rest of the things may vary depending upon the characteristic of the curve. So, it is useful to add the point at infinity, and we will see more clear in a closely y, the point is sitting, the this point of the

Infinity is a kind of point, which is generally believe like you sitting at the both the top of the y-axis and also bottom. So, therefore, it is it lies kind of at both the spaces. So, therefore, for example, if I draw a point or rather draw an elliptic curve, an elliptic

Curve would essentially look like this, and it would be compassed with, encompassed with various points which lie on the curve, but we assume that there is a point which is both at the top and both at the end, and this point is also commonly known as the o point, or

The point at infinity, the same point, this point o is also at the top. So, therefore, if I draw vertical line, then we know that by the original diophantus technique what we have seen is that, if there are two points, and if I draw a vertical line, then

It should also intersect the elliptic curve again at a third point right, but in this case, since it will not actually intersect on the elliptic curve, we assume that it intersects the elliptic curve again at a point on infinity and that is the concept of the point o.

So, therefore, it both at the top and at the bottom of the y-axis, and any line is said to pass through this point when it is vertical, so any vertical line will intersect this point at infinity, which is also assumed to lie on the elliptic curve based on this definition,

Ok. And it is a very useful point, because it helps us to define the concept of a group, because it helps us to create a group, which is very much needed in order to apply this elliptic curve for various applications, right.

So, let us see some of the reasons why the elliptic curve should form an abelian group. Now, this is slightly a recapitulation of the abelian group and we know that given two points P and Q, which lies saying E of F p, so it is an elliptic curve, it is defined

About the say finite, say on the plane fields. Then there is a third point, which is denoted by P plus Q, which also lays on E of F p, that is if I take P and if I take Q, we have

To define an operation, call it a class operation, or an addition operation on these two points, such that it also lies on E f P, that is it should be closed, right and the following relations also has to hold for all P, Q and R, which lies on E of F p .

So, what are the fundamental requirements? The first thing is the commutativity property, that is we know that if I add P and Q, this should be the same as adding Q and P, it also should satisfy the property of associativity, which means that if I take P plus Q, and then

I add R, it is a same as giving P and adding it to the sum of Q and R, ok. That is, it does not matter in which order we are doing the addition operation. Similarly,

There should be an identity element, that is if I take P and if I add O, then I should get back, and it is the same as O plus P, because of the commutativity property, and both of them should actually go back to P, that is there O, that is essentially here

With the identity element, and O is generally referred to as the point of infinity. So, that means, the point of infinity is assumed to be the identity element in the plus operation that we are trying to define here. Similarly, the I mean another point is that there exists

A minus P, such that if I take P and I add minus p, that is the additive inverse of P, then I should get back O, that is the point on infinity again. So, therefore, minus P is the additive inverse of P.

So, let us try to see how we can do this operation. So, consider an elliptic curve y square equal to x cube minus x plus 1, this is the form of an elliptic curve that we have chosen here.

So, if P 1 and P 2 are two points on this curve, so consider that there is another point called P 2, and let us try to define an addition operation on P 1 and P 2, ok.

So, what we essentially expect by the diophantus technique is that if I take P 1 and P 2 and draw a straight line through this, it should intersect the elliptic curve on a third point like. So, do the same technique which we had, what we have seen in context to solving the

First puzzle that we started with, right. So, now, for our addition purpose, instead we do not take this as the sum, but we take the deflection of this point on the x-axis

As the sum, that is we call P 3 to be the sum of P 1 and P 2. So, we take P 1 and P 2, and remember that when we started with was taking 0 0 and 1 1, and it intersected

The curve at a point ½, 1/2 and instead of telling 1/2, 1/2, we actually took the point of 1/2 minus 1/2. So, this has got a very close relationship with the diophantus technique, right, so we take this point and we draw this line.

So, now we will try to make it more formal, so therefore immediately you understand that. If I tell that P 3 is a sum of P 1 and P 2, and then we can actually obtain the ordinates of P 1 of P 1 plus P 2 by using the techniques of coordinate geometry.

So, that actually, I mean motivates us to apply coordinate geometric techniques here. So, consider that there is a point p, and there is a point Q, and the point P at Q, and Q have got the ordinates as shown in the diagram as x 1, y 1 and x 2, y 2.

So, what we have to telling is that we have got say a curve, and this curve has got two points, say one point is P, the other point is Q, and we are considering this line, that

Is the sum of P and Q. So p, the ordinate of P is x 1, y 1 and the ordinate of Q is x 2, y 2, and it is intersecting the curve again at a third point, so call it minus P

Plus Q, because you will consider the reflection of these point on the x-axis as the sum of P plus Q. So, therefore, this point is the point R, which is the sum of P and Q. So, what we do is that we actually consider the slope of

This line first, that is considering the slope of this particular line, which is actually used to join this P and Q. So, therefore, call this line as some line like y equal to m x plus c, ok. So, what is the value of m here, which is

The slope? m is immediately obtained as y 2 minus y 1 divided by x 2 minus x 1. So, therefore, of course, here when you are writing the m like this, we are making an assumption

That x 1 and x 2 are not the same. So, under these assumptions we know that we can write the value of m like this. So, now we will try to find out the intersection of this line with the with the cubic equation. So, we started with the cubic form and the

The the equation of the curve is like y square equal to x cube plus a x plus p. So, what we do is that we take this and we substitute this that is we take y 2 as m into x 2 minus

X 1 plus y 1, so is that straight away from the equation. So, we know that if in this equation, if there is a point which has got a coordinate of x or ordinate of x, then the y ordinate is obtained by this, right. So, therefore, if we substitute x here, this

Is the corresponding ordinate for y. So, we can take this y and substitute this in the equation right, so that means that we can actually take this and this, and we can combine these two equations, and we can write them as m x minus x 1 plus y 1 whole square is

Equal to x cube plus a x plus b, right, we can write them in this form. And therefore, we can actually rearrange them, and we can write them as 0 equal to x cube minus m square x square plus some more terms, which we are not actually bothered with. So, again you

See that there is a very close similarity with the diophantus method, right. Because, what we are now doing is that we have and we know that there are two points x 1 and x 2, and the third sum, if I believe it to be x, right is if I add these two things

I get m square, so that means that x is nothing but m square minus x 1 plus x 2 right. So, if I know that this particular curve is satisfied by three experts because of the cubic thing, right. So, two of them are x 1 and x 2, the third

Point if it is x, then if I add them, then it is nothing but the minus of minus m square, which is m square that is x, is nothing but m square minus x 1 plus x 2, right. So, we

Take this x, and that is why my essentially the ordinate of the of the third point of minus P plus Q ok. So, that is the way, I mean, so therefore we know that if we can obtain this point, similarly we can obtain the x coordinate of

This one also, because it is the same, right. The x-axis is the, it is since we are taking a reflection here, this point x x value and this point’s x value are the same x values.

So, therefore, the x is given by this m square minus x 1 plus x 2, where m is actually obtained by this equation. So, if we combine this, this will essentially look like this, that is I will come to this actually later on, I mean, the combined form do you know when

We summarize the results. Now, the obvious question is what about the case when P and Q are actually the same point. That is if the point P and Q are actually the same point, then we cannot write an m value like this, right. So, the obvious answer

To this is, since we are drawing a tangent here we do a differentiation, right, we tend to, we differentiate the value of of y with respect to x to obtain the corresponding equation of the tangent ok.

So, what we do is that we take this line, that is y square equal to y square equal to x x cube plus a x plus B, which is the equation of the curve, and we draw a line, like we

What we do is that we take a line and write 2y d y by d x is nothing but 3 x square plus A right. So, therefore, d y by d x at the point at any point x 1, y 1 is defined as

1 x 1 square plus a divided by 2 of y 1. That means that if I take this equation of the curve here, then if I draw a tangent here, and the tangent is defined at the point of

X 1, y 1 then this is the slope which is we are which we are defining here, right. So, therefore, this becomes the slope of this tangent, right So, therefore, now the question is, that is what how do I actually obtain the third point.

So, therefore, after this we can do a similar technique as what we have seen previously, and we can actually obtain the ordinate of the third point, right. So, what we do is that we again assume that this particular equation y equal to m x is substituted y x,

Is substituted into the curve equation, ok. And we again solve for the three point sum, and we obtain the coordinate of the third point. So, in this case, if I do so, then again I get this form like 0 equal to x cube minus m square x square plus so on. So, therefore,

In this case the two points that is x plus x, if I take that this point is x 1, y 1, so it is 2 x 1 actually, x 1 plus x 1 plus the third point is x sums to m square, like

What we have seen previously. So that means that x is nothing but m square minus 2 of x 1, where m is actually obtained by 3 x 1 square plus A divided by 2 y 1, right. So, therefore, this is the way how we can actually obtain the value of x, and similarly

Again we can draw reflection of the of this point and obtain the corresponding some point right, of this of the same x and y. So, actually this particular I mean way of obtaining the third point is called there is a doubling of two points, the this is actually commonly

Called as a doubling, this particular operation, whereas this is called commonly as the addition operation or the point addition operation. And these are actually forms two fundamental operations of the elliptic curve, of elliptic elliptic curve cryptography.

So, now, we again have this question like what happens when P 2 is equal to the point at infinity, then what happens. So, that means that what we are saying here is that, we will take P 1 and the second point is the point at infinity.

So, we basically what we do is that we take P 1 and we draw a vertical line through P 1, and therefore it intersects the P 1 at the the third point. So, what we need to do

After this is, we need to reflect this point, and if I reflect this point then we get back the value of P 1. So, therefore, this is actually I mean, means that the point at infinity satisfies whether behaves as the, it it behaves as what the

Group property should should should, would want it to satisfy, right. That is, O is essentially serving as your you’re your inverts, because we are adding P p P 1 to O, and you are getting back the value of P 1. So, therefore, this means that

The point at infinity with the additional point of infinity, we are actually ensuring that the point on the elliptic curve satisfies the group properties. So, actually there are, there is a, so what we fundamentally required is these four properties, we need that it should essentially satisfy the property of commutativity, which follows

From the way the addition has been done, because if I draw two lines it does not matter which way you are drawing it. Similarly the point or rather the existences of an identity element is also understood by the way what what we saw just now.

Similarly, the point of inverse is also understood, because if you take P and minus P, it will again intersect the point at infinity, if you reflect that point at infinity, you still get that infinity, because the it is assumed that the point infinity exist both at the

Top and both at the bottom, right. But what is not so obvious is this associativity… why it is associative also? But, this is actually a quite lengthy proof, and it is actually beyond the scope of this discussion. So, therefore, we will assume that it also satisfies the

Associativity property, and therefore the elliptic curve elements at, where the elliptic curve points satisfies the group properties or or other forms a group, forms an abelian group on that the addition operation. And how is the addition operation defined?

Now, we take two points, and we draw a straight line through it, intersect if there any third point, reflect that point on the x-axis to obtain the corresponding sum, or if the point P and Q are the same, then we draw a tangent through that point, again obtain the the point

Of intersection, reflect that and obtain the corresponding output. So, that is the corresponding, either how we do a P plus Q, where P and Q are different, or we do Q P, or that is how we do the addition property- addition operation or how we do the doubling operation. So, that

Is how it is defined, ok. So, therefore, this is a summary of what we have seen, that is if that points are different, that is if I take two points like x 1,y 1 and x 2,y 2 in elliptic curve, then your slope

Is defined like this, where x 1 and x 2 are not the same, but if x 1 and x 2 are indeed the same, then this is the way the slope is defined, and for both these cases your third

Point is actually lambda square minus x 1 minus x 2, and the corresponding y ordinate is obtained like this. So, therefore, we users take this and you substitute this and obtain the corresponding y sum. So, this is a cryptographic description,

Of you take P and Q, you add it, it intersects at the third point R, you again reflect that point to obtain the corresponding R. So, therefore, you can actually engage coordinate geometry to do this operation. Similarly, for for other cases like this, so this is a case of a doubling operation,

Like we take a P point, and we do want to do a 2 P operation, we are we are what we have done is that we have considered a tangent at this point, intersects at a point R, meant of reflecting it to obtain the corresponding some value. So, therefore, this can be similarly

Performed. So, this is again like doing P plus minus P, so therefore you take P and you add with this additive inverse minus P, it intersects at the point at infinity, and the point at infinity is, if we again reflect this, we

Again obtain point at infinity. So, as the result of the above cases, you have got this property. So, O is called the additive identity of the elliptic curve group, and hence all the elliptic curves have an additive identity O.

So, now we will come to the concluding part of this that is talking about, discussing about projective coordinates. Now, we shall see in our few unix linux class, that when we discuss that, when we are actually talking about the scalar operations, then what we

Essentially need to perform is repeated additions and repeated doubling. Now, because of this, we essentially, we I mean if we try to understand the elliptic curve operations that are underlying, we have actually to perform the finite field arithmetic operations, like we have to either perform finite field multiplication, finite field

Addition, finite field inverses and such kind of operations. So, therefore, if you if you want to make our implementations efficient, which is also very important, it is important that the underlying field operations need to be done in a clever way, that is we need to minimize the underlying field operations.

So, one of the most complexes underlying field operations is the multiplicative inverse. So, therefore, it is always an objective of how when we are implementing or finite elliptic curve scalar multiplication, is actually by repeatedly is by reducing the number of inverses. Now, what is scalar multiplication is like, is defined like this.

That is, see consider a scalar quantity, which is defined as lambda. So, what we are interested in, is in performing, so we consider a point or base point p, and we are interested in computing lambda p, that is we need to multiply this P with this scalar quantity called lambda.

So that essentially we know that we can do it by doing P plus P and so on lambda number of times that is we can perform this addition lambda number of times. That means if I take

Or start with an elliptic curve, and I take another a point p, then the first in which I have do is P plus p. So, that means I need to draw a tangent through this, and this intersects the curve at a point, you take a deflection, this becomes twice

P. So, now we take this, and we want to add this with further p, so that means that I take this, and I again intersect this line with this point p, this again intersects the curve at a third point, we again take its deflection, that becomes 3 p, right, so we

Continue this process, right. That is we performed repeated additions or repeated doublings on additions to compute the value of lambda p. Now, there is, I mean we can actually do it in a efficient way, like by as we have seen in the case of raising

Value to its exponent, we have used the square and multiply in a if we in order to do it efficiently. Similarly, here we can actually engage a double on addition algorithm to do this efficiently. But that we will see in the next class, but

What is important to understand is that, when you are doing this P plus P or P plus a third point as different point, then underlying operations which we are doing actually encompasses finite field, multiplies finite fields inverses and so on.

So, what is important is that when you are doing this operation, it is to reduce the combustion operations, right. So, what we will try is if we consider that the underlying, the underlying finite field operations, the underlying finite field operations, because

Elliptic curve is always defined on a finite field right in this case. So, therefore, the underlying finite field operations are inverses multipliers and additions. So, if we assume that this is the most cumbersome operation, then our objective will be to reduce these

Inverse operations as much as possible. So, in order to do that, there is a very effective transformation of the elliptic curves, which is from the affine coordinates, that what we have seen into a coordinate system, which is called the projective coordinate systems,

Which is actually very helpful in reducing the number of inverse operations. So, what we do here in this projective coordinate system is that, instead of considering a point x, y, we actually elaborate this and consider a point has to be made up of three important

Component that is x y and z, ok. So, it is assumed that various points which are actually lying on the curve, or instead of having two ordinates, have got three ordinates x y and z. And this transformation from the affine coordinates to this projective coordinates,

So what we first do is that we take the affine coordinates, transform it into the projective coordinates, perform all the operations in a projective coordinates, and then finally again bring it back to the affine coordinates. Now, what has been seen is that in this process

The number of inverses gets completely removed, that is when you are doing your addition and doubling operation in the projective coordinate system, then it is completely divide of the multiplicative inverse. It is made of… multiplications are required, but there are no multiplicative

Inverses which are employed here. But of course, when you are doing this final transformation back into the affine coordinate, you require to do one finite field inverse operation at that point. That means, that the entire purpose, or entire efficiency of these coordinate, system transformation is that we are actually reducing the number of

Multiplicative inverse at the cost of some extra multiplication operations. So, therefore, more formally this looks like this, that is the two dimensional projective space over k is given by the equivalence class of triples x, y, z, with x, y, z in k and

At least one of where where, at least one of this x, y, z is non zero, ok. So, therefore, this essentially is actually made up of equivalence classes, so x, y, z these are the three points, and they are actually equivalent when under certain conditions, that is with x, y, z in

K and at least one of the x, y, z being non zero. Now, when do we say that two points like x 1, y 1, z 1 and x 2, y 2, z 2 are equivalent, we say two triples like this to equivalent, if there exists a nonzero element lambda,

Which is also in k, such that x 1, y 1 and z 1 is equal to lambda x 2 into lambda y 2 into lambda z 2, that is if I take x 2, y 2 and z 2 and multiply each of them by lambda,

I get the I get the second point, ok. So, therefore, it is also commonly referred or this equivalence class, is actually defined only by the ratio of x, y and z and therefore, it is also commonly referred or written as x is to y is to z, which indicates that we

Are actually interested in the ratio of the x ordinate, y ordinate and the z ordinate. So, therefore, all the all the all such triples where this ratio is maintained are actually believed to, belong to one particular equivalence class, if there is another point which also

Maintains a ratio, then that belongs to another equivalence class. So, similarly, you can actually divide your entire coordinate or rather entire set of points into equivalence classes, so that is how it is defined. So, for example, I mean continuing further, like if z is not equal to 0, then you can

Always write x is to y is to z as same as that of x by z is to y by z is to 1, because that also maintains the same ratio, right, here x is to y is to z here and here are the

Same. But what about the point at z equal to 0, then you can cannot write like this, right. So, therefore, it is we obtain, so we say that this is the point at infinity, so we obtain the point of infinity if we do like

That. So, therefore, the two dimensional affine plane, so what we have seen previously was this, that is x, y was the corresponding affine plane, and that we defined, and x also belongs to k, and y also belongs to k, so therefore, the ordered pair x, y is the subset of the

Cartesian product of k and k, right. Now, if we use a transformation like this, where we transform this x and y to x is to y is to 1, there actually this gets transformed into the projective coordinate system. That means, what we are trying to say here is that

The A k square and the P k square, both of them are actually you can actually define a 1 to 1 relationship, right. You can transform any element here into this, and you can actually take any element here and and convert it back into the affine space. Now, there are some

Advantages as I told you, is that with projective coordinates which we will see in the next class also, there are certain distinct advantages of using projective coordinates, that is it reduces the number of inverse operations, which are required from the implementation point of view, ok.

That is, for example, if you need, if you see these operations like when we have considered these equations, then instead of, when we are when we are performing this right, that is all of them are finite field elements. So, therefore, we need finite field squarers,

We need finite field multiplies, we need finite field division operation, which is nothing but based based on finite field inverse operations. So, the objective of transforming into the projective space and then performing operations is to reduce the number, is to basically it

Is to reduce the number of multiplicative inverses. So, that is the objective of y, I mean one of the important- one of the important advantages of transforming a elliptic curve system into projective coordinates and performing the operations in the projective space.

So, I would like to, just going to make some comments about the about the similarity of an elliptic curve, so for an elliptic curve y square equal to f x, where we defined, we can actually find out the similarity in this fashion, that is you actually rewrite the

Equation as F(x, y) is equal to y square minus f x, and a singularity of the elliptic curve is a point x 0, y 0 such that your delta f delta x and delta f delta y are the same, ok. So, we actually find out delta f delta x with

Respect… so this is basically differentiate this curve y square minus f x with respect to x. So, if we do so, then we actually obtain, so we can actually do that, similarly we can also we also differentiate this in respective with y. For example, here we differentiate,

If we differentiate this with respect to y, what do we get? We get 2 y, right, so we get 2 y 0, because that is the point. So, what we are doing is that, we get x, because in

That case, the effects if we differentiate this with respect to y, that is 0. So, therefore, we get 2 y 0 and the other thing is f x 0 minus f x is here. So, what we are doing is

This, that is we are taking the curve F(x, y) and that is equal to y square minus f of x, we are differentiation del f with respect to del x, so del f with respect to del x will be equal to 2 y 0.

Similarly, your del f with respect to, sorry del f with respect del y, del f with del with respect to del x will be minus of f dash x, right. So, therefore, here what we are saying

Is that 2 y 0 and minus of f dash x is equal to 0 at the singularity point. So, we say that 2 y is equal to minus f dash x equal to 0, now when we are saying that 2 y 0 is

0, that is y 0 is equal to 0, it means that y 0 square is also 0, right. That means, your if you substitute this in this graph right, then it means that your f x 0 is also equal

To 0, that means we can write that… f we can write f x 0 and f dash x 0, both are equal to 0 here. So, that that is why we call this point at singularity as a double root, where it is also satisfy this and also the differentiation is satisfied, ok.

So, if you draw a curve like this, which has actually has got a singularity point and then you will find that it is quite distinctly defined from the other elliptic curve operations. So, it may have something like this kind of edge over there, right, so it is this point basically.

So, it is evenly assumed that the elliptic curve has got no singular point. So, therefore, I mean, we can go through this, and we can find out that if we go through this, then if we assume that f has got double roots, then if we take this x cube plus A x plus

P, and the for double roots, therefore we know that we will actually equate the differentiation of this equation with respect to x, this is 3 x square plus a with 0, and therefore your x square is minus a by 3 from this equation, right.

So, if you substitute this, therefore you know that x power of 4 plus A x square plus B x is equal to 0, because that is also a root right. So, therefore, if x cube plus

A x plus B is 0, you can always multiply this by x, and that is still 0, right. So, therefore, you get x to the power of 4 plus A x square plus B x is equal to 0, if you take this,

And now substitute this one into this, then you get a square by 9 minus A square by 3 plus B x is equal to 0 and therefore, x is equal to 2 A square by 9 B.

So, now, if you take this value of x, and substitute back into this equation of 3 x square plus a equal to 0, then you get that 4 A- this equation which is called as 4 A cube plus 27 B square is equal to 0. Now, this is commonly referred as the discriminant

Of an elliptic curve, that is, what we have seen in quadratic equations, we know that when it as when the discriminant is 0, then the two roots are same. Similarly, in cubic curves, when f has got double roots for the singularity conditions,

That it satisfies four A cube plus 27 B square, is also is equal to 0. So, this we can prove in a slightly different way also, but I mean one of the very important curve equation is for characteristic two, and it is useful for implementations. So,

We can consider again the weierstrass equation, and depending upon the facts right, we can actually again modify and we can actually reduce it into various forms, like one f the various very common forms is again y square equal to x cube plus A x plus B.

So, I am not going into these detailed deductions or it can also be done in the similar fashion, as we have seen for the other characteristic. Now, I just throw some points to think on, like we have thought about the discriminant point, so you see that, suppose that the cubic

Polynomial, this is factored as this, then we can actually show that 4 A cube plus 27 B square is equal to 0, if and only if two and more of the E 1, E 2 and E 3 are the same,

So you can actually reduce both the curve equations and this one, and and compare the coefficients. If you simplify, you will again get back these equations, this is another way of proving the discriminant that is if two roots are same, then you’ll get this equation

To be satisfied. The other thing is that we can sketch these two curves, and we can note that the second curve E 2 is actually not an elliptic curve, because it has a singular point. So, if you do this, you will get one feel about the singular

Point, the singularity of the curve. So, we stop at this point, and here are some of the references that I have used in my video, so I have used the stinson’s book and the washington’s book and also this book. And in next day, we can actually discuss about

The applications of elliptic curves to cryptography.

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