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Merge pull request #234 from zcash/book-point-addition
[book] Point addition and compression background
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@ -152,17 +152,27 @@ corresponding numerators such that $X/Z = x'$ and $Y/Z = y'$. This completely av
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need to perform an inversion when doubling, and something analogous to this can be done
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when adding two distinct points.
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### TODO: Point addition
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### Point addition
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We now add two points with distinct $x$-coordinates, $P = (x_0, y_0)$ and $Q = (x_1, y_1),$
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where $x_0 \neq x_1,$ to obtain $R = P + Q = (x_2, y_2).$ The line $\overline{PQ}$ has slope
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$$\lambda = frac{y_1 - y_0}{x_1 - x_0} \implies y - y_0 = \lambda \cdot (x - x_0).$$
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Using the expression for $\overline{PQ}$, we compute $y$-coordinate $-y_2$ of $-R$ as:
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$$-y_2 - y_0 = \lambda \cdot (x_2 - x_0) \implies \boxed{y_2 = (x_0 - x_2) - y_0}.$$
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Plugging the expression for $\overline{PQ}$ into the curve equation $y^2 = x^3 + b$ yields
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$$
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\begin{aligned}
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P + Q &= R\\
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(x_p, y_p) + (x_q, y_q) &= (x_r, y_r) \\
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\lambda &= \frac{y_q - y_p}{x_q - x_p} \\
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x_r &= \lambda^2 - x_p - x_q \\
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y_r &= \lambda(x_p - x_r) - y_p
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y^2 = x^3 + b &\implies (\lambda \cdot (x - x_0) + y_0)^2 = x^3 + b \\
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&\implies x^3 - \lambda^2 x^2 + \cdots = 0 \leftarrow\text{(rearranging terms)} \\
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&= (x - x_0)(x - x_1)(x - x_2) \leftarrow\text{(known roots $x_0, x_1, x_2$)} \\
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&= x^3 - (x_0 + x_1 + x_2)x^2 + \cdots.
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\end{aligned}
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$$
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Comparing coefficients for the $x^2$ term gives us
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$\lambda^2 = x_0 + x_1 + x_2 \implies \boxed{x_2 = \lambda^2 - x_0 - x_1}$.
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----------
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Important notes:
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@ -191,7 +201,42 @@ complicated, but when the curve has a prime $q$ number of points (and thus a pri
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$\mathbb{F}_q$ which is also a primitive cube root $\zeta_q$ in the scalar field.
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## Curve point compression
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TODO
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Given a point on the curve $P = (x,y)$, we know that its negation $-P = (x, -y)$ is also
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on the curve. To uniquely specify a point, we need only encode its $x$-coordinate along
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with the sign of its $y$-coordinate.
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### Serialization
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As mentioned in the [Fields](./fields.md) section, we can interpret the least significant
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bit of a field element as its "sign", since its additive inverse will always have the
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opposite LSB. So we record the LSB of the $y$-coordinate as `sign`.
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Pallas and Vesta are defined over the $\mathbb{F}_p$ and $\mathbb{F}_q$ fields, which
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elements can be expressed in $255$ bits. This conveniently leaves one unused bit in a
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32-byte representation. We pack the $y$-coordinate `sign` bit into the highest bit in
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the representation of the $x$-coordinate:
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```text
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<----------------------------------- x --------------------------------->
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Enc(P) = [_ _ _ _ _ _ _ _] [_ _ _ _ _ _ _ _] ... [_ _ _ _ _ _ _ _] [_ _ _ _ _ _ _ sign]
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^ <-------------------------------------> ^
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LSB 30 bytes MSB
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```
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The "point at infinity" $\mathcal{O}$ that serves as the group identity, does not have an
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affine $(x, y)$ representation. However, it turns out that there are no points on either
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the Pallas or Vesta curve with $x = 0$ or $y = 0$. We therefore use the "fake" affine
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coordinates $(0, 0)$ to encode $\mathcal{O}$, which results in the all-zeroes 32-byte
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array.
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### Deserialization
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When deserializing a compressed curve point, we first read the most significant bit as
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`ysign`, the sign of the $y$-coordinate. Then, we set this bit to zero to recover the
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original $x$-coordinate.
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If $x = 0, y = 0,$ we return the "point at infinity" $\mathcal{O}$. Otherwise, we proceed
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to compute $y = \sqrt{x^3 + b}.$ Here, we read the least significant bit of $y$ as `sign`.
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If `sign == ysign`, we already have the correct sign and simply return the curve point
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$(x, y)$. Otherwise, we negate $y$ and return $(x, -y)$.
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## Cycles of curves
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Let $E_p$ be an elliptic curve over a finite field $\mathbb{F}_p,$ where $p$ is a prime.
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