Provide CP-256, CP-384, CP-521 Y point resolution from an OKP serialized X point - https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-dnhpke-02#section-4.1.1

eccompressor.go

package main

import (
	"crypto/elliptic"
	"encoding/hex"
	"fmt"
	"math/big"
)

// https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-dnhpke-02#section-4.1.1
func main() {
	X, _ := hex.DecodeString("8f0cace46987ad7efec5959a925e05bf2c3e0f3d1637a861f79226715be99c41")
	Y, _ := hex.DecodeString("2c704aa459ef59c319bc1f872f5a7c532143f2f94b56d5565666488bb64c1748")

	fmt.Println("X: " + hex.EncodeToString(X))
	fmt.Println("Y: " + hex.EncodeToString(Y))
	fmt.Println("D: " + hex.EncodeToString(D))

	c := elliptic.P256()

	// https://crypto.stackexchange.com/a/20640
	// y = ((x^3 + a*x + b)^((p + 1)/4)) mod p
	x := big.NewInt(0).SetBytes(X)
	b := c.Params().B
	p := c.Params().P

	x3 := new(big.Int).Mul(x, x)
	x3.Mul(x3, x)
	threeX := new(big.Int).Lsh(x, 1)
	threeX.Add(threeX, x)
	x3.Sub(x3, threeX)
	x3.Add(x3, b)

	p1 := new(big.Int).Add(p, big.NewInt(1))
	p1.Div(p1, big.NewInt(4))

	y := big.NewInt(0).Exp(x3, p1, p)
	fmt.Println("Y' : " + hex.EncodeToString(y.Bytes()))
}