liftX static method

ProjectiveECCPoint liftX(BigInt x)

Lift the x-coordinate of a public key for P2TR.

Parameters:

• x: the x-coordinate of public key.

Throws:

• AddressConverterException if the x-coordinate cannot be lifted.

Implementation

static ProjectiveECCPoint liftX(BigInt x) {
  final BigInt p = Curves.curveSecp256k1.p;
  if (x >= p) {
    throw AddressConverterException.addressKeyValidationFailed(
      reason: "Invalid public key.",
    );
  }
  final ySq = (x.modPow(BigInt.from(3), p) + BigInt.from(7)) % p;
  final y = ySq.modPow((p + BigInt.one) ~/ BigInt.from(4), p);
  if (y.modPow(BigInt.two, p) != ySq) {
    throw AddressConverterException.addressKeyValidationFailed(
      reason: "Invalid public key.",
    );
  }
  final BigInt result = (y & BigInt.one) == BigInt.zero ? y : p - y;
  return ProjectiveECCPoint(
    curve: Curves.curveSecp256k1,
    x: x,
    y: result,
    z: BigInt.one,
  );
}