/// <summary>バイト配列より点を作成する (SEC1, 2.3.4)</summary>
public ECPoint(ECGroup group, byte[] data)
{
_group = group;
_field = group.FiniteField;
switch (data[0])
{
case 0: { // 無限遠点
ECPoint tmp = _field.GetInfinityPoint(_group);
_x = tmp._x;
_y = tmp._y;
_z = tmp._z;
return;
}
case 2:
case 3: { // 点圧縮済みデータ
int keyBits = group.P.BitCount();
int keyBytes = (keyBits >> 3) + ((keyBits & 7) == 0 ? 0 : 1);
if (data.Length != keyBytes + 1)
{
throw new ArgumentException();
}
Number x = _field.ToElement(new Number(data, 1, keyBytes, false));
Number y2 = _field.Add(_field.Multiply(_field.Add(_field.Multiply(x, x), _group.A), x), _group.B); // (x^2 + a)*x + b
Number y = _field.Sqrt(y2);
if (_field.ToNormal(y).GetBit(0) != data[0] - 2)
{
y = _field.Modulus - y;
}
_x = x;
_y = y;
_z = _field.ToElement(Number.One);
return;
}
case 4: { // 非圧縮データ
int keyBits = group.P.BitCount();
int keyBytes = (keyBits >> 3) + ((keyBits & 7) == 0 ? 0 : 1);
_x = _field.ToElement(new Number(data, 1, keyBytes, false));
_y = _field.ToElement(new Number(data, 1 + keyBytes, keyBytes, false));
_z = _field.ToElement(Number.One);
return;
}
default:
throw new ArgumentException();
}
}
/// <summary>バイト配列より点を作成する (SEC1, 2.3.4)</summary>
public ECPoint (ECGroup group, byte[] data)
{
_group = group;
_field = group.FiniteField;
switch (data[0]) {
case 0: { // 無限遠点
ECPoint tmp = _field.GetInfinityPoint (_group);
_x = tmp._x;
_y = tmp._y;
_z = tmp._z;
return;
}
case 2:
case 3: { // 点圧縮済みデータ
int keyBits = group.P.BitCount ();
int keyBytes = (keyBits >> 3) + ((keyBits & 7) == 0 ? 0 : 1);
if (data.Length != keyBytes + 1)
throw new ArgumentException ();
Number x = _field.ToElement (new Number (data, 1, keyBytes, false));
Number y2 = _field.Add (_field.Multiply (_field.Add (_field.Multiply (x, x), _group.A), x), _group.B); // (x^2 + a)*x + b
Number y = _field.Sqrt (y2);
if (_field.ToNormal (y).GetBit (0) != data[0] - 2)
y = _field.Modulus - y;
_x = x;
_y = y;
_z = _field.ToElement (Number.One);
return;
}
case 4: { // 非圧縮データ
int keyBits = group.P.BitCount ();
int keyBytes = (keyBits >> 3) + ((keyBits & 7) == 0 ? 0 : 1);
_x = _field.ToElement (new Number (data, 1, keyBytes, false));
_y = _field.ToElement (new Number (data, 1 + keyBytes, keyBytes, false));
_z = _field.ToElement (Number.One);
return;
}
default:
throw new ArgumentException ();
}
}
