PointGFp OS2ECP(const byte data[], size_t data_len,
const CurveGFp& curve)
{
if(data_len <= 1)
return PointGFp(curve); // return zero
const byte pc = data[0];
BigInt x, y;
if(pc == 2 || pc == 3)
{
//compressed form
x = BigInt::decode(&data[1], data_len - 1);
const bool y_mod_2 = ((pc & 0x01) == 1);
y = decompress_point(y_mod_2, x, curve);
}
else if(pc == 4)
{
const size_t l = (data_len - 1) / 2;
// uncompressed form
x = BigInt::decode(&data[1], l);
y = BigInt::decode(&data[l+1], l);
}
else if(pc == 6 || pc == 7)
{
const size_t l = (data_len - 1) / 2;
// hybrid form
x = BigInt::decode(&data[1], l);
y = BigInt::decode(&data[l+1], l);
const bool y_mod_2 = ((pc & 0x01) == 1);
if(decompress_point(y_mod_2, x, curve) != y)
throw Illegal_Point("OS2ECP: Decoding error in hybrid format");
}
else
throw Invalid_Argument("OS2ECP: Unknown format type");
PointGFp result(curve, x, y);
if(!result.on_the_curve())
throw Illegal_Point("OS2ECP: Decoded point was not on the curve");
return result;
}
std::pair<BigInt, BigInt> OS2ECP(const uint8_t data[], size_t data_len,
const BigInt& curve_p,
const BigInt& curve_a,
const BigInt& curve_b)
{
if(data_len <= 1)
throw Decoding_Error("OS2ECP invalid point");
const uint8_t pc = data[0];
BigInt x, y;
if(pc == 2 || pc == 3)
{
//compressed form
x = BigInt::decode(&data[1], data_len - 1);
const bool y_mod_2 = ((pc & 0x01) == 1);
y = decompress_point(y_mod_2, x, curve_p, curve_a, curve_b);
}
else if(pc == 4)
{
const size_t l = (data_len - 1) / 2;
// uncompressed form
x = BigInt::decode(&data[1], l);
y = BigInt::decode(&data[l+1], l);
}
else if(pc == 6 || pc == 7)
{
const size_t l = (data_len - 1) / 2;
// hybrid form
x = BigInt::decode(&data[1], l);
y = BigInt::decode(&data[l+1], l);
const bool y_mod_2 = ((pc & 0x01) == 1);
if(decompress_point(y_mod_2, x, curve_p, curve_a, curve_b) != y)
throw Illegal_Point("OS2ECP: Decoding error in hybrid format");
}
else
throw Invalid_Argument("OS2ECP: Unknown format type " + std::to_string(pc));
return std::make_pair(x, y);
}
