Repro

gistfile1.txt

[Fact]
public void SumOfMultipliesBug()
{
	var curve = Org.BouncyCastle.Asn1.Sec.SecNamedCurves.GetByName("secp256k1");
	var a = new Org.BouncyCastle.Math.BigInteger("69382933896972716708744287348684382108685473997297686356748931369614199359285");

	var o = X9IntegerConverter.IntegerToBytes
				(new Org.BouncyCastle.Math.BigInteger("27188564572527577826207139682381325092978633561404879297236674360940833429317"),
				0x21);
	o[0] = (byte)0x02;
	var q = curve.Curve.DecodePoint(o);

	var b = new Org.BouncyCastle.Math.BigInteger("100771008261047044268288327751827424543746424910943046077177469841741341064373");

	var r1 = ECAlgorithms.SumOfTwoMultiplies(curve.G, a, q, b);
	var r2 = ECAlgorithms.SumOfTwoMultiplies2(curve.G, a, q, b);
	Assert.Equal(r1.X, r2.X);
	Assert.Equal(r1.Y, r2.Y); //Crash here
}