Hi Chris,
It's not a bug in ToBigInteger per se, but one does have to be careful
about the length of an encoding of the value. Note that it must be
exactly 32 (for secp256k1), so one has to deal with a possible leading
"sign byte", and also the less likely occurrence of a very small field
element value leading to < 32 bytes encoding.
When I get to porting the latest EC stuff over from Java, there will be
a GetEncoded method on ECFieldElement, but for now you can use the
Org.BouncyCastle.Asn1.X9.X9IntegerConverter utility class:
Snippet
ECFieldElement fe = ...;
int length= X9IntegerConverter.GetByteLength(fe);
byte[] encoding =
X9IntegerConverter.IntegerToBytes(fe.ToBigInteger(), length);
BTW, if you are trying to produce a standard point encoding, you can
just use ECPoint.GetEncoded() which does the above internally and also
adds the leading point format byte.
Snippet Regards,
Pete Dettman
On 26/10/2013 10:28 PM, Mankowski, Chris wrote:
EDIT: corrections in subject and body (bits vs bytes)
There seems to be a bug in ToBigInteger, that causes the output of
ECFieldElement to sometimes be 33 bits long.
The problem seems to be a leading zero is appended (or left in) during
the conversion. This bit seems to be located in byte[0]. So far it
seems that the value is always zero with these outside-in tests,
however further investigation is warranted.
Then again, if I do have any error in my code, please let me know.
while(true)
{
ECKeyPairGeneratorgen = newECKeyPairGenerator("ECDSA");
Org.BouncyCastle.Asn1.X9.X9ECParameters ecp =
Org.BouncyCastle.Asn1.Sec.SecNamedCurves.GetByName("secp256k1");
ECDomainParameters ecSpec = new ECDomainParameters(ecp.Curve, ecp.G,
ecp.N, ecp.H, ecp.GetSeed());
SecureRandom secureRandom = new SecureRandom();
ECKeyGenerationParametersecgp = newECKeyGenerationParameters(ecSpec,
secureRandom);
gen.Init(ecgp);
AsymmetricCipherKeyPair eckp = gen.GenerateKeyPair();
// Cast it
var publicKey = (ECPublicKeyParameters)(eckp.Public);
var privateKey = (ECPrivateKeyParameters)(eckp.Private);
// Get X and Y
var pubxBigInt = publicKey.Q.X.ToBigInteger();
var pubyBigInt = publicKey.Q.Y.ToBigInteger();
// Get Byte Array... whoa, why is the length 33? Can I simply trim the
leading zero?
var pubxBigIntArray = pubyBigInt.ToByteArray();
var pubyBigIntArray = pubyBigInt.ToByteArray();
if (pubyBigIntArray.Length > 32 || pubxBigIntArray.Length > 32)
{
StringBuilder sb = new StringBuilder();
if (pubxBigIntArray.Length > 32)
{
sb.Append(String.Format("X is {0} bits.
Bit[0] = {1}", pubxBigIntArray.Length, pubxBigIntArray[0]));
}
if (pubyBigIntArray.Length > 32)
{
sb.Append(String.Format(" Y is {0} bits.
Bit[0] = {1}",pubyBigIntArray.Length, pubyBigIntArray[0]));
}
Console.WriteLine(sb.ToString());
}
else
{
Console.WriteLine("OK");
}
}
Chris Mankowski
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