/// <summary>
/// Derives a shared secret key from a private key and another persons public key
/// </summary>
/// <param name="myPrivateKey">the private key which is used</param>
/// <param name="otherPartyPublicKey">the public key of the other person</param>
/// <returns></returns>
public byte[] DeriveKey(byte[] myPrivateKey, byte[] otherPartyPublicKey)
{
DHPrivateKeyParameters privKey = null;
try
{
privKey = (DHPrivateKeyParameters)CreateAsymmetricKeyParameterFromPrivateKeyInfo(myPrivateKey);
}
catch (InvalidCastException exception)
{
string message = "Private Key Import Failed!\n" +
$"{exception.Message}.\n" +
"The contents of the source do not represent a valid DH key parameter\n" +
"Verify that the key is not corrupted.\n" +
"- or - Verify that the correct key is selected.";
throw new CryptoException(message, exception);
}
var a1 = new DHAgreement();
a1.Init(privKey);
BigInteger m1 = a1.CalculateMessage();
DHPublicKeyParameters pubKey = null;
try
{
pubKey = (DHPublicKeyParameters)CreateAsymmetricKeyParameterFromPublicKeyInfo(otherPartyPublicKey);
}
catch (InvalidCastException exception)
{
string message = "Public Key Import Failed!\n" +
$"{exception.Message}.\n" +
"The contents of the source do not represent a valid DH key parameter\n" +
"Verify that the key is not corrupted.\n" +
"- or - Verify that the correct key is selected.";
throw new CryptoException(message, exception);
}
//Both party keys must share the same DHParameters to be able to calculate the agreement
BigInteger k1 = a1.CalculateAgreement(pubKey, m1);
return(k1.ToByteArrayUnsigned());
}
private void doTestDH(
int size,
BigInteger g,
BigInteger p)
{
DHKeyPairGenerator kpGen = getDHKeyPairGenerator(g, p);
//
// generate first pair
//
AsymmetricCipherKeyPair pair = kpGen.GenerateKeyPair();
DHPublicKeyParameters pu1 = (DHPublicKeyParameters)pair.Public;
DHPrivateKeyParameters pv1 = (DHPrivateKeyParameters)pair.Private;
//
// generate second pair
//
pair = kpGen.GenerateKeyPair();
DHPublicKeyParameters pu2 = (DHPublicKeyParameters)pair.Public;
DHPrivateKeyParameters pv2 = (DHPrivateKeyParameters)pair.Private;
//
// two way
//
DHAgreement e1 = new DHAgreement();
DHAgreement e2 = new DHAgreement();
e1.Init(pv1);
e2.Init(pv2);
BigInteger m1 = e1.CalculateMessage();
BigInteger m2 = e2.CalculateMessage();
BigInteger k1 = e1.CalculateAgreement(pu2, m2);
BigInteger k2 = e2.CalculateAgreement(pu1, m1);
if (!k1.Equals(k2))
{
Fail(size + " bit 2-way test failed");
}
}
private void doTestGPWithRandom(
DHKeyPairGenerator kpGen)
{
//
// generate first pair
//
AsymmetricCipherKeyPair pair = kpGen.GenerateKeyPair();
DHPublicKeyParameters pu1 = (DHPublicKeyParameters)pair.Public;
DHPrivateKeyParameters pv1 = (DHPrivateKeyParameters)pair.Private;
//
// generate second pair
//
pair = kpGen.GenerateKeyPair();
DHPublicKeyParameters pu2 = (DHPublicKeyParameters)pair.Public;
DHPrivateKeyParameters pv2 = (DHPrivateKeyParameters)pair.Private;
//
// two way
//
DHAgreement e1 = new DHAgreement();
DHAgreement e2 = new DHAgreement();
e1.Init(new ParametersWithRandom(pv1, new SecureRandom()));
e2.Init(new ParametersWithRandom(pv2, new SecureRandom()));
BigInteger m1 = e1.CalculateMessage();
BigInteger m2 = e2.CalculateMessage();
BigInteger k1 = e1.CalculateAgreement(pu2, m2);
BigInteger k2 = e2.CalculateAgreement(pu1, m1);
if (!k1.Equals(k2))
{
Fail("basic with random 2-way test failed");
}
}
