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output/Libraries/BouncyCastle.Crypto/Org/BouncyCastle/Crypto/Agreement/DHAgreement.cs

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+using System;
+using Org.BouncyCastle.Crypto.Generators;
+using Org.BouncyCastle.Crypto.Parameters;
+using Org.BouncyCastle.Math;
+using Org.BouncyCastle.Security;
+
+namespace Org.BouncyCastle.Crypto.Agreement;
+
+public class DHAgreement
+{
+	private DHPrivateKeyParameters key;
+
+	private DHParameters dhParams;
+
+	private BigInteger privateValue;
+
+	private SecureRandom random;
+
+	public void Init(ICipherParameters parameters)
+	{
+		AsymmetricKeyParameter asymmetricKeyParameter;
+		if (parameters is ParametersWithRandom)
+		{
+			ParametersWithRandom parametersWithRandom = (ParametersWithRandom)parameters;
+			random = parametersWithRandom.Random;
+			asymmetricKeyParameter = (AsymmetricKeyParameter)parametersWithRandom.Parameters;
+		}
+		else
+		{
+			random = new SecureRandom();
+			asymmetricKeyParameter = (AsymmetricKeyParameter)parameters;
+		}
+		if (!(asymmetricKeyParameter is DHPrivateKeyParameters))
+		{
+			throw new ArgumentException("DHEngine expects DHPrivateKeyParameters");
+		}
+		key = (DHPrivateKeyParameters)asymmetricKeyParameter;
+		dhParams = key.Parameters;
+	}
+
+	public BigInteger CalculateMessage()
+	{
+		DHKeyPairGenerator dHKeyPairGenerator = new DHKeyPairGenerator();
+		dHKeyPairGenerator.Init(new DHKeyGenerationParameters(random, dhParams));
+		AsymmetricCipherKeyPair asymmetricCipherKeyPair = dHKeyPairGenerator.GenerateKeyPair();
+		privateValue = ((DHPrivateKeyParameters)asymmetricCipherKeyPair.Private).X;
+		return ((DHPublicKeyParameters)asymmetricCipherKeyPair.Public).Y;
+	}
+
+	public BigInteger CalculateAgreement(DHPublicKeyParameters pub, BigInteger message)
+	{
+		if (pub == null)
+		{
+			throw new ArgumentNullException("pub");
+		}
+		if (message == null)
+		{
+			throw new ArgumentNullException("message");
+		}
+		if (!pub.Parameters.Equals(dhParams))
+		{
+			throw new ArgumentException("Diffie-Hellman public key has wrong parameters.");
+		}
+		BigInteger p = dhParams.P;
+		BigInteger y = pub.Y;
+		if (y == null || y.CompareTo(BigInteger.One) <= 0 || y.CompareTo(p.Subtract(BigInteger.One)) >= 0)
+		{
+			throw new ArgumentException("Diffie-Hellman public key is weak");
+		}
+		BigInteger bigInteger = y.ModPow(privateValue, p);
+		if (bigInteger.Equals(BigInteger.One))
+		{
+			throw new InvalidOperationException("Shared key can't be 1");
+		}
+		return message.ModPow(key.X, p).Multiply(bigInteger).Mod(p);
+	}
+}