How to use BouncyCastle's Diffie-Hellman in C#?

问题

I'm writing an app that'll exchange data between a phone and a Windows PC, and I want to protect the data sent with key generated with a Diffie-Hellman exchange.

I'm trying to use BouncyCastle for that, but the almost non-existant documentation for the C# implementation has me stumped.

What I want to know is: what's the workflow for generating a DH key and computing a shared key when the other side's key is received? (I'm assuming I can send my key as a string and I can work with the other side's key as a string.) What objects/methods do I use in C# for that?

回答1:

Alright, after a lot of trial, I got it working. Posting answer in case someone else needs it.

I'll assume the reader (1) knows what Diffie-Hellman is and what it's useful for (read here for details) and (2) already imported Bouncycastle to a .NET project via NuGet.

Imports you'll need:

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;

How to generate g and p:

public DHParameters GenerateParameters()
{
    var generator = new DHParametersGenerator();
    generator.Init(BitSize, DefaultPrimeProbability, new SecureRandom());
    return generator.GenerateParameters();
}

Wanna get g and p as strings?

public string GetG(DHParameters parameters)
    {
        return parameters.G.ToString();
    }

public string GetP(DHParameters parameters)
    {
        return parameters.P.ToString();
    }

How to generate a and A:

public AsymmetricCipherKeyPair GenerateKeys(DHParameters parameters)
    {
    var keyGen = GeneratorUtilities.GetKeyPairGenerator("DH");
    var kgp = new DHKeyGenerationParameters(new SecureRandom(), parameters);
    keyGen.Init(kgp);
    return keyGen.GenerateKeyPair();
}

Wanna read a and A as a string?

// This returns A
public string GetPublicKey(AsymmetricCipherKeyPair keyPair)
{
    var dhPublicKeyParameters = _generatedKey.Public as DHPublicKeyParameters;
    if (dhPublicKeyParameters != null)
    {
        return dhPublicKeyParameters.Y.ToString();
    }
    throw new NullReferenceException("The key pair provided is not a valid DH keypair.");
}

// This returns a
public string GetPrivateKey(AsymmetricCipherKeyPair keyPair)
{
    var dhPrivateKeyParameters = _generatedKey.Private as DHPrivateKeyParameters;
    if (dhPrivateKeyParameters != null)
    {
        return dhPrivateKeyParameters.X.ToString();
    }
    throw new NullReferenceException("The key pair provided is not a valid DH keypair.");
}

To import the parameters from strings just do:

var importedParameters = new DHParameters(p, g);

To generate b and B just use GenerateKeys() with importedParameters instead of the generated parameters.

Let's say you generated b and B and already got p, g and A. To compute the shared secret:

public BigInteger ComputeSharedSecret(string A, AsymmetricKeyParameter bPrivateKey, DHParameters internalParameters)
{
    var importedKey = new DHPublicKeyParameters(new BigInteger(A), internalParameters);
    var internalKeyAgree = AgreementUtilities.GetBasicAgreement("DH");
    internalKeyAgree.Init(bPrivateKey);
    return internalKeyAgree.CalculateAgreement(importedKey);
}

Repeat for A and now you have a shared secret between 2 clients, ready to be used to encrypt communications.

Hope this is useful.

来源：https://stackoverflow.com/questions/29089311/how-to-use-bouncycastles-diffie-hellman-in-c

标签

bouncycastle

diffie-hellman