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POBouncy Castle Diffie-Hellman DHParams Issue
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copied!<p>I am trying to use the bouncy castle .net libraries to perform Diffie-Hellman key exchange and am running into a problem when generating the DHParams object. </p> <p>My solution will include a central authority which will generate a separate DH key/pair for each client that connects. The idea is that I will keep a separate DH key agreement for each client that connects. I will then send the p,g value to the client where the client will calculate it's dh key agreement. I want to generate different p,g values for each client. I am using BigInteger for that but am running into some problems.</p> <p>When I try to create a new DHParameters object it throws the following exception whenever I use a bit length other than 768: </p> <pre><code>System.ArgumentException was unhandled Message="generator must in the range [2, p - 2]\r\nParameter name: g" Source="BouncyCastle.Crypto" ParamName="g" StackTrace: at Org.BouncyCastle.Crypto.Parameters.DHParameters..ctor(BigInteger p, BigInteger g, BigInteger q, Int32 m, Int32 l, BigInteger j, DHValidationParameters validation) at Org.BouncyCastle.Crypto.Parameters.DHParameters..ctor(BigInteger p, BigInteger g, BigInteger q, Int32 l) at TestDH.Program.Main(String[] args) in C:\dev\source\TestDH\TestDH\Program.cs:line 30 at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart() InnerException: "generator must in the range [2, p - 2]" </code></pre> <p>I'm not sure if it matters but I've tried all kinds of different values for certainty in the BigInteger constructor.</p> <p>Here is my code:</p> <pre><code> SecureRandom sr = new SecureRandom(); // p,g generation, done by central authority BigInteger g512 = new BigInteger(512, 30, sr); BigInteger p512 = new BigInteger(512, 30, sr); // p,g is then sent to client from central authority // common - performed by both server and client sides IAsymmetricCipherKeyPairGenerator keyGen = GeneratorUtilities.GetKeyPairGenerator("DH"); DHParameters dhParams = new DHParameters(p512, g512, null, 512); // Here is where I get the exception if the first parameter if BigInteger is not 768 or lager </code></pre> <p>The issue is that it takes too long to generate the 768 bit prime number - upwards of 5 seconds on a dual core 2.1Ghz processor with no other processes running. That's just too big of a penalty to incur with each client that initiates a connection. I would like to use smaller bit lengths for the BigInteger.</p> <p>I'm probably doing this completely wrong - there is scant documentation on how to do DH with bouncy castle and the test/examples just don't match my usage case. I don't want to have pre-generated p,g values.</p> <p><strong>EDIT</strong> It seems even the 768 bit length gives errors once in awhile. After restarting my machine I could get no bit lengths other than 1024 to work, and even then only about 80% of the time. I think I'm doing something wrong.</p>

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