Exception / hang on Mono.Security Diffie-Hellman key generation for SSH Transport

Question

I'm trying to do pushes through SSH on NGit. I am specifying my own keys by using the method set out here: NGit making a connection with a private key file

It works well with a clone, and on the first push. After that, it fails 30% of the time, always in this method in Mono.Security:

Mono.Security.dll!Mono.Math.BigInteger.Kernel.MinusEq(Mono.Math.BigInteger big, Mono.Math.BigInteger small)

Either it fails with the message: "Error occurred during a cryptographic function", or it hangs indefinitely in this function. Again, it seems completely random whether it succeeds or not. It is being called by function in NGit:

DiffieHellmanManaged dh = new DiffieHellmanManaged (pspec.P.GetBytes (), pspec.G.GetBytes (), 0);

in GenerateKeyPair (KeyPairGenerator.cs)

Any ideas? If not, are there any ways to generate the Diffie-Hellman keys through some other method?

Thanks so much,

Cheers,

UPDATE: I have discovered a strange solution that I don't entirely understand:

Previously, I created a new thread, as I was calling it from the UI and didn't want it to hang. This was an STA thread, as we are building a VSTO application and it doesn't support MTA / BackgroundWorker() etc. from UI components (pretty much limiting you to the old Thread(), as you can specify it is STA as required). I found the problem happened more frequently when the user clicked around whilst the push was in progress (thus doing some git checkouts), so I disabled the threading and ran it directly from the UI. Since then, it freezes the UI like hell, but doesn't hang pr crash on the calculation of BigInt. I am completely perplexed as to how this is the case unless: Office (which we are building addons for) somehow won't provide a thread with enough memory for the BigInt calculations (or mismanages this memory, which wouldn't be entirely surprising), or, for some reason, BigInt calculations simply can't be called in a threaded context (maybe Thread->Ngit->Sharpen->Mono.Security->Mono.Math(32 digit number) broke threading??)

Any idea what could be the cause of this?

Answer 1

I had the exact same issues when using NGit + SSH private key to fetch/pull from a Git repository, which could be requested from multiple threads.

I tracked it down to being an issue with the Mono.Security Mono.Math.BigInteger when it is run in a multi-threaded context (the bug report and code to replicate it can be found here). Though for whatever reason I could only replicate the issue in a Windows environment. On Mono in OSX it seemed to work all fine.

To workaround, I ended up changing NSch to use a modified implementation of DiffieHellmanManaged which calculated the ModPow on the BigIntegers itself using the same logic as System.Numerics.BigInteger.

public static BigInteger ModPower(this BigInteger value, BigInteger exp, BigInteger n)
    {
        if (exp < 0)
            throw new ArgumentOutOfRangeException("exponent", "power must be >= 0");
        if (n == 0)
            throw new DivideByZeroException();

        BigInteger result = new BigInteger(1) % n;
        while (exp != 0)
        {
            if (exp % 2 != 0)
            {
                result = result * value;
                result = result % n;
            }
            if (exp == 1)
                break;
            value = value * value;
            value = value % n;
            exp >>= 1;
        }
        return result;
    }