Use native block size for pbkdf2 and increase iteration count.

Pbkdf2Perf

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using System;
using System.Diagnostics;
using System.Security.Cryptography;

namespace Pbkdf2Perf
{
    /// <summary>
    /// As mentioned in the comments of http://security.stackexchange.com/questions/17994/with-pbkdf2-what-is-an-optimal-hash-size-in-bytes-what-about-the-size-of-the-s,
    /// and http://security.stackexchange.com/a/51430/28362 with related info https://hashcat.net/forum/thread-2238.html, when using pbkdf2 for password hashing, the configured
    /// size of the output hash should *not* exceed the native block size of the underlying hash algorithm.
    /// 
    /// The reason is that this only harms you, the defender, since only you experience the slowdown.  The attacker only has to check the first block, but you the defender
    /// have to compute all the blocks.
    /// 
    /// Instead, for password hashing, use the native block size, but increase the pbkdf2 iteration count, which makes the attacker's job harder.
    /// 
    /// This program demonstrates the slowdown experienced by the defender solely as a result of using block sizes greater than native.
    /// </summary>
    class BlockSizeTiming
    {
        static void Main(string[] args)
        {
            int blockSize = 20; //.Net implementation of Rfc2898DeriveBytes currently uses SHA1, which has a block size of 20.

            int iterationCount = 1000;
            int saltSize = 32;

            int benchmarkRepetitions = 4;

            string password = Guid.NewGuid().ToString();

            double blockSizeTicks = (double)Benchmark(password, blockSize, iterationCount, saltSize, benchmarkRepetitions);

            for (int i = 2; i <= 10000; ++i)
            {
                int outputBytes = blockSize * i;

                var elapsedTicks = (double)Benchmark(password, outputBytes, iterationCount, saltSize, benchmarkRepetitions);

                Console.WriteLine(string.Format("OutputBytes {0} \t ElapsedTicks {1} \t SlowFactor {2}", outputBytes, elapsedTicks, elapsedTicks / blockSizeTicks));
            }
        }

        static long Benchmark(string password, int pbkdf2OutputBytes, int pbkdf2IterationCount, int pbkdf2SaltSize, int benchmarkRepetitions)
        {
            var stopwatch = new Stopwatch();
            stopwatch.Start();

            for (int i = 0; i < benchmarkRepetitions; ++i)
            {
                RNGCryptoServiceProvider csprng = new RNGCryptoServiceProvider();
                byte[] salt = new byte[pbkdf2SaltSize];
                csprng.GetBytes(salt);

                Rfc2898DeriveBytes pbkdf2 = new Rfc2898DeriveBytes(password, salt);
                pbkdf2.IterationCount = pbkdf2IterationCount;
                pbkdf2.GetBytes(pbkdf2OutputBytes);
            }

            stopwatch.Stop();

            return stopwatch.ElapsedTicks;
        }
    }
}