DevTut

# Hash Functions

# MD5

Hash functions map binary strings of an arbitrary length to small binary strings of a fixed length.

The MD5 (opens new window) algorithm is a widely used hash function producing a 128-bit hash value (16 Bytes, 32 Hexdecimal characters).

The ComputeHash (opens new window) method of the System.Security.Cryptography.MD5 (opens new window) class returns the hash as an array of 16 bytes.

Example:

using System;
using System.Security.Cryptography;
using System.Text;

internal class Program
{
    private static void Main()
    {
        var source = "Hello World!";

        // Creates an instance of the default implementation of the MD5 hash algorithm.
        using (var md5Hash = MD5.Create())
        {
            // Byte array representation of source string
            var sourceBytes = Encoding.UTF8.GetBytes(source);

            // Generate hash value(Byte Array) for input data
            var hashBytes = md5Hash.ComputeHash(sourceBytes);

            // Convert hash byte array to string
            var hash = BitConverter.ToString(hashBytes).Replace("-", string.Empty);

            // Output the MD5 hash
            Console.WriteLine("The MD5 hash of " + source + " is: " + hash);
        }
    }
}

Output: The MD5 hash of Hello World! is: ED076287532E86365E841E92BFC50D8C

Security Issues: (opens new window)

Like most hash functions, MD5 is neither encryption nor encoding. It can be reversed by brute-force attack and suffers from extensive vulnerabilities against collision and preimage attacks.

# SHA1

using System;
using System.Security.Cryptography;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string source = "Hello World!";
            using (SHA1 sha1Hash = SHA1.Create())
            {
                //From String to byte array
                byte[] sourceBytes = Encoding.UTF8.GetBytes(source);
                byte[] hashBytes = sha1Hash.ComputeHash(sourceBytes);
                string hash = BitConverter.ToString(hashBytes).Replace("-",String.Empty);

                Console.WriteLine("The SHA1 hash of " + source + " is: " + hash);     
            }
        } 
    }
 }

Output:

The SHA1 hash of Hello Word! is: 2EF7BDE608CE5404E97D5F042F95F89F1C232871

# SHA256

using System;
using System.Security.Cryptography;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string source = "Hello World!";
            using (SHA256 sha256Hash = SHA256.Create())
            {
                //From String to byte array
                byte[] sourceBytes = Encoding.UTF8.GetBytes(source);
                byte[] hashBytes = sha256Hash.ComputeHash(sourceBytes);
                string hash = BitConverter.ToString(hashBytes).Replace("-", String.Empty);

                Console.WriteLine("The SHA256 hash of " + source + " is: " + hash);
            }
        }
    }
}

Output:

The SHA256 hash of Hello World! is: 7F83B1657FF1FC53B92DC18148A1D65DFC2D4B1FA3D677284ADDD200126D9069

# SHA384

using System;
using System.Security.Cryptography;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string source = "Hello World!";
            using (SHA384 sha384Hash = SHA384.Create())
            {
                //From String to byte array
                byte[] sourceBytes = Encoding.UTF8.GetBytes(source);
                byte[] hashBytes = sha384Hash.ComputeHash(sourceBytes);
                string hash = BitConverter.ToString(hashBytes).Replace("-", String.Empty);

                Console.WriteLine("The SHA384 hash of " + source + " is: " + hash);
            }
        }
    }
}

Output:

The SHA384 hash of Hello World! is: BFD76C0EBBD006FEE583410547C1887B0292BE76D582D96C242D2A792723E3FD6FD061F9D5CFD13B8F961358E6ADBA4A

# SHA512

using System;
using System.Security.Cryptography;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string source = "Hello World!";
            using (SHA512 sha512Hash = SHA512.Create())
            {
                //From String to byte array
                byte[] sourceBytes = Encoding.UTF8.GetBytes(source);
                byte[] hashBytes = sha512Hash.ComputeHash(sourceBytes);
                string hash = BitConverter.ToString(hashBytes).Replace("-", String.Empty);

                Console.WriteLine("The SHA512 hash of " + source + " is: " + hash);
            }
        }
    }
}

Output: The SHA512 hash of Hello World! is: 861844D6704E8573FEC34D967E20BCFEF3D424CF48BE04E6DC08F2BD58C729743371015EAD891CC3CF1C9D34B49264B510751B1FF9E537937BC46B5D6FF4ECC8

# PBKDF2 for Password Hashing

PBKDF2 ("Password-Based Key Derivation Function 2") is one of the recommended hash-functions for password-hashing. It is part of rfc-2898 (opens new window).

.NET's Rfc2898DeriveBytes-Class is based upon HMACSHA1.


   using System.Security.Cryptography;

    ...

    public const int SALT_SIZE = 24; // size in bytes
    public const int HASH_SIZE = 24; // size in bytes
    public const int ITERATIONS = 100000; // number of pbkdf2 iterations

    public static byte[] CreateHash(string input)
    {
        // Generate a salt
        RNGCryptoServiceProvider provider = new RNGCryptoServiceProvider();
        byte[] salt = new byte[SALT_SIZE];
        provider.GetBytes(salt);

        // Generate the hash
        Rfc2898DeriveBytes pbkdf2 = new Rfc2898DeriveBytes(input, salt, ITERATIONS);
        return pbkdf2.GetBytes(HASH_SIZE);
    }

PBKDF2 requires a salt (opens new window) and the number of iterations.

Iterations:

A high number of iterations will slow the algorithm down, which makes password cracking a lot harder. A high number of iterations is therefor recommended. PBKDF2 is order of magnitudes slower than MD5 for example.

Salt:

A salt will prevent the lookup of hash values in rainbow tables. (opens new window) It has to be stored alongside the password hash. One salt per password (not one global salt) is recommended.

# Complete Password Hashing Solution using Pbkdf2

using System;
using System.Linq;
using System.Security.Cryptography;

namespace YourCryptoNamespace
{
    /// <summary>
    /// Salted password hashing with PBKDF2-SHA1.
  /// Compatibility: .NET 3.0 and later.
  /// </summary>
  /// <remarks>See http://crackstation.net/hashing-security.htm for much more on password hashing.</remarks>
  public static class PasswordHashProvider
  {
    /// <summary>
    /// The salt byte size, 64 length ensures safety but could be increased / decreased
    /// </summary>
    private const int SaltByteSize = 64;
    /// <summary>
    /// The hash byte size, 
    /// </summary>
    private const int HashByteSize = 64;
    /// <summary>
    ///  High iteration count is less likely to be cracked
    /// </summary>
    private const int Pbkdf2Iterations = 10000;

    /// <summary>
    /// Creates a salted PBKDF2 hash of the password.
    /// </summary>
    /// <remarks>
    /// The salt and the hash have to be persisted side by side for the password. They could be persisted as bytes or as a string using the convenience methods in the next class to convert from byte[] to string and later back again when executing password validation.
    /// </remarks>
    /// <param name="password">The password to hash.</param>
    /// <returns>The hash of the password.</returns>
    public static PasswordHashContainer CreateHash(string password)
    {
      // Generate a random salt
      using (var csprng = new RNGCryptoServiceProvider())
      {
        // create a unique salt for every password hash to prevent rainbow and dictionary based attacks
        var salt = new byte[SaltByteSize];
        csprng.GetBytes(salt);

        // Hash the password and encode the parameters
        var hash = Pbkdf2(password, salt, Pbkdf2Iterations, HashByteSize);

        return new PasswordHashContainer(hash, salt);
      }
    }
    /// <summary>
    /// Recreates a password hash based on the incoming password string and the stored salt
    /// </summary>
    /// <param name="password">The password to check.</param>
    /// <param name="salt">The salt existing.</param>
    /// <returns>the generated hash based on the password and salt</returns>
    public static byte[] CreateHash(string password, byte[] salt)
    {
      // Extract the parameters from the hash
      return Pbkdf2(password, salt, Pbkdf2Iterations, HashByteSize);
    }

    /// <summary>
    /// Validates a password given a hash of the correct one.
    /// </summary>
    /// <param name="password">The password to check.</param>
    /// <param name="salt">The existing stored salt.</param>
    /// <param name="correctHash">The hash of the existing password.</param>
    /// <returns><c>true</c> if the password is correct. <c>false</c> otherwise. </returns>
    public static bool ValidatePassword(string password, byte[] salt, byte[] correctHash)
    {
      // Extract the parameters from the hash
      byte[] testHash = Pbkdf2(password, salt, Pbkdf2Iterations, HashByteSize);
      return CompareHashes(correctHash, testHash);
    }
    /// <summary>
    /// Compares two byte arrays (hashes)
    /// </summary>
    /// <param name="array1">The array1.</param>
    /// <param name="array2">The array2.</param>
    /// <returns><c>true</c> if they are the same, otherwise <c>false</c></returns>
    public static bool CompareHashes(byte[] array1, byte[] array2)
    {
      if (array1.Length != array2.Length) return false;
      return !array1.Where((t, i) => t != array2[i]).Any();
    }
  
    /// <summary>
    /// Computes the PBKDF2-SHA1 hash of a password.
    /// </summary>
    /// <param name="password">The password to hash.</param>
    /// <param name="salt">The salt.</param>
    /// <param name="iterations">The PBKDF2 iteration count.</param>
    /// <param name="outputBytes">The length of the hash to generate, in bytes.</param>
    /// <returns>A hash of the password.</returns>
    private static byte[] Pbkdf2(string password, byte[] salt, int iterations, int outputBytes)
    {
      using (var pbkdf2 = new Rfc2898DeriveBytes(password, salt))
      {
        pbkdf2.IterationCount = iterations;
        return pbkdf2.GetBytes(outputBytes);
      }
    }
  }

  /// <summary>
  /// Container for password hash and salt and iterations.
  /// </summary>
  public sealed class PasswordHashContainer
  {
    /// <summary>
    /// Gets the hashed password.
    /// </summary>
    public byte[] HashedPassword { get; private set; }
    /// <summary>
    /// Gets the salt.
    /// </summary>
    public byte[] Salt { get; private set; }

    /// <summary>
    /// Initializes a new instance of the <see cref="PasswordHashContainer" /> class.
    /// </summary>
    /// <param name="hashedPassword">The hashed password.</param>
    /// <param name="salt">The salt.</param>
    public PasswordHashContainer(byte[] hashedPassword, byte[] salt)
    {
      this.HashedPassword = hashedPassword;
      this.Salt = salt;
    }
  }

  /// <summary>
  /// Convenience methods for converting between hex strings and byte array.
  /// </summary>
  public static class ByteConverter
  {
    /// <summary>
    /// Converts the hex representation string to an array of bytes
    /// </summary>
    /// <param name="hexedString">The hexed string.</param>
    /// <returns></returns>
    public static byte[] GetHexBytes(string hexedString)
    {
      var bytes = new byte[hexedString.Length / 2];
      for (var i = 0; i < bytes.Length; i++)
      {
        var strPos = i * 2;
        var chars = hexedString.Substring(strPos, 2);
        bytes[i] = Convert.ToByte(chars, 16);
      }
      return bytes;
    }
    /// <summary>
    /// Gets a hex string representation of the byte array passed in.
    /// </summary>
    /// <param name="bytes">The bytes.</param>
    public static string GetHexString(byte[] bytes)
    {
      return BitConverter.ToString(bytes).Replace("-", "").ToUpper();
    }
  }
}

/* 
 * Password Hashing With PBKDF2 (http://crackstation.net/hashing-security.htm).
 * Copyright (c) 2013, Taylor Hornby
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without 
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, 
 * this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 * this list of conditions and the following disclaimer in the documentation 
 * and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE 
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
 * POSSIBILITY OF SUCH DAMAGE.
 */

Please see this excellent resource Crackstation - Salted Password Hashing - Doing it Right (opens new window) for more information. Part of this solution (the hashing function) was based on the code from that site.

# Remarks

MD5 and SHA1 are insecure and should be avoided. The examples exist for educational purposes and due to the fact that legacy software may still use these algorithms.

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