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Use Visual C# to compute and compare hash values

This step-by-step article shows you how to obtain a hash value and how to compare two hash values to check whether they're identical by using Visual C#. It also provides a code sample to show how to do this task.

Original product version:   Visual C#
Original KB number:   307020

Summary

This article refers to the following Microsoft .NET Framework Class Library namespaces:

  • System.Security.Cryptography
  • System.Text

The System.Security.Cryptography class in the .NET Framework makes it easy to compute a hash value for your source data.

Compute a hash value

It's easy to generate and compare hash values using the cryptographic resources contained in the System.Security.Cryptography namespace. Because all hash functions take input of type Byte[], it might be necessary to convert the source into a byte array before it's hashed. To create a hash for a string value, follow these steps:

  1. Open Visual Studio .NET or Visual Studio.

  2. Create a new Console Application in Visual C# .NET or in Visual C# creates a public class for you along with an empty Main() method.

    Note

    In Visual C#. NET, Class1.cs is created by default. In Visual C#, Program.cs is created by default.

  3. Use the using directive on the System, System.Security.Cryptography, and System.Text namespaces so that you aren't required to qualify declarations from these namespaces later in your code. These statements must be used before any other declarations.

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

  4. Declare a string variable to hold your source data, and two byte arrays (of undefined size) to hold the source bytes and the resulting hash value.

    string sSourceData;
byte[] tmpSource;
byte[] tmpHash;

  5. Use the GetBytes() method of the System.Text.ASCIIEncoding class to convert your source string into an array of bytes (required as input to the hashing function).

    sSourceData = "MySourceData";
//Create a byte array from source data.
tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData);

  6. Compute the MD5 hash for your source data by calling ComputeHash on an instance of the MD5CryptoServiceProvider class.

    Note

    To compute another hash value, you will need to create another instance of the class.

    //Compute hash based on source data.
tmpHash = new MD5CryptoServiceProvider().ComputeHash(tmpSource);

  7. The tmpHash byte array now holds the computed hash value (128-bit value=16 bytes) for your source data. It's often useful to display or store a value like this as a hexadecimal string, which the following code accomplishes:

    Console.WriteLine(ByteArrayToString(tmpHash));
static string ByteArrayToString(byte[] arrInput)
{
    int i;
    StringBuilder sOutput = new StringBuilder(arrInput.Length);
    for (i=0;i < arrInput.Length; i++)
    {
        sOutput.Append(arrInput[i].ToString("X2"));
    }
    return sOutput.ToString();
}

  8. Save and then run your code to see the resulting hexadecimal string for the source value.

Compare two hash values

The purposes of creating a hash from source data are:

  • Providing a way to see if data has changed over time.
  • Comparing two values without ever working with the actual values.

In either case, you need to compare two computed hashes. It's easy if they're both stored as hexadecimal strings (as in the last step of the above section). But it's possible that they'll both be in the form of byte arrays. The following code, which continues from the code created in the previous section, shows how to compare two arrays of bytes.

  1. Just below the creation of a hexadecimal string, create a new hash value based on new source data.

    sSourceData = "NotMySourceData";
tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData);
byte[] tmpNewHash;
tmpNewHash = new MD5CryptoServiceProvider().ComputeHash(tmpSource);

  2. The most straightforward way to compare two arrays of bytes is to loop through the arrays, comparing each individual element to its counterpart from the second value. If any elements are different, or if the two arrays aren't the same size, the two values aren't equal.

    bool bEqual = false;
if (tmpNewHash.Length == tmpHash.Length)
{
    int i=0;
    while ((i < tmpNewHash.Length) && (tmpNewHash[i] == tmpHash[i]))
    {
        i += 1;
    }
    if (i == tmpNewHash.Length)
    {
        bEqual = true;
    }
}

if (bEqual)
    Console.WriteLine("The two hash values are the same");
else
    Console.WriteLine("The two hash values are not the same");
Console.ReadLine();

  3. Save and then run your project to view the hexadecimal string created from the first hash value. Find out if the new hash is equal to the original.

Complete code listing

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

namespace ComputeAHash_csharp
{
    /// <summary>
    /// Summary description for Class1.
    /// </summary>
    class Class1
    {
        static void Main(string[] args)
        {
            string sSourceData;
            byte[] tmpSource;
            byte[] tmpHash;
            sSourceData = "MySourceData";
            //Create a byte array from source data
            tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData);

            //Compute hash based on source data
            tmpHash = new MD5CryptoServiceProvider().ComputeHash(tmpSource);
            Console.WriteLine(ByteArrayToString(tmpHash));

            sSourceData = "NotMySourceData";
            tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData);

            byte[] tmpNewHash;

            tmpNewHash = new MD5CryptoServiceProvider().ComputeHash(tmpSource);

            bool bEqual = false;
            if (tmpNewHash.Length == tmpHash.Length)
            {
                int i=0;
                while ((i < tmpNewHash.Length) && (tmpNewHash[i] == tmpHash[i]))
                {
                    i += 1;
                }
                if (i == tmpNewHash.Length)
                {
                    bEqual = true;
                }
            }

            if (bEqual)
                Console.WriteLine("The two hash values are the same");
            else
                Console.WriteLine("The two hash values are not the same");
            Console.ReadLine();
        }

        static string ByteArrayToString(byte[] arrInput)
        {
            int i;
            StringBuilder sOutput = new StringBuilder(arrInput.Length);
            for (i=0;i < arrInput.Length; i++)
            {
                sOutput.Append(arrInput[i].ToString("X2"));
            }
            return sOutput.ToString();
        }
    }
}

References

For more information about how to use the cryptographic features of the .NET Framework, see .NET.

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