Microsoft KB Archive/301053

= How to compute and compare hash values by using Visual Basic .NET or Visual Basic 2005 =

Article ID: 301053

Article Last Modified on 5/12/2007

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APPLIES TO


 * Microsoft Visual Basic 2005
 * Microsoft Visual Basic .NET 2003 Standard Edition
 * Microsoft Visual Basic .NET 2002 Standard Edition

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This article was previously published under Q301053





For a Microsoft Visual C# .NET version of this article, see 307020.

IN THIS TASK
SUMMARY
 * Requirements
 * Compute a Hash Value
 * Compare Two Hash Values
 * Complete Code Listing

REFERENCES



SUMMARY
The System.Security.Cryptography classes in the Microsoft .NET Framework make it easy to compute a hash value for your source data. This article shows how to obtain a hash value and how to compare two hash values to check whether they are identical.

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Requirements
The following list outlines the recommended hardware, software, network infrastructure, and service packs that you will need:
 * Microsoft Visual Studio .NET or Microsoft Visual Studio 2005

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Compute a Hash Value
It is 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 is hashed. To create a hash for a string value, follow these steps:  Open Visual Studio .NET or Visual Studio 2005. Create a new Console Application in Visual Basic .NET or in Visual Basic 2005. Visual Studio .NET and Visual Studio 2005 create a Module for you along with an empty Main procedure. Make sure that the project references the System and System.Security namespaces.  Use the Imports statement on the System, System.Security, System.Security.Cryptographic, and System.Text namespaces so that you are not required to qualify declarations from these namespaces later in your code. These statements must be used prior to any other declarations. Imports System Imports System.Security Imports System.Security.Cryptography Imports System.Text   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. Dim sSourceData As String Dim tmpSource As Byte Dim tmpHash As Byte </li>  Use the GetBytes function, which is part of the System.Text.ASCIIEncoding.ASCII class, to convert your source string into an array of bytes (required as input to the hashing function). sSourceData = &quot;MySourceData&quot; 'Create a byte array from source data. tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData) </li>  Compute the MD5 hash for your source data by calling ComputeHash on an instance of the MD5CryptoServiceProvider class. Note that 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) </li>  The tmpHash byte array now holds the computed hash value (128-bit value=16 bytes) for your source data. It is often useful to display or store a value like this as a hexadecimal string, which the following code accomplishes: Console.WriteLine(ByteArrayToString(tmpHash))

Private Function ByteArrayToString(ByVal arrInput As Byte) As String Dim i As Integer Dim sOutput As New StringBuilder(arrInput.Length) For i = 0 To arrInput.Length - 1 sOutput.Append(arrInput(i).ToString(&quot;X2&quot;)) Next Return sOutput.ToString End Function </li> Save and then run your code to see the resulting hexadecimal string for the source value.</li></ol>

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Compare Two Hash Values
One of the purposes of creating a hash from source data is to provide a way to see if data has changed over time, or to compare two values without ever working with the actual values. In either case, you need to compare two computed hashes, which is easy if they are both stored as hexadecimal strings (as in the last step of the above section). However, it is quite possible that they will 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 byte arrays. <ol>  Just below the creation of a hexadecimal string, create a new hash value based on new source data. sSourceData = &quot;NotMySourceData&quot; tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData)

Dim tmpNewHash As Byte Dim bEqual As Boolean = False tmpNewHash = New MD5CryptoServiceProvider.ComputeHash(tmpSource) </li>  The most straightforward way to compare two byte arrays 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 are not the same size, the two values are not equal. If tmpNewHash.Length = tmpHash.Length Then Dim i As Integer Do While (i < tmpNewHash.Length) AndAlso (tmpNewHash(i) = tmpHash(i)) i += 1 Loop If i = tmpNewHash.Length Then bEqual = True End If End If

If bEqual Then Console.WriteLine(&quot;The two hash values are the same&quot;) Else Console.WriteLine(&quot;The two hash values are not the same&quot;) End If Console.ReadLine </li> Save and then run your project to view the hexadecimal string created from the first hash value, and to find out if the new hash is equal to the original.</li></ol>

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Complete Code Listing
Imports System Imports System.Security Imports System.Security.Cryptography Imports System.Text

Module Module1

Sub Main Dim sSourceData As String Dim tmpSource As Byte Dim tmpHash As Byte sSourceData = &quot;MySourceData&quot; '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 = &quot;NotMySourceData&quot; tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData)

Dim tmpNewHash As Byte Dim bEqual As Boolean = False tmpNewHash = New MD5CryptoServiceProvider.ComputeHash(tmpSource)

If tmpNewHash.Length = tmpHash.Length Then Dim i As Integer Do While (i < tmpNewHash.Length) AndAlso (tmpNewHash(i) = tmpHash(i)) i += 1 Loop If i = tmpNewHash.Length Then bEqual = True End If       End If

If bEqual Then Console.WriteLine(&quot;The two hash values are the same&quot;) Else Console.WriteLine(&quot;The two hash values are not the same&quot;) End If       Console.ReadLine End Sub

Private Function ByteArrayToString(ByVal arrInput As Byte) As String Dim i As Integer Dim sOutput As New StringBuilder(arrInput.Length) For i = 0 To arrInput.Length - 1 sOutput.Append(arrInput(i).ToString(&quot;X2&quot;)) Next Return sOutput.ToString End Function End Module back to the top

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