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

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

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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

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:
  1. Open Visual Studio .NET or Visual Studio 2005.
  2. 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.
  3. Make sure that the project references the System and System.Security namespaces.
  4. 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
  5. 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
  6. 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 = "MySourceData"
    'Create a byte array from source data.
    tmpSource = ASCIIEncoding.ASCII.GetBytes(sSourceData)
  7. 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)
  8. 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("X2"))
    Next
    Return sOutput.ToString()
    End Function
  9. Save and then run your code to see the resulting hexadecimal string for the source value.

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.
  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)

    Dim tmpNewHash() As Byte
    Dim bEqual As Boolean = False
    tmpNewHash = New MD5CryptoServiceProvider().ComputeHash(tmpSource)
  2. 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("The two hash values are the same")
    Else
    Console.WriteLine("The two hash values are not the same")
    End If
    Console.ReadLine()
  3. 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.

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 = "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)

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("The two hash values are the same")
Else
Console.WriteLine("The two hash values are not the same")
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("X2"))
Next
Return sOutput.ToString()
End Function
End Module

References

For more information on using the cryptographic features of the Microsoft .NET Framework, and on cryptography in general, see the following links:
Properties

Article ID: 301053 - Last Review: May 12, 2007 - Revision: 1

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