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KeyedHashAlgorithm Class

Updated: March 2011

Represents the abstract class from which all implementations of keyed hash algorithms must derive.

Namespace:  System.Security.Cryptography
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
<ComVisibleAttribute(True)> _
Public MustInherit Class KeyedHashAlgorithm _
	Inherits HashAlgorithm
'Usage
Dim instance As KeyedHashAlgorithm

Hash functions map binary strings of an arbitrary length to small binary strings of a fixed length. A cryptographic hash function has the property that it is computationally infeasible to find two distinct inputs that hash to the same value. Small changes to the data result in large, unpredictable changes in the hash.

A keyed hash algorithm is a key-dependent, one-way hash function used as a message authentication code. Only someone who knows the key can verify the hash. Keyed hash algorithms provide authenticity without secrecy.

Hash functions are commonly used with digital signatures and for data integrity. The HMACSHA1 class is an example of a keyed hash algorithm.

The following code example demonstrates how to derive from the KeyedHashAlgorithm class.

Imports System
Imports System.Security.Cryptography
 _

Public Class TestHMACMD5

    Private Shared Sub PrintByteArray(ByVal arr() As [Byte])
        Dim i As Integer
        Console.WriteLine(("Length: " + arr.Length.ToString()))
        For i = 0 To arr.Length - 1
            Console.Write("{0:X}", arr(i))
            Console.Write("    ")
            If (i + 9) Mod 8 = 0 Then
                Console.WriteLine()
            End If 
        Next i
        If i Mod 8 <> 0 Then
            Console.WriteLine()
        End If 
    End Sub 

    Public Shared Sub Main()
        ' Create a key. 
        Dim key1 As Byte() = {&HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB}
        ' Pass the key to the constructor of the HMACMD5 class.   
        Dim hmac1 As New HMACMD5(key1)

        ' Create another key. 
        Dim key2 As Byte() = System.Text.Encoding.ASCII.GetBytes("KeyString")
        ' Pass the key to the constructor of the HMACMD5 class.   
        Dim hmac2 As New HMACMD5(key2)

        ' Encode a string into a byte array, create a hash of the array, 
        ' and print the hash to the screen. 
        Dim data1 As Byte() = System.Text.Encoding.ASCII.GetBytes("Hi There")
        PrintByteArray(hmac1.ComputeHash(data1))

        ' Encode a string into a byte array, create a hash of the array, 
        ' and print the hash to the screen. 
        Dim data2 As Byte() = System.Text.Encoding.ASCII.GetBytes("This data will be hashed.")
        PrintByteArray(hmac2.ComputeHash(data2))
    End Sub 
End Class
 _

Public Class HMACMD5
    Inherits KeyedHashAlgorithm
    Private hash1 As MD5
    Private hash2 As MD5
    Private bHashing As Boolean = False 

    Private rgbInner(64) As Byte
    Private rgbOuter(64) As Byte


    Public Sub New(ByVal rgbKey() As Byte)
        HashSizeValue = 128
        ' Create the hash algorithms.
        hash1 = MD5.Create()
        hash2 = MD5.Create()
        ' Get the key. 
        If rgbKey.Length > 64 Then
            KeyValue = hash1.ComputeHash(rgbKey)
            ' No need to call Initialize; ComputeHash does it automatically. 
        Else
            KeyValue = CType(rgbKey.Clone(), Byte())
        End If 
        ' Compute rgbInner and rgbOuter. 
        Dim i As Integer = 0
        For i = 0 To 63
            rgbInner(i) = &H36
            rgbOuter(i) = &H5C
        Next i
        i = 0
        For i = 0 To KeyValue.Length - 1
            rgbInner(i) = rgbInner(i) Xor KeyValue(i)
            rgbOuter(i) = rgbOuter(i) Xor KeyValue(i)
        Next i
    End Sub 


    Public Overrides Property Key() As Byte()
        Get 
            Return CType(KeyValue.Clone(), Byte())
        End Get 
        Set(ByVal Value As Byte())
            If bHashing Then 
                Throw New Exception("Cannot change key during hash operation")
            End If 
            If value.Length > 64 Then
                KeyValue = hash1.ComputeHash(value)
                ' No need to call Initialize; ComputeHash does it automatically. 
            Else
                KeyValue = CType(value.Clone(), Byte())
            End If 
            ' Compute rgbInner and rgbOuter. 
            Dim i As Integer = 0
            For i = 0 To 63
                rgbInner(i) = &H36
                rgbOuter(i) = &H5C
            Next i
            For i = 0 To KeyValue.Length - 1
                rgbInner(i) ^= KeyValue(i)
                rgbOuter(i) ^= KeyValue(i)
            Next i
        End Set 
    End Property 


    Public Overrides Sub Initialize()
        hash1.Initialize()
        hash2.Initialize()
        bHashing = False 
    End Sub 


    Protected Overrides Sub HashCore(ByVal rgb() As Byte, ByVal ib As Integer, ByVal cb As Integer)
        If bHashing = False Then
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0)
            bHashing = True 
        End If
        hash1.TransformBlock(rgb, ib, cb, rgb, ib)
    End Sub 


    Protected Overrides Function HashFinal() As Byte()
        If bHashing = False Then
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0)
            bHashing = True 
        End If 
        ' Finalize the original hash.
        hash1.TransformFinalBlock(New Byte(0) {}, 0, 0)
        ' Write the outer array.
        hash2.TransformBlock(rgbOuter, 0, 64, rgbOuter, 0)
        ' Write the inner hash and finalize the hash.
        hash2.TransformFinalBlock(hash1.Hash, 0, hash1.Hash.Length)
        bHashing = False 
        Return hash2.Hash
    End Function 
End Class

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

Date

History

Reason

March 2011

Replaced the example.

Information enhancement.

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