KeyedHashAlgorithm Class
Updated: March 2011
Represents the abstract class from which all implementations of keyed hash algorithms must derive.
Assembly: mscorlib (in mscorlib.dll)
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
System.Security.Cryptography.HashAlgorithm
System.Security.Cryptography.KeyedHashAlgorithm
System.Security.Cryptography.HMAC
System.Security.Cryptography.MACTripleDES
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
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