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MACTripleDES Class

Computes a Message Authentication Code (MAC) using TripleDES for the input data CryptoStream.

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

'Declaration
<ComVisibleAttribute(True)> _
Public Class MACTripleDES _
	Inherits KeyedHashAlgorithm

The MACTripleDES type exposes the following members.

  NameDescription
Public methodMACTripleDESInitializes a new instance of the MACTripleDES class.
Public methodMACTripleDES(Byte())Initializes a new instance of the MACTripleDES class with the specified key data.
Public methodMACTripleDES(String, Byte())Initializes a new instance of the MACTripleDES class with the specified key data and using the specified implementation of TripleDES.
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  NameDescription
Public propertyCanReuseTransformGets a value indicating whether the current transform can be reused. (Inherited from HashAlgorithm.)
Public propertyCanTransformMultipleBlocksWhen overridden in a derived class, gets a value indicating whether multiple blocks can be transformed. (Inherited from HashAlgorithm.)
Public propertyHashGets the value of the computed hash code. (Inherited from HashAlgorithm.)
Public propertyHashSizeGets the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
Public propertyInputBlockSizeWhen overridden in a derived class, gets the input block size. (Inherited from HashAlgorithm.)
Public propertyKeyGets or sets the key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.)
Public propertyOutputBlockSizeWhen overridden in a derived class, gets the output block size. (Inherited from HashAlgorithm.)
Public propertyPaddingGets or sets the padding mode used in the hashing algorithm.
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  NameDescription
Public methodClearReleases all resources used by the HashAlgorithm class. (Inherited from HashAlgorithm.)
Public methodComputeHash(Byte())Computes the hash value for the specified byte array. (Inherited from HashAlgorithm.)
Public methodComputeHash(Stream)Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm.)
Public methodComputeHash(Byte(), Int32, Int32)Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
Public methodDisposeReleases all resources used by the current instance of the HashAlgorithm class. (Inherited from HashAlgorithm.)
Protected methodDispose(Boolean)Releases the resources used by the MACTripleDES instance. (Overrides KeyedHashAlgorithm.Dispose(Boolean).)
Public methodEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodHashCoreRoutes data written to the object into the TripleDES encryptor for computing the Message Authentication Code (MAC). (Overrides HashAlgorithm.HashCore(Byte(), Int32, Int32).)
Protected methodHashFinalReturns the computed Message Authentication Code (MAC) after all data is written to the object. (Overrides HashAlgorithm.HashFinal.)
Public methodInitializeInitializes an instance of MACTripleDES. (Overrides HashAlgorithm.Initialize.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodTransformBlockComputes the hash value for the specified region of the input byte array and copies the specified region of the input byte array to the specified region of the output byte array. (Inherited from HashAlgorithm.)
Public methodTransformFinalBlockComputes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
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  NameDescription
Protected fieldHashSizeValueRepresents the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
Protected fieldHashValueRepresents the value of the computed hash code. (Inherited from HashAlgorithm.)
Protected fieldKeyValueThe key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.)
Protected fieldStateRepresents the state of the hash computation. (Inherited from HashAlgorithm.)
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A MAC can be used to determine whether a message sent over an insecure channel has been tampered with, provided that the sender and receiver share a secret key. The sender computes the MAC for the original data, and sends both as a single message. The receiver recomputes the MAC on the received message, and checks that the computed MAC matches the transmitted MAC.

Any change to the data or the MAC results in a mismatch, because knowledge of the secret key is required to change the message and reproduce the correct MAC. Therefore, if the codes match, the message is authenticated.

MACTripleDES uses a key that is 16 or 24 bytes long, and produces a hash sequence that is 8 bytes long.

The following example creates a MAC for a file named input.txt, which is located in the folder that contains the example executable. The MAC and the original text are written to a file named encrypted.hsh in the same folder. The signed file is then read, and the MAC is calculated for the text portion of the file and compared to the MAC that is included with the text.

Imports System
Imports System.IO
Imports System.Security.Cryptography

Public Class MACTripleDESexample

    Public Shared Sub Main(ByVal Fileargs() As String)
        Dim dataFile As String 
        Dim signedFile As String 
        'If no file names are specified, create them. 
        If Fileargs.Length < 2 Then
            dataFile = "text.txt"
            signedFile = "signedFile.enc" 

            If Not File.Exists(dataFile) Then 
                ' Create a file to write to. 
                Using sw As StreamWriter = File.CreateText(dataFile)
                    sw.WriteLine("Here is a message to sign")
                End Using 
            End If 

        Else
            dataFile = Fileargs(0)
            signedFile = Fileargs(1)
        End If 
        Try 
            ' Create a random key using a random number generator. This would be the 
            '  secret key shared by sender and receiver. 
            Dim secretkey() As Byte = New [Byte](23) {}
            'RNGCryptoServiceProvider is an implementation of a random number generator. 
            Using rng As New RNGCryptoServiceProvider()
                ' The array is now filled with cryptographically strong random bytes.
                rng.GetBytes(secretkey)

                ' Use the secret key to encode the message file.
                SignFile(secretkey, dataFile, signedFile)

                ' Take the encoded file and decode
                VerifyFile(secretkey, signedFile)
            End Using 
        Catch e As IOException
            Console.WriteLine("Error: File not found", e)
        End Try 

    End Sub 'Main

    ' Computes a keyed hash for a source file and creates a target file with the keyed hash 
    ' prepended to the contents of the source file.  
    Public Shared Sub SignFile(ByVal key() As Byte, ByVal sourceFile As String, ByVal destFile As String)
        ' Initialize the keyed hash object. 
        Using myhmac As New MACTripleDES(key)
            Using inStream As New FileStream(sourceFile, FileMode.Open)
                Using outStream As New FileStream(destFile, FileMode.Create)
                    ' Compute the hash of the input file. 
                    Dim hashValue As Byte() = myhmac.ComputeHash(inStream)
                    ' Reset inStream to the beginning of the file.
                    inStream.Position = 0
                    ' Write the computed hash value to the output file.
                    outStream.Write(hashValue, 0, hashValue.Length)
                    ' Copy the contents of the sourceFile to the destFile. 
                    Dim bytesRead As Integer 
                    ' read 1K at a time 
                    Dim buffer(1023) As Byte 
                    Do 
                        ' Read from the wrapping CryptoStream.
                        bytesRead = inStream.Read(buffer, 0, 1024)
                        outStream.Write(buffer, 0, bytesRead)
                    Loop While bytesRead > 0
                End Using 
            End Using 
        End Using 
        Return 

    End Sub 'SignFile
    ' end SignFile 

    ' Compares the key in the source file with a new key created for the data portion of the file. If the keys  
    ' compare the data has not been tampered with. 
    Public Shared Function VerifyFile(ByVal key() As Byte, ByVal sourceFile As String) As Boolean 
        Dim err As Boolean = False 
        ' Initialize the keyed hash object.  
        Using hmac As New MACTripleDES(key)
            ' Create an array to hold the keyed hash value read from the file. 
            Dim storedHash(hmac.HashSize / 8) As Byte 
            ' Create a FileStream for the source file. 
            Using inStream As New FileStream(sourceFile, FileMode.Open)
                ' Read in the storedHash.
                inStream.Read(storedHash, 0, storedHash.Length - 1)
                ' Compute the hash of the remaining contents of the file. 
                ' The stream is properly positioned at the beginning of the content,  
                ' immediately after the stored hash value. 
                Dim computedHash As Byte() = hmac.ComputeHash(inStream)
                ' compare the computed hash with the stored value 
                Dim i As Integer 
                For i = 0 To storedHash.Length - 2
                    If computedHash(i) <> storedHash(i) Then
                        err = True 
                    End If 
                Next i
            End Using 
        End Using 
        If err Then
            Console.WriteLine("Hash values differ! Signed file has been tampered with!")
            Return False 
        Else
            Console.WriteLine("Hash values agree -- no tampering occurred.")
            Return True 
        End If 

    End Function 'VerifyFile 
End Class 'MACTripleDESexample 'end VerifyFile
'end class

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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