MACTripleDES Class
Updated: May 2011
Computes a Message Authentication Code (MAC) using TripleDES for the input data CryptoStream.
System.Security.Cryptography.HashAlgorithm
System.Security.Cryptography.KeyedHashAlgorithm
System.Security.Cryptography.MACTripleDES
Assembly: mscorlib (in mscorlib.dll)
The MACTripleDES type exposes the following members.
| Name | Description | |
|---|---|---|
![]() | MACTripleDES | Initializes a new instance of the MACTripleDES class. |
![]() | MACTripleDES(Byte()) | Initializes a new instance of the MACTripleDES class with the specified key data. |
![]() | MACTripleDES(String, Byte()) | Initializes a new instance of the MACTripleDES class with the specified key data using the specified implementation of TripleDES. |
| Name | Description | |
|---|---|---|
![]() | CanReuseTransform | Gets a value indicating whether the current transform can be reused. (Inherited from HashAlgorithm.) |
![]() | CanTransformMultipleBlocks | When overridden in a derived class, gets a value indicating whether multiple blocks can be transformed. (Inherited from HashAlgorithm.) |
![]() | Hash | Gets the value of the computed hash code. (Inherited from HashAlgorithm.) |
![]() | HashSize | Gets the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.) |
![]() | InputBlockSize | When overridden in a derived class, gets the input block size. (Inherited from HashAlgorithm.) |
![]() | Key | Gets or sets the key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.) |
![]() | OutputBlockSize | When overridden in a derived class, gets the output block size. (Inherited from HashAlgorithm.) |
![]() | Padding | Gets or sets the padding mode used in the hashing algorithm. |
| Name | Description | |
|---|---|---|
![]() | Clear | Releases all resources used by the HashAlgorithm class. (Inherited from HashAlgorithm.) |
![]() | ComputeHash(Byte()) | Computes the hash value for the specified byte array. (Inherited from HashAlgorithm.) |
![]() | ComputeHash(Stream) | Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm.) |
![]() | ComputeHash(Byte(), Int32, Int32) | Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.) |
![]() | Dispose | Releases all resources used by the current instance of the HashAlgorithm class. (Inherited from HashAlgorithm.) |
![]() | Dispose(Boolean) | Releases the unmanaged resources used by the MACTripleDES and optionally releases the managed resources. (Overrides KeyedHashAlgorithm.Dispose(Boolean).) |
![]() | Equals(Object) | Determines whether the specified Object is equal to the current Object. (Inherited from Object.) |
![]() | Finalize | Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.) |
![]() | GetHashCode | Serves as a hash function for a particular type. (Inherited from Object.) |
![]() | GetType | Gets the Type of the current instance. (Inherited from Object.) |
![]() | HashCore | Routes data written to the object into the TripleDES encryptor for computing the Message Authentication Code (MAC). (Overrides HashAlgorithm.HashCore(Byte(), Int32, Int32).) |
![]() | HashFinal | Returns the computed Message Authentication Code (MAC) after all data is written to the object. (Overrides HashAlgorithm.HashFinal.) |
![]() | Initialize | Initializes an instance of MACTripleDES. (Overrides HashAlgorithm.Initialize.) |
![]() | MemberwiseClone | Creates a shallow copy of the current Object. (Inherited from Object.) |
![]() | ToString | Returns a string that represents the current object. (Inherited from Object.) |
![]() | TransformBlock | Computes 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.) |
![]() | TransformFinalBlock | Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.) |
| Name | Description | |
|---|---|---|
![]() | HashSizeValue | Represents the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.) |
![]() | HashValue | Represents the value of the computed hash code. (Inherited from HashAlgorithm.) |
![]() | KeyValue | The key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.) |
![]() | State | Represents the state of the hash computation. (Inherited from HashAlgorithm.) |
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 will result 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 of length 16 or 24 bytes, and produces a hash sequence of length 8 bytes.
The following example creates a MAC for a file named input.txt, which is located in the folder that contains the sample 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 contained 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
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.



