MACTripleDES Class

Definition

Namespace:

System.Security.Cryptography

Assembly:

mscorlib.dll

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

public ref class MACTripleDES : System::Security::Cryptography::KeyedHashAlgorithm

[System.Runtime.InteropServices.ComVisible(true)]
public class MACTripleDES : System.Security.Cryptography.KeyedHashAlgorithm

[<System.Runtime.InteropServices.ComVisible(true)>]
type MACTripleDES = class
    inherit KeyedHashAlgorithm

Public Class MACTripleDES
Inherits KeyedHashAlgorithm

Inheritance

Object

HashAlgorithm

KeyedHashAlgorithm

MACTripleDES

Attributes

ComVisibleAttribute

Examples

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.

using System;
using System.IO;
using System.Security.Cryptography;

public class MACTripleDESexample
{

    public static void Main(string[] Fileargs)
    {
        string dataFile;
        string signedFile;
        //If no file names are specified, create them.
        if (Fileargs.Length < 2)
        {
            dataFile = @"text.txt";
            signedFile = "signedFile.enc";

            if (!File.Exists(dataFile))
            {
                // Create a file to write to.
                using (StreamWriter sw = File.CreateText(dataFile))
                {
                    sw.WriteLine("Here is a message to sign");
                }
            }
        }
        else
        {
            dataFile = Fileargs[0];
            signedFile = Fileargs[1];
        }
        try
        {
            // Create a random key using a random number generator. This would be the
            //  secret key shared by sender and receiver.
            byte[] secretkey = new Byte[24];

            using (RandomNumberGenerator rng = RandomNumberGenerator.Create())
            {
                // The array is now filled with cryptographically strong random bytes.
                rng.GetBytes(secretkey);

                // Use the secret key to sign the message file.
                SignFile(secretkey, dataFile, signedFile);

                // Verify the signed file
                VerifyFile(secretkey, signedFile);
            }
        }
        catch (IOException e)
        {
            Console.WriteLine("Error: File not found", e);
        }
    }  //end 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 static void SignFile(byte[] key, String sourceFile, String destFile)
    {
        // Initialize the keyed hash object.
        using (MACTripleDES hmac = new MACTripleDES(key))
        {
            using (FileStream inStream = new FileStream(sourceFile, FileMode.Open))
            {
                using (FileStream outStream = new FileStream(destFile, FileMode.Create))
                {
                    // Compute the hash of the input file.
                    byte[] hashValue = hmac.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.
                    int bytesRead;
                    // read 1K at a time
                    byte[] buffer = new byte[1024];
                    do
                    {
                        // Read from the wrapping CryptoStream.
                        bytesRead = inStream.Read(buffer, 0, 1024);
                        outStream.Write(buffer, 0, bytesRead);
                    } while (bytesRead > 0);
                }
            }
        }
        return;
    } // 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 static bool VerifyFile(byte[] key, String sourceFile)
    {
        bool err = false;
        // Initialize the keyed hash object.
        using (MACTripleDES hmac = new MACTripleDES(key))
        {
            // Create an array to hold the keyed hash value read from the file.
            byte[] storedHash = new byte[hmac.HashSize / 8];
            // Create a FileStream for the source file.
            using (FileStream inStream = new FileStream(sourceFile, FileMode.Open))
            {
                // Read in the storedHash.
                inStream.Read(storedHash, 0, storedHash.Length);
                // 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.
                byte[] computedHash = hmac.ComputeHash(inStream);
                // compare the computed hash with the stored value

                for (int i = 0; i < storedHash.Length; i++)
                {
                    if (computedHash[i] != storedHash[i])
                    {
                        err = true;
                    }
                }
            }
        }
        if (err)
        {
            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 VerifyFile
} //end class

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) {}

            Using rng As RandomNumberGenerator = RandomNumberGenerator.Create()
                ' 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

    ' 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
    ' 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
'end class

Remarks

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.

Constructors

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 and using the specified implementation of TripleDES.

Fields

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)

Properties

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.

Methods

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(Byte[], Int32, Int32)	Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm)
ComputeHash(Stream)	Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm)
ComputeHashAsync(Stream, CancellationToken)	Asynchronously computes the hash value for the specified Stream object. (Inherited from HashAlgorithm)
Dispose()	Releases all resources used by the current instance of the HashAlgorithm class. (Inherited from HashAlgorithm)
Dispose(Boolean)	Releases the resources used by the MACTripleDES instance.
Equals(Object)	Determines whether the specified object is equal to the current object. (Inherited from Object)
GetHashCode()	Serves as the default hash function. (Inherited from Object)
GetType()	Gets the Type of the current instance. (Inherited from Object)
HashCore(Byte[], Int32, Int32)	Routes data written to the object into the TripleDES encryptor for computing the Message Authentication Code (MAC).
HashCore(ReadOnlySpan<Byte>)	Routes data written to the object into the hash algorithm for computing the hash. (Inherited from HashAlgorithm)
HashFinal()	Returns the computed Message Authentication Code (MAC) after all data is written to the object.
Initialize()	Initializes an instance of MACTripleDES.
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(Byte[], Int32, Int32, Byte[], Int32)	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(Byte[], Int32, Int32)	Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm)
TryComputeHash(ReadOnlySpan<Byte>, Span<Byte>, Int32)	Attempts to compute the hash value for the specified byte array. (Inherited from HashAlgorithm)
TryHashFinal(Span<Byte>, Int32)	Attempts to finalize the hash computation after the last data is processed by the hash algorithm. (Inherited from HashAlgorithm)