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SymmetricAlgorithm Class

Represents the abstract base class from which all implementations of symmetric algorithms must inherit.

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

'Declaration
<ComVisibleAttribute(True)> _
Public MustInherit Class SymmetricAlgorithm _
	Implements IDisposable
'Usage
Dim instance As SymmetricAlgorithm

The classes that derive from the SymmetricAlgorithm class use a chaining mode called cipher block chaining (CBC), which requires a key (Key) and an initialization vector (IV) to perform cryptographic transformations on data. To decrypt data that was encrypted using one of the SymmetricAlgorithm classes, you must set the Key property and the IV property to the same values that were used for encryption. For a symmetric algorithm to be useful, the secret key must be known only to the sender and the receiver.

RijndaelManaged, DESCryptoServiceProvider, RC2CryptoServiceProvider, and TripleDESCryptoServiceProvider are implementations of symmetric algorithms.

Note that when using derived classes, it is not enough, from a security perspective, to simply force a garbage collection after you have finished using the object. You must explicitly call the Clear method on the object to zero out any sensitive data within the object before it is released. Note that garbage collection does not zero out the contents of collected objects but simply marks the memory as available for reallocation. Thus the data contained within a garbage collected object may still be present in the memory heap in unallocated memory. In the case of cryptographic objects, this data could contain sensitive information such as key data or a block of plain text.

All cryptographic classes in the .NET Framework that hold sensitive data implement a Clear method. When called, the Clear method overwrites all sensitive data within the object with zeros and then releases the object so that it can be safely garbage collected. When the object has been zeroed and released, you should then call the Dispose method with the disposing parameter set to True to dispose of all managed and unmanaged resources associated with the object.

Notes to Inheritors:

When you inherit from the SymmetricAlgorithm class, you must override the following members: CreateDecryptor, CreateEncryptor, GenerateIV, and GenerateKey.

The following code example uses the RijndaelManaged class with the specified Key property and initialization vector (IV) to encrypt a file specified by inName, and outputs the encrypted result to the file specified by outName. The desKey and desIV parameters to the method are 8-byte arrays. You must have the high encryption pack installed to run this example.

Private Shared Sub EncryptData(inName As String, outName As String, _
    rijnKey() As Byte, rijnIV() As Byte)

        'Create the file streams to handle the input and output files. 
        Dim fin As New FileStream(inName, FileMode.Open, FileAccess.Read)
        Dim fout As New FileStream(outName, FileMode.OpenOrCreate, _
           FileAccess.Write)
        fout.SetLength(0)

        'Create variables to help with read and write. 
        Dim bin(100) As Byte 'This is intermediate storage for the encryption.
        Dim rdlen As Long = 0 'This is the total number of bytes written.
        Dim totlen As Long = fin.Length 'Total length of the input file.
        Dim len As Integer 'This is the number of bytes to be written at a time.
        'Creates the default implementation, which is RijndaelManaged. 
        Dim rijn As SymmetricAlgorithm = SymmetricAlgorithm.Create()
        Dim encStream As New CryptoStream(fout, _
           rijn.CreateEncryptor(rijnKey, rijnIV), CryptoStreamMode.Write)

        Console.WriteLine("Encrypting...")

        'Read from the input file, then encrypt and write to the output file. 
        While rdlen < totlen
            len = fin.Read(bin, 0, 100)
            encStream.Write(bin, 0, len)
            rdlen = Convert.ToInt32(rdlen + len)
            Console.WriteLine("{0} bytes processed", rdlen)
        End While

        encStream.Close()
	fout.Close()
	fin.Close()
    End Sub

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, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

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

.NET Compact Framework

Supported in: 3.5, 2.0

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