SymmetricAlgorithm Class

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.

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 static void EncryptData(String inName, String outName, byte[] rijnKey, byte[] rijnIV)
 {    
     //Create the file streams to handle the input and output files.
     FileStream fin = new FileStream(inName, FileMode.Open, FileAccess.Read);
     FileStream fout = new FileStream(outName, FileMode.OpenOrCreate, FileAccess.Write);
     fout.SetLength(0);

     //Create variables to help with read and write.
     byte[] bin = new byte[100]; //This is intermediate storage for the encryption.
     long rdlen = 0;              //This is the total number of bytes written.
     long totlen = fin.Length;    //This is the total length of the input file.
     int len;                     //This is the number of bytes to be written at a time.

     SymmetricAlgorithm rijn = SymmetricAlgorithm.Create(); //Creates the default implementation, which is RijndaelManaged.         
     CryptoStream encStream = 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 = rdlen + len;
         Console.WriteLine("{0} bytes processed", rdlen);
     }

     encStream.Close();  
     fout.Close();
     fin.Close();                   
 }

.NET Framework

.NET Framework Client Profile

Windows 8 Consumer Preview, Windows Server 8 Beta, Windows 7, Windows Server 2008 SP2, 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.

Reference

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