The classes that derive from the System.Security.Cryptography.SymmetricAlgorithm class use a chaining mode called cipher block chaining (CBC), which requires a key (SymmetricAlgorithm.Key) and an initialization vector (SymmetricAlgorithm.IV) to perform cryptographic transformations on data. To decrypt data that was encrypted using one of the System.Security.Cryptography.SymmetricAlgorithm classes, you must set the SymmetricAlgorithm.Key property and the SymmetricAlgorithm.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.

System.Security.Cryptography.RijndaelManaged, System.Security.Cryptography.DESCryptoServiceProvider, System.Security.Cryptography.RC2CryptoServiceProvider, and System.Security.Cryptography.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 SymmetricAlgorithm.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.

Namespace: System.Security.Cryptography
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0