Programming Symmetrical Encryption (part 1)

The .NET Framework takes the same basic approach to defining symmetric algorithms; abstract classes extend the System.Security.Cryptography.SymmetricAlgorithm class for each of the supported algorithms. Individual implementations of the algorithms extend the abstract class, supporting the possibility of more than one implementation of an algorithm, as represented by Figure 1.

Figure 1. The .NET Framework class hierarchy for symmetric encryption algorithms

1. The SymmetricAlgorithm Class

The SymmetricAlgorithm class allows you to configure an algorithm (select the block size, padding mode, etc.) and create instances of the classes that encrypt and decrypt data; this class, and the derived implementation classes, are not used to process data directly;

Table 1. Members of the SymmetricAlgorithm Class

Member	Description
Properties
BlockSize	Gets or sets the block size used by the cipher function.
FeedbackSize	Gets or sets the block size used to create feedback when encrypting data.
KeySize	Gets or sets the size in bits of the secret key used by the algorithm.
IV	Get and set the values of the secret key and initialization vector, expressed as an array of bytes.
Key
LegalBlockSizes	Return the range of block and secret key sizes that the algorithm supports.
LegalKeySizes
Mode	Gets and sets the cipher mode used to prepare data.
Padding	Gets or sets the padding mode that will fill out partial blocks of data.
Methods
Create	Creates a new instance of the SymmetricAlgorithm class by name. See the following section for further details.
CreateEncryptor	Create instances of the classes used to encrypt and decrypt data. See Section 14.3.4 for details.
CreateDecryptor
GenerateIV	Generate random secret keys and initialization vectors.
GenerateKey
ValidKeySize	Determines if a key of a given length is valid for the algorithm.

2. Instantiating the Algorithm

You can instantiate the implementation classes for symmetric algorithms in the same way. The preferred way to create an instance is by using the Create method of the SymmetricAlgorithm class:

# C#

SymmetricAlgorithm x_alg = SymmetricAlgorithm.Create("RC2");

# Visual Basic .NET

Dim x_alg As SymmetricAlgorithm = SymmetricAlgorithm.Create("RC2")

The Create method instantiates an implementation class based on the value of the argument; Table 14-3 shows the list of supported argument strings and the implementation classes that they create. If no argument is supplied, the Create method will instantiate the RijndaelManaged class and return null (C#) or NothingTable 14-3. The system administrator configures the mapping between the string values and the instantiated classes. (Visual Basic .NET) if the argument is not one of those listed in

Algorithms can be instantiated directly, an approach that is useful if you wish to ensure that a specific implementation class is used or if you are using algorithms for which no mappings are available:

# C#

SymmetricAlgorithm x_alg = new RC2CryptoServiceProvider(  );

# Visual Basic .NET

Dim x_alg As SymmetricAlgorithm = New RC2CryptoServiceProvider(  )

Table 2. Mapping string values to algorithm classes

String value	Algorithm-implementation class
DES	DESCryptoServiceProvider
System.Security.Cryptography.DES	DESCryptoServiceProvider
3DES	TripleDESCryptoServiceProvider
TripleDES	TripleDESCryptoServiceProvider
Triple DES	TripleDESCryptoServiceProvider
System.Security.Cryptography.TripleDES	TripleDESCryptoServiceProvider
RC2	RC2CryptoServiceProvider
System.Security.Cryptography.RC2	RC2CryptoServiceProvider
Rijndael	RijndaelManaged
System.Security.Cryptography.Rijndael	RijndaelManaged