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In addition, the symmetric algorithm classes share the following methods (standard object methods have been omitted): To decrypt messages, you must create a symmetric algorithm object and call this method to create an ICryptoTransform object that a CryptoStream object can use to decrypt the stream.
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Creates a symmetric encryptor object used by CryptoStream objects to encrypt a stream.
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CreateEncryptor GenerateIV Generates a random IV to be used for the algorithm. Generally, there is no need to call this method because IVs are randomly generated automatically unless you specifically define them. You call this method only if you have defined an IV and later need to use a different random IV.
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Lesson 3: Encrypting and Decrypting Data
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Generates a random key to be used for the algorithm. Like GenerateIV, you need to call this method only if you already have defined the Key property and later need to use a random key.
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Determines whether the specified key size is valid for the current algorithm and returns a boolean value. Use this method when you are working with an unknown symmetric algorithm class to verify that your key is valid for the given algorithm.
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BEST PRACTICES
Choosing a Symmetric Key Algorithm
Use the Rijndael algorithm whenever both the encryptor and decryptor are running on Windows XP or later operating systems; otherwise, use Triple DES. Of all symmetric key algorithms supported by the .NET Framework, the U.S. government approved Rijndael algorithm is considered the most secure. This algorithm supports 128-, 192-, and 256-bit keys. Another reason to choose Rijndael is that it is natively supported by the .NET Framework. Other than the less flexible AesManaged class, the other algorithms must make calls to unmanaged code.
How to Establish a Symmetric Key
Before you can encrypt and decrypt messages by using symmetric encryption, both the encryptor and decryptor must have the same key. You can t use just any piece of data as a key, however. Symmetric encryption algorithms must use keys of a specific length, so you cannot simply set the Key property to a user-provided password. To generate a random key, simply create and use a symmetric algorithm object. If you specify a value for the Key property and later want to use a random key, call the GenerateKey method. You can also generate a valid key based on a user-provided password if you can rely on users to transfer the password between the encryptor and decryptor. You cannot use passwords directly as encryption keys, but you can use the System.Security.Cryptography .Rfc2898DeriveBytes class to turn a password into a key. This is particularly useful when a shared secret has already been established between an encryptor and a decryptor. For example, if you create a custom authentication mechanism and your application is privy to the user s username and password, you could concatenate the user s own username and password to derive the same key at both the encryptor and decryptor. Rfc2898DeriveBytes requires three values in addition to the user s password: a salt value, an IV, and the number of iterations used to generate the key. Ideally, all these values are generated randomly. Changing any of these values produces a different key,
12
User and Data Security
so you are required to use the same values at both the encryptor and decryptor. Therefore, when random values are used, the values must be exchanged in the same way the password is exchanged. For this reason, it usually is not possible to securely exchange these values in addition to the password. Instead, you can specify static values that both the encryptor and decryptor applications have stored within their source code, but it is more secure to generate the values based on other shared secret information, such as the password. Creating symmetric keys based on a password requires several different values to be synchronized between the encryptor and decryptor: The password The salt value The IV The number of iterations used to generate the key (or you can accept the default) The simplest way to specify these values is to pass them to the Rfc2898DeriveBytes constructor. After initialization, you can retrieve a key by calling the Rfc2898DeriveBytes .GetBytes method. GetBytes accepts the number of bytes to return as an integer. When deriving a key, determine the length based on the number of bits required by the algorithm object s KeySize property. Note that KeySize is defined as a number of bits, whereas the Rfc2898DeriveBytes.GetBytes method requires a number of bytes. You must divide the number of bits required for the key by 8 to determine the number of bytes required. Besides the key, the encryption algorithm must also have the same IV specified at both the encryptor and decryptor. For optimal security, when only a password is shared between the encryptor and decryptor, you should also generate the IV based on the password. Whereas the length of the key being generated must be based on the KeySize property, the length of the IV must be based on the encryption algorithm s BlockSize property. Like KeySize, BlockSize is defined as a number of bits, so you need to divide the number of bits by 8 to determine the number of bytes required. The following sample code generates a key and IV for the SymmetricAlgorithm object named myAlg using a static password, but in real-world use, the password should be provided by the user:
' VB ' In practice, the user would provide the password Dim password As String = "P@S5w0r]>" ' Create an algorithm object
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