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Padding A PaddingMode enumeration value, this property determines how
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the encryption algorithm fills out any difference between the algorithm s block size and the length of the plain text. You should generally not change this property. Additionally, the symmetric algorithm classes each share the following methods (standard object methods have been omitted):
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CreateDecryptor To decrypt messages, you must create a symmetric algorithm
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object and call this method to create an ICryptoTransform object that a CryptoStream object can use to decrypt the stream.
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CreateEncryptor
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Creates a symmetric encryptor object used by CryptoStream objects to encrypt a stream.
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GenerateIV Generates a random IV to be used for the algorithm. Generally,
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there is no need to call this method because IVs are automatically randomly generated unless you specifically define them. You will call this method only if you have defined IV and later need to use a different random IV.
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GenerateKey Generates a random key to be used for the algorithm. Like Gener-
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ateIV, you need to call this method only if you have already defined the Key property and later need to use a random key.
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ValidKeySize 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.
Lesson 3: Encrypting and Decrypting Data
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. Out 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 the only implementation that is natively supported by the .NET Framework. 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 directly use passwords 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 user name and password, you could concatenate the user s own user name and password to derive the same key at both the encryptor and decryptor.
NOTE .NET 2.0
Rfc2898DeriveBytes is new to the .NET Framework. It functions almost exactly like PasswordDeriveBytes, but is preferred because it is standards-based. PasswordDeriveBytes also gives you the ability to specify the hashing algorithm, should you need that level of control.
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 randomly generated. Changing any of these values produces a different key, requiring you 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 is usually not possible to securely exchange
12
User and Data Security
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 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 and BlockSize properties. Note that both KeySize and BlockSize are 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. The following sample code generates a key and IV for the myAlg SymmetricAlgorithm object using a static password, which should be provided by the user in practice:
' VB ' In practice, the user would provide the password Dim password As String = "P@S5w0r]>" ' Create an algorithm object Dim myAlg As RijndaelManaged = New RijndaelManaged() ' Derive the key and use it to define the algorithm Dim salt As Byte() = System.Text.Encoding.ASCII.GetBytes("This is my sa1t") Dim key As Rfc2898DeriveBytes = New Rfc2898DeriveBytes(password, salt) myAlg.Key = key.GetBytes(myAlg.KeySize / 8) myAlg.IV = key.GetBytes(myAlg.BlockSize / 8)
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