Cryptography in VB.NET

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Cryptography
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he .NET Micro Framework provides two encryption algorithms: the symmetrical encryption algorithm XTEA (eXtended Tiny Encryption Algorithm), which uses the same key for encoding and decoding, and the common asymmetrical method RSA, which operates with public and private keys. Further, you can verify signed data with the .NET Micro Framework. Unfortunately, however, there are no methods to sign data with a .NET Micro Framework application. Encryption is necessary whenever data is to be exchanged with other devices such as PCs, PDAs, smartphones, or other .NET Micro Framework devices over a network, WLAN, Bluetooth, ZigBee, or the serial interface. Data signing is often used for license files or for validating the sender of a message: only users with the private key can create and sign a license file or message. But any user with the public key can validate data against it. Sender validation is used to check if the data comes from an expected sender. In the previous chapters, you learned how to communicate over a network and how to communicate wirelessly with other devices. This chapter explores cryptography and demonstrates how you can use symmetric and asymmetric encryption and authentication with the .NET Micro Framework to keep those communications secure. It will also explain how to communicate securely with other devices like PCs, PDAs, or smartphones that run the full .NET or .NET Compact Framework.
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XTEA: A Symmetric Algorithm
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The XTEA (eXtended Tiny Encryption Algorithm) was developed in 1997 by David Wheeler and Roger Needham of the Cambridge Computer Laboratory. Like the name suggests, the XTEA, which is not patented, descends from the TEA and fixes some weak points of the TEA. XTEA is a block-based encryption method with a block size of 8 bytes and a key length of 16 bytes (128 bits). The algorithm uses a symmetrical key, which means data is encrypted and decrypted with the same key. This method is simple to implement and is relatively well performing. The implementation of the algorithm in the .NET Micro Framework uses cipher block chaining (CBC) with cipher byte stealing (CBS). With the CBC, blocks are encrypted successively, and for each block an XOR operation is performed (that is, it s XORed) before encryption with the previous, already encrypted block. The first block is XORed with an initialization vector. Decoding is done the same way, in reverse order. If the length of the data to encrypt is not exactly divisible by the block length (8 bytes), then with the CBS, the last incomplete block is filled
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CHAPTER 8 CRY PTOGRAPHY
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up with the bytes of the preceding complete block, so that it possesses a block that s exactly 8 bytes in length. After the incomplete block is filled, the two last blocks are exchanged, and the last block is trimmed so that the message again has its original length. The length of the cipher data is thus identical to the original data. Known text attacks can be prevented by CBC and CBS.
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Using XTEA
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XTEA is absent from the full .NET Framework but is represented in the .NET Micro Framework by the Key_TinyEncryptionAlgorithm class in the Microsoft.SPOT.Cryptography namespace in the Microsoft.SPOT.Native.dll assembly. The following code snippet shows encryption and deciphering with XTEA: byte[] key = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 }; byte[] iv = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 }; Key_TinyEncryptionAlgorithm xtea = new Key_TinyEncryptionAlgorithm(key); string plainText = "Hello World!"; //original message, min length is 8 bytes byte[] plainBytes = System.Text.Encoding.UTF8.GetBytes(plainText); //Encryption byte[] cipherBytes = xtea.Encrypt(plainBytes, 0, plainBytes.Length, iv); //Decryption byte[] restoredBytes = xtea.Decrypt(cipherBytes, 0, cipherBytes.Length, iv); The key is passed to the constructor when creating an instance of the algorithm class. The key must not be null. If the used key is shorter than 16 bytes, it is filled up at the end with zerobytes. If it is longer than 16 bytes, only the first 16 bytes are used. The encryption is done with the Encrypt method and the decryption with Decrypt. You should consider that the minimum length of the data to encrypt or decipher must equal the length of one block, that is, 8 bytes. With less than 8 bytes, no exception is thrown, and the data will not be extended with zero-bytes, but an array with 8 zero-bytes will be returned. For both encryption and deciphering, an initialization vector is specified, and you naturally need to use the same initialization vector with both operations. 8 bytes, the length of one block, are thus used by the initialization vector for the XOR operation of the first block. If either zero or less than 8 bytes are passed, then either 8 zero-bytes are used or the block is filled up at the end with zero-bytes. With more than 8 bytes, only the first 8 bytes are important. A zero-byte does not change the data in the XOR operation at all. The Key_TinyEncryptionAlgorithm class possesses the following method: public String GetActivationString (ushort region, ushort model) with which passwords of 16 characters (digits and upper-case characters) can be created in a simple manner. If you convert these with Encoding.UTF8.GetBytes into a byte array, you can use that array as XTEA key. The characters of the passwords are generated on the basis of regional code and a hardware or system model ID.
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