vb.net barcode scanner webcam Symmetric Algorithms: Block Versus Stream Ciphers in Software

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key value that is the same size from one use to the next, but your key might vary from 64 bits to 128 to 192 or even 256 bits. For that reason, you build a key table (which is bigger than the biggest possible key size) from the key. It s easier to create a constant-sized key table at the beginning of your encryption session than to do it repeatedly while encrypting data. Another reason to use a key table is to prevent attacks on the algorithm. Recall that there are two ways to break security: a brute-force attack and attacks on an algorithm s weaknesses. If you use a big, pseudo-random key table, it s easier to do serious scrambling. With good scrambling, the ciphertext looks nothing like the plaintext. If the algorithm cannot do a good job of creating gibberish unless it has a good key, that is be an algorithmic weakness. A good algorithm will simply expand the key into a bigger value and make sure that no matter what key it s given, the key table is random. An attacker could try a brute-force attack on the key table, but that would be more time-consuming than an attack on the key. The user should give the algorithm a good key. But even with a bad key, it is possible to create a good key table. Just as a PRNG produces good numbers no matter what the seed is, a good encryption algorithm produces a good key table no matter what the key is. With a good key table, the algorithm produces a good scramble, the resulting ciphertext is not at all close to the plaintext, and the attacker cannot exploit an algorithm s weakness.
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Symmetric Algorithms: Block Versus Stream Ciphers
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If you re using symmetric-key cryptography, how do you choose a good algorithm There are two types of symmetric-key algorithms: block and stream ciphers. What are they, and which is better
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A block cipher operates on blocks of data. When you give the algorithm a chunk of data to encrypt or decrypt, it breaks the plaintext into blocks and operates on each block independently (see Figure 2-10). Usually, blocks are 8 or 16 bytes long.
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Figure 2-10 A block cipher grabs each block of the input data (usually 8 or 16 bytes) and uses the key table to produce a unique block of output, continuing until all the blocks are encrypted
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Suppose that your plaintext is 227 bytes long and the cipher you re using operates on 16-byte blocks. The algorithm grabs the first 16 bytes of data, encrypts them using the key table, and produces 16 bytes of ciphertext. Then it starts over, encrypting the next 16 bytes of plaintext. No matter which block it is working with, the cipher encrypts it by starting over from scratch. The key table does not change from block to block. After encrypting 14 blocks (224 bytes), the algorithm is left with 3 more bytes. But your block cipher cannot operate on 3 bytes; it needs 16 bytes. To encrypt the last 3 bytes, you must pad the data: add extra bytes to an incomplete block to make it complete. Whoever decrypts the ciphertext must be able to recognize (and ignore) the padding. The most popular padding scheme determines the number of bytes to be padded and repeats that value in the final bytes in the data. In our example, the padding scheme must add 13 bytes to the plaintext so that it has a full block. So it repeats the byte 13 in each of the final 13 otherwise empty spaces. During decryption, you look at the last byte of decrypted
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