vb.net data matrix reader The Three Important Digest Algorithms in Software

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The Three Important Digest Algorithms
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There are many digest algorithms, but three have dominated the market: MD2, MD5, and SHA-1.
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Ron Rivest created a digest algorithm and named it MD. Then he thought he could do better and so developed the next generation, MD2. Because MD2 produces a 128-bit (16-byte) digest, it has 2128 possible digest values. MD2 has been widely used, but over the years, analysts found flaws with it. Eventually, a few collisions were discovered. Nobody was able to find collisions on demand with any arbitrary message, but certain classes of messages produced collisions. Hence, MD2 isn t used very much anymore except on old certificates created before MD2 lost favor ( 6 describes certificates). Most of those old certificates have probably expired or will expire soon. No good cryptographer would recommend using MD2 in new applications.
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Rivest wanted a faster digest, and when MD2 began to show weaknesses, he also wanted one that was stronger. He started creating new digests. MD3 was a bust, and when he showed MD4 to the world it was quickly shown to be weak. (Despite that weakness, at least one application used it. See Crypto Blunders on the accompanying CD for that story.) MD5 was more successful. MD5, a lot faster and much stronger than MD2, became the dominant algorithm and is still in common use. Like MD2, MD5 is a 16-byte digest. Over the years, research has led to potential weaknesses. MD5 isn t broken, and no one has found collisions; rather, some of the internals of the algorithm are vulnerable. If a component or two were missing from the algorithm, it would be broken. But because those components are there, the algorithm survives. Some people say that it doesn t matter that the algorithm would be weak if certain pieces were missing; the pieces are there, so it s not weak. Others say that you don t break an algorithm all at once; you break it piece by piece. Now that there are only a few pieces (maybe one or two) preventing a total collapse, they argue, it would be better to move on to another algorithm.
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The Digital Signature
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The SHA-1 algorithm looks a lot like MD5 (Ron Rivest played a role in the design of SHA-1). SHA-1 contains stronger internals than MD5, and it produces a longer digest (160 bits compared with 128 bits). Size alone makes it stronger. SHA-1 has survived cryptanalysis and comes highly recommended by the crypto community. In development are SHA-1 variants that produce 192-bit and 256-bit digests.
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If you re looking for something to produce a representative of a larger amount of data, it s easy to see that a message digest does that job fairly well. First, the output of a digest algorithm is usually smaller than the data itself, and no matter how big the data gets, the digest as a representative will always be the same size. If someone tries to surreptitiously change the original message, the new, fake message will not produce the same digest. If the digest produced by the algorithm does not represent the data, you know that something went wrong (see Figure 5-7). Maybe the data has been altered, maybe the digest is wrong. You might not know what exactly happened, but you do know something happened. Here s how an application can check a digest. Pao-Chi is sending Daniel some data, such as an e-mail or a contract; for this example, it s the message about selling four units to Satomi. Before Pao-Chi sends the message, he digests it. Now he sends the data and the digest. When Daniel gets the data, he also digests it. If his digest matches Pao-Chi s, he knows the data has not been changed in transit. If Satomi had intercepted and altered the message, the digest that Daniel produced would not have matched the digest Pao-Chi produced. Daniel would know that something happened and would not trust the data. Your immediate response might be, If Satomi could alter the data, she could alter the digest. That s true, but there are two ways to prevent that. One is to use a digital signature, a topic we ll return to shortly. For now, let s look at the second way: a keyed digest. The most common keyed digest is called HMAC.
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