vb.net data matrix reader The Digital Signature in Software

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The Digital Signature
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Figure 5-10 The RSA signature
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it to the bottom of a different message, something that makes it much more difficult to forge a signature. Think of it this way. Two people (a sender and a receiver) each have a copy of a message. Are they really copies or was the receiver s copy altered in transit To find out, they digest the two messages and compare them. If the digests are the same, both parties know that the two versions match. If the digests don t match, something went wrong. How do you know that the sender s digest was not altered You know that because it
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was encrypted with the sender s private key. How do you know that it was encrypted with the sender s private key You know it because the public key decrypts it. In addition, you can make a couple of other checks. In the real world, there will almost certainly be some digest algorithm identifier bytes (discussed in the next paragraph) and some pad bytes in addition to the digest. A signer will encrypt a block of data that is the padding, the digest algorithm identifier, and the digest. That encrypted value is the signature. Figure 5-11 shows an example. Using the appropriate public key, that signature decrypts to the padded value. The verifier checks not only for the digest but also the pad bytes and the SHA-1 algorithm identifier. (Technically, the program the verifier runs will make these checks.) Having three checks makes it harder to spoof. The algorithm identifier bytes prevent an attacker from substituting an alternative digest algorithm. Suppose that Satomi looks at Pao-Chi s message and its correct digest. She then finds a second message and digests it using a different algorithm. Further suppose that this second algorithm on the second message produces the same digest as the first algorithm on the first message. If the signature were the encryption of the digest only, that one signature would look as if it also came from the second algorithm. But if you tie a signature to a digest and the algorithm, you thwart such an attack. On the one hand, it doesn t seem likely that someone would ever be able to generate the same digest from a different algorithm. On the other hand, might MD2 be broken completely someday It doesn t cost anything to make the second check, so you might as well use it.
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Trying to Cheat
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Two people-Satomi and Pao-Chi-might try to cheat. Here s how they can try. First, suppose that Satomi intercepts the message and replaces 4 with 5. She figures she ll pay Pao-Chi for four units but Daniel will send her five, and she ll get an extra press for free. In this scheme, when Daniel gets the e-mail, he digests it and gets the following value. (Using the same algorithm Pao-Chi used-SHA-1-that information is part of the e-mail although not part of the message digested.)
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The Digital Signature
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Figure 5-11 A digested message and the RSA signature. The private key used for this example is listed in 4
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Daniel must find out whether that value is the same one Pao-Chi got when he digested the message he sent. To find out, Daniel uses Pao-Chi s public key to decrypt the signature. After decryption, he gets a chunk of data. Does this data have the correct padding He sees that the padding is correct, so he just throws that away. The next bytes are the identifying marks indicating that the algorithm is SHA-1; that s correct. Finally, he has the digest.
46 73 a5 85 89 ba 86 58 44 ac 5b e8 48 7a cd 12 63 f8 c1 5a
Daniel compares the digest value in the decrypted signature to his digest value (the value he just computed from the purported message) and sees that they are different. Something s not right. What went wrong Daniel doesn t know exactly what caused the discrepancy, but he knows that the message he received is not the same message Pao-Chi sent. Because Daniel doesn t trust the message, he ignores it, asking Pao-Chi to try again. Meanwhile, Daniel doesn t send Satomi anything and she doesn t get her extra unit. Now let s look at Pao-Chi s attempt at cheating. Suppose he made a mistake and quoted Satomi a price for two units. He got paid for two but told Daniel to ship four. He doesn t want to take the heat for the error, so he claims he wrote 2 instead of 4 in his e-mail. He figures he can shift the blame to Daniel or maybe just technology some gremlin on the Internet that garbled the message. Daniel points out that the signature attached to his e-mail matches the message with the number of presses to ship at four. Because that s PaoChi s signature and because each signature is unique to a message and private key, Daniel claims that Pao-Chi vouched for the information and can t back out now. To counter this, Pao-Chi could claim that the signature was forged. To forge a signature would mean that someone was able to create a blob of data, through other means, that was the same as a signature. This would mean that some unknown forger had broken the RSA algorithm. That is highly unlikely (see 4). No, Pao-Chi signed the message, and he can t claim otherwise. Or Pao-Chi could try another approach, claiming that someone stole his private key. Maybe it was protected on his hard drive using PBE, and someone cracked his password. Maybe it was stored on a smart card or other token, and someone broke that device or was able to log on as PaoChi (possibly by breaking a password). If that really is the case, Pao-Chi
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