vb.net data matrix reader The Key Distribution Problem and Public-Key Cryptography in Software

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The Key Distribution Problem and Public-Key Cryptography
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The S/MIME (Secure/Multipurpose Internet Mail Extensions) standard specifies a way to encrypt e-mail. MIME is a widely adopted e-mail standard, and S/MIME is an extension that adds encryption. S/MIME solves the key distribution problem by using RSA digital envelopes. If your e-mail package is S/MIME-enabled, you can create a digital envelope. All you need to do is get your correspondent s public key and flip the switch to encrypt the message. If you send e-mail through Netscape Communicator, for example, you can use S/MIME. Here s how. First, launch the Netscape browser. Click the Security button and then click Messenger (along the left-hand column). You ll get a window that looks like the one in Figure 4-29. Click the option Encrypt Mail Messages, When It Is Possible. (The signing options are the topic of 5.) To encrypt a message, you need to select your correspondent s public key, which you ll find inside a certificate. If you don t already have the certificate, you can search for it in a directory (see Figure 4-30). To get to this menu, click Security Info. Under Certificates (along the left-hand column in the resulting window), click People. Then click Search Directory. After you select the public key, any e-mail you send to that individual will be encrypted using a digital envelope. If you use Microsoft Outlook 98, click Tools, then Options, and then the Security tab. You ll see a window that looks like the one in Figure 4-31. As with the Communicator program, there is an option to encrypt outgoing messages. Again, you ll need the other party s public key to do that. 6 talks about certificates and their directories. For now, you can see that applications today are using public key cryptography to solve the key distribution problem.
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Figure 4-29
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Netscape Communicator s menu for encrypting e-mail using S/MIME
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Figure 4-30 A Netscape Communicator menu for finding a public key to use when creating the digital envelope
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The Key Distribution Problem and Public-Key Cryptography
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Figure 4-31 The S/MIME menu in Microsoft Outlook 98
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CHAPTER
The Digital Signature
Public-key cryptography helps to solve the key distribution problem. It also addresses two other cryptography issues: authentication and nonrepudiation. Authentication allows someone in the electronic world to confirm data and identities, and nonrepudiation prevents people from going back on their electronic word. One way to implement these features is to use a digital signature. When you use the RSA algorithm, it means that anything encrypted with the public key can be decrypted only with the private key. What would happen if you encrypted plaintext with a private key Is that possible And if so, which key would you use to decrypt It turns out that RSA works from private to public as well as public to private. So you can encrypt data using the private key, and in that case, only the public key can be used to decrypt the data (see Figure 5-1). You may ask, What good is that After all, if you encrypt data with your private key, anyone can read it because your public key, which is publicly available, can be used to decrypt it. It s true that using RSA in this direction does not let you keep secrets, but it is a way to vouch for the contents of a message. If a public key properly decrypts data, then it must have been encrypted with the private key. In the crypto community, this technique is conventionally called a digital signature. If we didn t all agree to call it a digital signature, it wouldn t be, it would be just an interesting exercise in math and computer science. But the crypto community
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Figure 5-1
5
If you encrypt plaintext with an RSA private key, you can use the public key to decrypt it
called it such, the rest of the computer community (hardware and software vendors) have agreed to this nomenclature, and governments are starting to come on board. At the state and national level, laws are being passed that declare a digital signature as a legally binding way to sign documents. This means that anything you encrypt with your private key is a digital signature. So you shouldn t go around encrypting things with your private key unless you re willing to vouch for them.
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