vb.net code 128 reader Michelle acts as a trusted third party, distributing keys between Pao-Chi and Gwen in Software

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Michelle acts as a trusted third party, distributing keys between Pao-Chi and Gwen
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KEK with Pao-Chi
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Michelle
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RNG or PRNG
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Gwen
KEK with Michelle
KEK with Michelle
Decrypt
Shared session Key
Decrypt
project, but it is more manageable than having a single companywide TTP. If two correspondents are in the same office, they can use the services of a shared TTP. If they are in separate offices, each one communicates with his or her own TTP. Then the two TTPs communicate with each other to bridge the gap (see Figure 4-3).
Problems With This Scheme
The first problem is that the TTP can read all the messages. The whole idea of encrypting messages is to limit their exposure to only the corre-
The Key Distribution Problem and Public-Key Cryptography
Figure 4-3 TTP Michelle (San Francisco) shares keys with TTP Alexander (New York), creating a hierarchy that serves Pao-Chi and Daniel in the two cities
KEK with Alexander Michelle RNG or PRNG
Encrypt
Pao-Chi and Daniel s session key KEK with Pao-Chi
Alexander KEK with Michelle Pao-Chi and Daniel s session key
Encrypt Alexander and Daniel s KEK
Pao-Chi Daniel
spondents. Now a third person has access. If the correspondents can live with that, this scheme will work. Otherwise, they d better look for another solution. The second problem is that when the TTP leaves the company it must hire a new TTP and start the process over from the ground up. Otherwise, the outgoing TTP can gain access to all sensitive materials. An alternative is to contract the job of TTP to an outside company. In this arrangement, the TTP is not an individual but a corporate entity. In this case, you must trust that the company has checks in place that prevent its employees from gaining access to the keys.
4
Public-Key Cryptography and the Digital Envelope
In the 1970s, researchers invented asymmetric-key cryptography, a new way to securely send keys. This scheme uses two different keys. Although they are related to each other they are partners they are significantly different. The relationship is mathematical, and what one key encrypts the other key decrypts. In symmetric crypto, the same key is used to encrypt and decrypt (hence the word symmetric the same on both sides); if you use any other key to decrypt, the result is gibberish. But with asymmetric crypto (see Figure 4-4), the key that s used to encrypt the data does not decrypt it; only its partner does (hence the word asymmetric, each side different). An analogy is the asymmetric lockers often found in airports, train stations, skating rinks, and many other public places. To securely store your belongings, you put them into the locker and lock it by inserting money. Just as your house key locks your front door, the money locks the locker in a sense, your money is the key. After you lock the door, you receive another key perhaps an actual key that looks like your house key or car key, or perhaps a piece of paper that contains a number. To reopen the locker, you use the key or enter the number on a key pad (sort of like using a temporary personal identification number or PIN). Suppose thieves want to steal your belongings. To open the locker, they need a key. The key you used to lock it was money. But if the thieves insert more money into the locker, it won t open. They can stuff money into it all day long, and it still won t open. The key that was used to lock the locker will not unlock it. Only the second, different key will unlock the door. Similarly, it s possible to create a cryptographic algorithm in which one key encrypts data and the other key decrypts it. Another term for this model (the term we use in this book) is public-key cryptography. Because both keys are needed to lock and unlock the data, one of them can be made public without jeopardizing security. This key is known as the public key. Its partner is called the private key. You encrypt data with the public key and decrypt it with the private key. Just as thieves can know what key was used to lock the asymmetric locker can even have access to that key and still not be able to open the door, an attacker can have access to a cryptographic public key and still not be able to decrypt the data. Only the private key can be used to decrypt it, and if the owner of
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