applied cryptography - protocols, algorithms, and source code in c

Alice tries to go secure she generates a key as

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Unformatted text preview: (A, B, C)) (4) Bob computes K4 in the same manner Alice did. Assuming all the names match and the check is correctly formed, he accepts it for payment. A really clever thing about this protocol is that the encryption key for each message depends on the previous message. Each message doubles as an authenticator for all previous messages. This means that someone can’t replay an old message; the receiver could never decrypt it. I am impressed with this idea and expect that it will see wider use once it becomes widely known. Another clever thing about this protocol is that it enforces correct implementation. If the application developer doesn’t implement this protocol correctly, it just won’t work. Both cards store records of every transaction. When the cards eventually go online to communicate with the bank—the merchant to deposit his money and the customer to get more money—the bank uploads these records for auditing purposes. Tamperproof hardware prevents either participant from messing with the data; Alice cannot change the value of her card. Extensive audit trails provide data to identify and prosecute fraudulent transactions. There are universal secrets in the cards—MAC keys in the customer cards, functions to convert customer names to K1 and K2 in the merchant cards—but these are assumed to be difficult to reverse-engineer. This scheme is not meant to be perfect, only more secure than either paper checks or traditional debit cards. The threat of fraud is not from rival militaries, but from opportunistic customers and merchants. UEPS protects against that kind of abuse. The message exchange is an excellent example of a robust protocol: Every message names both parties, includes unique information to ensure freshness, and depends explicitly on all the messages that came before it. 24.16 Clipper The Clipper chip (also known as the MYK-78T) is an NSA-designed, tamper-resistant VLSI chip designed for encrypting voice conversations; it is one of the two chips that implements the...
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