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Cryptography Engineering
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Before we begin the detailed discussion of key management in the next chapter, we need to discuss one more primitive function: the clock. At first glance, this is a decidedly un-cryptographic primitive, but because the current time is often used in cryptographic systems, we need a reliable clock.
There are several cryptographic uses for a clock. Key management functions are often linked to deadlines. The current time can provide both a unique value and a complete ordering of events. We will discuss each of these uses in more detail.
In many situations, we want to limit the validity period of a document. In the real world, we often see limited validity periods too. Checks, open airline tickets, vouchers, coupons, and even copyrights all have limited validity periods. The standard way to limit the validity period of a digital document is to include the expiration time in the document itself. But to check whether a document...
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