Probability and Perfect Secrecy
In the previous chapter, we have described a number of historical cryptosystems. It turned out that they were all affine linear and therefore insecure. Are there cryptosystems that are provably secure? In 1949, Claude Shannon  was able to describe such systems. Unfortunately, those systems are not very efficient. Also, they are not secure against active attacks if no further cryptographic techniques are used. In this chapter, we present Shannon’s theory. At the same time, we will introduce a few notions and results of elementary probability theory.
KeywordsSample Space Elementary Event Active Attack Pseudorandom Number Generator Encryption Function
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