Actual sequence cipher


Using PRNG to construct cipher stream

Many pnrgs have good statistical properties, which is necessary for strong sequence ciphers. If we make a statistical test on the key stream sequence, the behavior of the output result should be very similar to the bit sequence obtained by flipping a coin. Therefore, it is natural to assume that PRNG can be used to generate a key stream. However, these are not enough for the sequence key, because the opponent Oscar is also very smart.

Using cspring to construct key sequence

The unpredictability of key stream means that given the key sequences_1, s_2,…s_nIt is impossible to calculate the first n output bits ofs_{n+1}, s_{n+2}, …. Unfortunately, a considerable number of pseudo-random number generators used outside cryptography are not cryptography secure. Therefore, in practice, we need to use a specially designed pseudo-random number generator to generate the key sequence cipher.

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