### 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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