How does a stream cipher work?
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A stream cipher works roughly like a one-time pad.
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Just as in one-time pad, it can be broken if (A) you have insight into what the pad looks like, or if (B) the pad is used multiple times.
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The difference is that the “pad” in a stream cipher is not random, but pseudorandom - called a “keystream”.
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The pad/keystream essentially comes from a PRNG, which is seeded by (secret key + nonce).
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So the PRNG seed has a constant secret half (the secret key), and a changing public half (the nonce).
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The constant secret half (the key) prevents the first kind of breakage (A: attacker having insight into the pad).
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The changing public half (the nonce) prevents the second kind of breakage (B: the pad being used multiple times).
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