My question is in Daniel J. Bernstein's ChaCha20,
only nonce and counter changes each block.
With nonce and counter, they are totally 128 bits,
however, it produces 512 bits keystream blocks.
If we use the concept of a random number generater,
the entropy is much smaller than output.
Is it secure? Or how to explain its security.
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This question might be better places in [crypto.stackexchange](https://crypto.stackexchange.com). – rossum May 13 '20 at 17:01