title: Cryptography： Question about streamcipher
body: <p>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,<br>
          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.</p>