rc4random_buf</b>() does not exist on your system,
  see <a class="Xr" title="Xr"                          href="../intro.html">intro(3monocypher)</a> for
  advice about how to generate cryptographically secure random bytes.
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Generate a pair of shared keys with your secret key and their public key. (This
  can help nonce management for full duplex communications.)
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const uint8_t their_pk     [32]; /* T