docs: public byte[][] keypair()
      {
          byte[][] pair = new byte[2][];
  
          byte[] publicKey = new byte[Size.PUBLICKEY.bytes()];
          byte[] secretKey = new byte[Size.SECRETKEY.bytes()];
  
          int rc = curve25519xsalsa20poly1305.crypto_box_keypair(publicKey, secretKey);
          assert (rc == 0);
  
          pair[0] = publicKey;
          pair[1] = secretKey;
  
          return pair;
      }