code: func nonceForChunkSecretBox(i encryptionBlockNumber) Nonce {
  	var n Nonce
  	copyEqualSizeStr(n[0:16], "saltpack_ploadsb")
  	binary.BigEndian.PutUint64(n[16:], uint64(i))
  	return n
  }
docs: // Construct the nonce for the ith block of encryption payload.