code: func nonceForChunkSigncryption(headerHash headerHash, isFinal bool, i encryptionBlockNumber) Nonce {
  	var n Nonce
  	off := len(n) - 8
  	copyEqualSize(n[:off], headerHash[:off])
  	// Set LSB of last byte based on isFinal.
  	n[off-1] &^= 1
  	if isFinal {
  		n[off-1] |= 1
  	}
  	binary.BigEndian.PutUint64(n[off:], uint64(i))
  	return n
  }
docs: // Construct the nonce for the ith block of signcryption
  // payload.