You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: machine.go
    path of file: ./repos/sliver/vendor/tailscale.com/types/key
    
    the code of the file until where you have to start completion: // Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

package key

import (
	"bytes"
	"crypto/subtle"
	"encoding/hex"

	"go4.org/mem"
	"golang
    
func (k MachinePrecomputedSharedKey) Open(ciphertext []byte) (cleartext []byte, ok bool) {
	if k == (MachinePrecomputedSharedKey{}) {
		panic("can't open with zero keys")
	}
	if len(ciphertext) < 24 {
		return nil, false
	}
	var nonce [24]byte
	copy(nonce[:], ciphertext)
	return box.OpenAfterPrecomputation(nil, ciphertext[len(nonce):], &nonce, &k.k)
}