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: secretbox.go
    path of file: ./repos/teller/vendor/golang.org/x/crypto/nacl/secretbox
    
    the code of the file until where you have to start completion: // Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license th
    
func Open(out, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool) {
	if len(box) < Overhead {
		return nil, false
	}

	var subKey [32]byte
	var counter [16]byte
	setup(&subKey, &counter, nonce, key)

	// The Poly1305 key is generated by encrypting 32 bytes of zeros. Since
	// Salsa20 works with 64-byte blocks, we also generate 32 bytes of
	// keystream as a side effect.
	var firstBlock [64]byte
	salsa.XORKeyStream(firstBlock[:], firstBlock[:], &counter, &subKey)

	var poly1305Key [32]byte
	copy(poly1305Key[:], firstBlock[:])
	var tag [poly1305.TagSize]byte
	copy(tag[:], box)

	if !poly1305.Verify(&tag, box[poly1305.TagSize:], &poly1305Key) {
		return nil, false
	}

	ret, out := sliceForAppend(out, len(box)-Overhead)
	if alias.AnyOverlap(out, box) {
		panic("nacl: invalid buffer overlap")
	}

	// We XOR up to 32 bytes of box with the keystream generated from
	// the first block.
	box = box[Overhead:]
	firstMessageBlock := box
	if len(firstMessageBlock) > 32 {
		firstMessageBlock = firstMessageBlock[:32]
	}
	for i, x := range firstMessageBlock {
		out[i] = firstBlock[32+i] ^ x
	}

	box = box[len(firstMessageBlock):]
	out = out[len(firstMessageBlock):]

	// Now decrypt the rest.
	counter[8] = 1
	salsa.XORKeyStream(out, box, &counter, &subKey)

	return ret, true
}