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/gitkube/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 that can be found in the LICENSE file.

/*
Package secretbox encrypts and authenticates small messages.

Secretbox uses XSalsa20 and Poly1305 to encrypt and authenticate messages with
secret-key cryptography. The length of messages is not hidden.

It is the caller's responsibility to ensure the uniqueness of noncesâ€”for
example, by using nonce 1 for the first message, nonce 2 for the second
message, etc. Nonces are long enough that randomly generated nonces have
negligible risk of collision.

Messages should be small because:

1. The whole message needs to be held in memory to be processed.

2. Using large messages pressures implementations on small machines to decrypt
and process plaintext before authenti
    
func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte {
	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[:])

	ret, out := sliceForAppend(out, len(message)+poly1305.TagSize)
	if subtle.AnyOverlap(out, message) {
		panic("nacl: invalid buffer overlap")
	}

	// We XOR up to 32 bytes of message with the keystream generated from
	// the first block.
	firstMessageBlock := message
	if len(firstMessageBlock) > 32 {
		firstMessageBlock = firstMessageBlock[:32]
	}

	tagOut := out
	out = out[poly1305.TagSize:]
	for i, x := range firstMessageBlock {
		out[i] = firstBlock[32+i] ^ x
	}
	message = message[len(firstMessageBlock):]
	ciphertext := out
	out = out[len(firstMessageBlock):]

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

	var tag [poly1305.TagSize]byte
	poly1305.Sum(&tag, ciphertext, &poly1305Key)
	copy(tagOut, tag[:])

	return ret
}