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: chacha20poly1305_generic.go
    path of file: ./repos/go-starter-kit/vendor/golang.org/x/crypto/chacha20poly1305
    
    the code of the file until where you have to start completion: // Copyright 2016 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 chacha20poly1305

import (
	"encoding/binary"

	"golang.org/x/crypto/chacha20poly1305/internal/chacha20"
	"golang.org/x/crypto/poly1305"
)

func roundTo16(n int) int {
	return 16 * ((n + 15) / 16)
}

func (c *chacha20poly1305) sealGeneric(dst, nonce, plaintext, additionalData []byte) []by
    
func (c *chacha20poly1305) openGeneric(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
	var tag [poly1305.TagSize]byte
	copy(tag[:], ciphertext[len(ciphertext)-16:])
	ciphertext = ciphertext[:len(ciphertext)-16]

	var counter [16]byte
	copy(counter[4:], nonce)

	var polyKey [32]byte
	chacha20.XORKeyStream(polyKey[:], polyKey[:], &counter, &c.key)

	polyInput := make([]byte, roundTo16(len(additionalData))+roundTo16(len(ciphertext))+8+8)
	copy(polyInput, additionalData)
	copy(polyInput[roundTo16(len(additionalData)):], ciphertext)
	binary.LittleEndian.PutUint64(polyInput[len(polyInput)-16:], uint64(len(additionalData)))
	binary.LittleEndian.PutUint64(polyInput[len(polyInput)-8:], uint64(len(ciphertext)))

	ret, out := sliceForAppend(dst, len(ciphertext))
	if !poly1305.Verify(&tag, polyInput, &polyKey) {
		for i := range out {
			out[i] = 0
		}
		return nil, errOpen
	}

	counter[0] = 1
	chacha20.XORKeyStream(out, ciphertext, &counter, &c.key)
	return ret, nil
}