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: aeskw.go
    path of file: ./repos/plumber/vendor/github.com/dvsekhvalnov/jose2go
    
    the code of the file until where you have to start completion: package jose

import (
	"errors"
	"fmt"
	"github.com/dvsekhvalnov/jose2go/aes"
	"github.com/dvsekhvalnov/jose2go/arrays"
)

func init() {
	RegisterJwa(&AesKW{ keySizeBits: 128})
	RegisterJwa(&AesKW{ keySi
    
func (alg *AesKW) Unwrap(encryptedCek []byte, key interface{}, cekSizeBits int, header map[string]interface{}) (cek []byte, err error) {
	
	if kek,ok:=key.([]byte); ok {
		
		kekSizeBits := len(kek) << 3
		
		if kekSizeBits != alg.keySizeBits {
			return nil,errors.New(fmt.Sprintf("AesKW.Unwrap(): expected key of size %v bits, but was given %v bits.", alg.keySizeBits, kekSizeBits))
		}	
		
		return aes.KeyUnwrap(encryptedCek, kek)
	}
		
	return nil,errors.New("AesKW.Unwrap(): expected key to be '[]byte' array")		
}