public String decrypt(String word){
	if(word == null) throw new IllegalArgumentException();
	String result = "";
	String wordLower = word.toLowerCase();
	for(int i=0; i<wordLower.length(); i++){
	    if(wordLower.charAt(i)<97 || wordLower.charAt(i)>122)
		throw new IllegalArgumentException();
	    int k = (wordLower.charAt(i)-97)-this.cipherKey;
	    if(k<0) k+=26;
	    k += 97;
	    result += Character.toString((char)k);
	}
	return result;
    }