public String encrypt(String word){
	if(word == null||word.length()==0) 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 a = (wordLower.charAt(i)-97)*this.keyA;
	    int b = a+this.keyB;
	    int k = (b%26)+97;
	    result += Character.toString((char)k);
	}
	return result;
    }