public String decrypt(String word){
	String result = "";
	String wordLower = word.toLowerCase();
	if(MathUtil.coPrime(this.keyA, 26)==false)
	    throw new IllegalArgumentException();
	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.keyB;
	    if(a<0)a+=26;
	    int b = a*MathUtil.modInverse(this.keyA);
	    int k = (b%26)+97;
	    result += Character.toString((char)k);
	}
	return result;
    }