仿射破译输出错误

Question

Affine解密代码失败的问题是我得到了一个完全错误的输出。

public class AffineCipherDecryption {

public void Affine(int a, int b, JTextComponent Message, JTextComponent DisplayResult){
        StringBuilder builder = new StringBuilder();
        int len = Message.getText().length();
        BigInteger dda = BigInteger.valueOf(a);
        BigInteger dd = dda.modInverse(BigInteger.valueOf(26));
            int bb = dd.intValue();

        for (int i = 0; i < len; i++){

          char currentChar = Message.getText().charAt(i);

          char finalChar = (char) ((bb*(currentChar - 'A') - b) % 26 + 'A');


           builder.append(finalChar);

         }

        String result = builder.toString().toLowerCase();


        DisplayResult.setText(result);


}

}

我修改了它但输出却错了......