加密解密时可以用到，只支持小写字母的转换。

 private string dmix(string str1)
    {
        int max_count = 20;
        char ca[] = str1.tochararray();
        int count = 0;
        for(int i = 0; i < ca.length; i++)
        {
            int a = ca[i] - count;
            ca[i] = (char)a;
            if(ca[i] < 'a')
                ca[i] += '/032';
            count = ++count % max_count;
        }

        stringbuffer sb = new stringbuffer();
        sb.append(ca);
        return sb.tostring();
    }
    public string mix(string str1)
    {
     int max_count = 20;
        char ca[] = str1.tochararray();
        int count = 0;
        for(int i = 0; i < ca.length; i++)
        {
         int a=0;
            a = ca[i] + count;
            ca[i] = (char)a;
            if(ca[i] > 'z')
                ca[i] -= '/032';
            count = ++count % max_count;
        }

        stringbuffer sb = new stringbuffer();
        sb.append(ca);
        return sb.tostring();
    }