密码溢出

我正在加密如下,但不知道如果它们移出范围,如何阻止大写字母进入其他符号,同样小写字母超出小写字母的范围.如何让它们围成一圈并停止溢出？谢谢

int size = strlen(plain_text);
int arrayelement = 0;
for(arrayelement = 0; arrayelement < size; arrayelement++)
{
    if (islower(plain_text[arrayelement]))
    {
        ciphered_text[arrayelement] = (int)(plain_text[arrayelement] + shiftkey);
    }
    else if (isupper(plain_text[arrayelement]))
    {
        ciphered_text[arrayelement] = (int)(plain_text[arrayelement] + shiftkey);
    }
}

ciphered_text[size] = '\0';
printf("%s",ciphered_text);

int tmp_ciphered = (my_char + shift) % 0xff;
char ciphered = (char)(tmp_ciphered);