怎么改成随机自定义五个字母然后加密

怎么改成自己随机五个字母加密的呢，要怎么定义。希望有人能够回答一下谢谢

#include <stdio.h>
#include <time.h>
int main()
{
    srand((unsigned int)time(NULL));
    char str[6] = {0};
    int i, j, t;
    for (i = 0; i < 5; i++)
    {
        t = rand() % 2; //用来随机切换大小写
        str[i] = rand() % 26 + ((t) ? 'a' : 'A'); //产生随机字符
    }
    printf("%s\n", str);
    t = rand() % 10+1;//加密值
    for (i = 0; i < 5; i++)
        str[i] += t;

    printf("+%d\n%s\n", t,str);
}