程序代码：

#include <stdio.h>

int main(void)
{
    FILE *      fp;
    int         ch, i = 0, j;
    static char letters[10000];
    char        pwd[4] = {0};
   

    if((fp = fopen("cipher1.txt", "r")) == NULL) {
        perror(NULL);
        return 0;
    }
   

    do {
        fscanf(fp, "%d", &ch);
        letters[i++] = (char)ch;
        ch = fgetc(fp);
    } while(EOF != ch);

   

    for(pwd[0] = 'a'; pwd[0] <= 'z'; pwd[0]++) {
        for(pwd[1] = 'a'; pwd[1] <= 'z'; pwd[1]++) {
            for(pwd[2] = 'a'; pwd[2] <= 'z'; pwd[2]++) {
                for(j = 0; j < i; j += 3) {
                    letters[j + 0] ^= pwd[0];
                    letters[j + 1] ^= pwd[1];
                    letters[j + 2] ^= pwd[2];
                }
                printf("Password: %s\n", pwd);
                for(j = 0; j < i; j++)
                    printf("%c", letters[j]);
                printf("\n\n");
                for(j = 0; j < i; j += 3) {
                    letters[j + 0] ^= pwd[0];
                    letters[j + 1] ^= pwd[1];
                    letters[j + 2] ^= pwd[2];
                }
            }
        }
    }
                   

    fclose(fp);
    return 0;
}