A sample int overflow crackme.

intOverflow.c

#include <stdio.h>
#include <stdlib.h>
#define KEYMAXLEN 64

void printKey(int c)
{
    char bits[4];
    char *buffer;
    int i = 0;
    while (c != -1) {
        bits[i++] = c & 0xFF;
        c >>= 8;
    }
    FILE* stream;
    stream = fopen("key.dat", "rb");
    if (!stream) {
        puts("not here!");
        exit(1);
    }
    buffer = calloc(KEYMAXLEN, sizeof(char));
    fread (buffer, sizeof(char), KEYMAXLEN, stream);
    while (*buffer) {
        if (i == 4) i = 0;
        printf("%c", (*buffer - 0xFA) ^ bits[i]);
        ++buffer;
        ++i;
    }
    puts("");
}

int main(int argc, char const *argv[])
{
    int a, b, c;
    puts("今天有 n 男 m 女 去露營，總共需要幾個睡袋？");
    printf("男生有幾人？ ");
    scanf("%d", &a);
    printf("女生有幾人？ ");
    scanf("%d", &b);
    c = a + b;
    if (a < 0 || b < 0) {
        puts("人數怎麼會有負數呢？");
        exit(0);
    }
    if (c >= 0) printf("有 %d 個男生 %d 個女生，總共需要 %d 個睡袋！\n", a, b, c);
    else puts("segmentation fault");
    if (c == 0x80000000) {
        printf("The key is ");
        printKey(c);
    }
    return 0;
}

keyGen.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    // intoverflowsoeasy
    FILE *stream;
    char *a;
    char bits[] = {0x00, 0x00, 0x00, 0x80};
    int i = 0;
    if (argc < 2) {
        puts("Usage: keygen <key>");
        return 1;
    }
    stream = fopen("key.dat", "wb");
    a = argv[1];
    while (*a) {
        fputc((*a++ ^ bits[i++]) + 0xFA, stream);
        if (i == 4) i = 0;
    }
    return 0;
}