easyctf-2017/lost-seeds/encrypt.c
2017-02-15 03:49:56 -06:00

37 lines
658 B
C

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#define RAND_MAX ((1U << 31) - 1)
#define MAXLENGTH 80
int realrand() {
int n;
FILE *fp = fopen("/dev/urandom", "r");
fread(&n, 4, 1, fp);
fclose(fp);
return n % MAXLENGTH;
}
int seed;
int pseudorand() {
return seed = (seed * 19394489 + 132241) & 0xff;
}
int main() {
char input[MAXLENGTH];
FILE *fp = fopen("flag.in", "r");
fread(&input, 1, MAXLENGTH, fp);
fclose(fp);
fp = fopen("flag.out", "wb");
seed = realrand();
int r;
char c;
for (int i = 0, l = strlen(input); i < l; ++i) {
c = ((char) input[i] ^ (r = pseudorand())) & 0xff;
fwrite(&c, 1, 1, fp);
}
fclose(fp);
}