AlpacaHack Mirage writeup

#include <stdio.h>
#include <string.h>
#include <stdint.h>

#define N 19

uint8_t enc[N] = {
    0x31, 0x54, 0x6c, 0x2f, 0x04, 0x52, 0x22, 0x41, 0x3f, 0x59,
    0x27, 0x45, 0x67, 0x79, 0x1a, 0x4e, 0x78, 0x2d, 0x19
};

uint16_t step(uint16_t s) {
    uint16_t bit = ((s >> 0) ^ (s >> 2) ^ (s >> 3) ^ (s >> 5)) & 1;
    return (s >> 1) | (bit << 15);
}

int main(void) {
    setvbuf(stdout, NULL, _IONBF, 0);
    char buf[64];
    printf("enter the flag: ");
    if (!fgets(buf, sizeof(buf), stdin))
        return 1;
    buf[strcspn(buf, "\n")] = 0;
    if (strlen(buf) != N) {
        printf("wrong\n");
        return 1;
    }
    uint16_t state = 0xACE1;
    for (int i = 0; i < N; i++) {
        state = step(state);
        if (((uint8_t)buf[i] ^ (state & 0x7F)) != enc[i]) {
            printf("wrong\n");
            return 1;
        }
    }
    printf("correct!\n");
    return 0;
}

#include <stdio.h>
#include <string.h>
#include <stdint.h>

uint16_t step(uint16_t s) {
    uint16_t bit = ((s >> 0) ^ (s >> 2) ^ (s >> 3) ^ (s >> 5)) & 1;
    return (s >> 1) | (bit << 15);
}

int main(void){
　　uint8_t enc[19] = {0x31, 0x54, 0x6c, 0x2f, 0x04, 0x52, 0x22, 0x41, 0x3f, 0x59,
    0x27, 0x45, 0x67, 0x79, 0x1a, 0x4e, 0x78, 0x2d, 0x19};
　　uint16_t state = 0xACE1;

　　uint8_t flag[20] = {0};
　　for (int i = 0; i < 19; i++) {
        　state = step(state);
        　flag[i] = enc[i] ^ (state & 0x7F);
　　}
　　printf("flag: %s\n",flag);
　　return 0;
}