Remote Coders — Missing Pieces

Overview

“Missing Pieces” by syyntax hides a flag behind a hex string XOR’d with a short key. Fix the incomplete C program, convert hex → bytes, XOR with the key, and print the result.

Provided Information

File > Download (4 KB)

SHA1 895ca32d397db50ee2fe3ad7021a63efc5dc7582

XOR primer

Convert the hex-encoded ciphertext to bytes, convert the ASCII key to bytes, then XOR: plain[i] = cipher[i] ^ key[i % len(key)]. Print as characters to reveal the flag.

C — fix and run to decode

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

const char *hex_string = "b52195a4a82bc5ade23e9c9c8725c79cb07d90f0ae";
const char *key = "d34df4c3";

int hex_char_to_int(char c) {
    if (c >= '0' && c <= '9') return c - '0';
    if (c >= 'a' && c <= 'f') return c - 'a' + 10;
    if (c >= 'A' && c <= 'F') return c - 'A' + 10;
    return -1;
}

void hex_string_to_bytes(const char *hex, unsigned char *bytes, size_t length) {
    for (size_t i = 0; i < length; ++i) {
        int hi = hex_char_to_int(hex[i * 2]);
        int lo = hex_char_to_int(hex[i * 2 + 1]);
        bytes[i] = (unsigned char)((hi << 4) | lo);
    }
}

void xor_bytes(unsigned char *data, size_t data_length, const unsigned char *key, size_t key_length) {
    for (size_t i = 0; i < data_length; ++i) {
        data[i] ^= key[i % key_length];
    }
}

int main(void) {
    size_t flag_length = strlen(hex_string) / 2;
    unsigned char flag[256] = {0};
    unsigned char key_bytes[4] = {0};

    hex_string_to_bytes(hex_string, flag, flag_length);
    hex_string_to_bytes(key, key_bytes, 4);

    xor_bytes(flag, flag_length, key_bytes, 4);

    printf("Resulting string: ");
    for (size_t i = 0; i < flag_length; ++i) {
        printf("%c", flag[i]);
    }
    printf("\\n");

    return 0;
}

Flag

flag{f1n1sh_Th3_c0d3}

MISSING PIECES