Language: C

Source code:

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <time.h> // The flag looks like FLAG{md5} void encode(const char *filename, const char *text) { int number; FILE *fd = fopen(filename, "wb"); if (fd == NULL) { printf("[*] Cannot open '%s'\n", filename); exit(EXIT_FAILURE); } srand(time(NULL)); for (size_t i = 0; i < strlen(text); i++) { number = rand() & 0xff; fputc(text[i] ^ number, fd); fputc(number, fd); } fclose(fd); } int main(int argc, char *argv[]) { if (argc < 3) { printf("Usage: %s <FILENAME> <TEXT>\n", argv[0]); exit(EXIT_FAILURE); } printf("[*] Encoding...\n"); encode(argv[1], argv[2]); printf("[*] Result in '%s'\n", argv[1]); exit(EXIT_SUCCESS); }

Flag to decode: encoded_flag3

Compiled Linux binary: encoder3
Compiled Windows binary: encoder3.exe