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) { FILE *fd = fopen(filename, "wb"); if (fd == NULL) { printf("[*] Cannot open '%s'\n", filename); exit(EXIT_FAILURE); } int c = 0x7f; printf("[*] Initialize number: 0x%02x\n", c); for (size_t i = 0; i < strlen(text); i++) { fputc(c ^ text[i], fd); c ^= text[i]; } 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_flag

Compiled Linux binary: encoder
Compiled Windows binary: encoder.exe