December 10th

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

int main() {
  const char * flag = "712249146f241d31651a504a1a7372384d173f7f790c2b115f47";
  char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";
  char *a;
  char an[]="";
  int len,alen,t, fl, lastfl,orig;

  len=(int)(strlen(flag)/2);
  alen=strlen(alphabet);

  printf("reversing input..\n");
  for(int i=0; i<len; i++){
    strncpy(an,flag+i*2,2);
    t=strtol(an,&a,16);

    for(int j=0; j<alen; j++){
      fl=alphabet[j];
        orig=fl;
	    if (i > 0) fl ^= lastfl;
		fl ^= fl >> 4;
		fl ^= fl >> 3;
		fl ^= fl >> 2;
		fl ^= fl >> 1;

        if (t == fl){
	    printf("%c",orig);
            break;
	}
    }
    lastfl=fl;
  }

  printf("\ndone.\n");
  return 0;
}