# Fibonacci
**Category**: Forensics \**Points**: 200
Opening this file in a hex editor:```$ xxd Fibonacci.7z | less00000000: 4242 4242 3742 7abc 42af 271c 0042 04c9  BBBB7Bz.B.'..B..00000010: 336d e569 0542 0000 0000 0000 5a00 0000  3m.i.B......Z...00000020: 0000 4200 0077 9ed7 14e0 0be2 0561 5d00  ..B..w.......a].00000030: 2168 b490 09c2 6442 c064 0461 7378 6c59  !h....dB.d.asxlY00000040: 041b 7ec3 e075 08cf 3b81 a186 1f0c 2557  ..~..u..;.....%W00000050: fded 72e7 04b6 20eb a242 dcc7 e2f1 ee76  ..r... ..B.....v00000060: 407f 6fa8 09dc e8db 19d4 ea35 0743 14a5  @.o........5.C..```
We can almost see the `7zip` header, but it seems to be crowded with `B`characters.
Looking closely, we see that there are `B` characters at these positions:```0 1 1 2 3 5 8 13 21 ...```
This is the fibonacci sequence, so let's just remove these characters and seewhat happens.
```pythondef gen_fib(n):    ans = [0, 1]    while ans[-1] < n:        ans.append(ans[-1] + ans[-2])    return ans
i = 0with open('out.7z', 'wb') as fout:    with open('Fibonacci.7z', 'rb') as fin:        fibs = gen_fib(1000)        fibs = set(fibs)
        while True:            b = fin.read(1)            if b == b'':                break
            if i not in fibs:                fout.write(b)
            i += 1```
Then we can just unzip:```$ 7z e out.7z$ cat Fibonacci | grep AFFCTF"AFFCTF{Hitchhiker}," said Deep Thought, with infinite majesty and calm.”```