We solved this problem mostly blackbox.

Running the binary we get no output, and if we input a string in flag format like `F#{asdf1234}`, we will get something like `H4c#hZSF0SWI{` with a long delay, where `H4c` is the start of this challenge's description `H4ck1ng Fl4m3s In th3 Sh3lL`. If we input the correct flag, we'll probably get the entire description string as output. So we can simply brutefore the flag byte by byte. Here is my original script:

```python
from pwn import *

context.log_level = 'debug'
chars = '_#G1VWk92{5toOnCQF6zXpifDh8SdYlev uq0RMajKsrHUx}IyTbgAm3L4BcNEZJ7w'
flag = 'F#{'
dest = 'H4ck1ng Fl4m3s In th3 Sh3lL'

def sim(a, b):
if len(a) * len(b) == 0:
return 0
i = 0
while a[i] == b[i]:
i+=1
return i

def test(ch):
s = process('crackme')
s.sendline(flag + ch)
ret = s.recv()
s.close()
return ret

for i in range(len(dest) ):
for c in chars:
if sim(test(c), dest) == i + 4:
flag += c
print flag
break
```
Runing the script, we found the program outputs with a long delay if we guessed wrong, and the program will output instantly if we guessed right. So we can just add a timeout when reading the output to accelerate the bruteforce:
`ret = s.recv(timeout=1)`
And we can get the entire flag in a few minutes:
`F#{M4k3_R3v3rse_gr3at_Ag41N}`

Full script (only timeout added):
```python
from pwn import *

context.log_level = 'debug'
chars = '_#G1VWk92{5toOnCQF6zXpifDh8SdYlev uq0RMajKsrHUx}IyTbgAm3L4BcNEZJ7w'
flag = 'F#{'
dest = 'H4ck1ng Fl4m3s In th3 Sh3lL'

def sim(a, b):
if len(a) * len(b) == 0:
return 0
i = 0
while a[i] == b[i]:
i+=1
return i

def test(ch):
s = process('crackme')
s.sendline(flag + ch)
ret = s.recv(timeout=1)
s.close()
return ret

for i in range(len(dest) ):
for c in chars:
if sim(test(c), dest) == i + 4:
flag += c
print flag
break
```