Tags: crypto
Rating:
# Description
Starfleet has received a transmission from Bynaus. However, the message apears to be blank. Is there some kind of hidden message here?
MD5(transmission.txt) = 736b9d6c408c3c75559c45083413c10a
# Files
- transmission.txt
## Solution
# Recon
- I started with a simple file analysis command `file transmission.txt` :
```
┌─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:48]
└─[$] <> file transmission.txt
transmission.txt: ASCII text
```
- Okay, let's run `cat transmission.txt` and see what this file contai... oh :
```
┌─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:50]
└─[$] <> cat transmission.txt
```
## Trial and Error
- I figured out this is some type of whitespace encoding, so I used the tool `stegsnow` and got nothing, literally :
```
┌─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:52]
└─[$] <> stegsnow transmission.txt
┌─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:52]
└─[$] <>
```
- After hexdumping the file with `xxd`, I saw something interesting :
```
┌─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:52]
└─[$] <> xxd transmission.txt
00000000: 2009 0909 2020 0909 0a20 0909 2009 2020 ... ... .. .
00000010: 200a 2009 0920 2020 0909 0a20 0909 0920 . .. ... ...
00000020: 0920 200a 2009 0920 2009 0920 0a20 0909 . . .. .. . ..
00000030: 0909 2009 090a 2009 0920 2020 2009 0a20 .. ... .. ..
00000040: 0920 0909 0909 090a 2009 0920 2020 0920 . ...... .. .
00000050: 0a20 0909 2009 0920 200a 2020 0909 2020 . .. .. . ..
00000060: 2009 0a20 0909 2009 0909 200a 2009 0920 .. .. ... . ..
00000070: 2009 2020 0a20 0920 0909 0909 090a 2009 . . . ...... .
00000080: 0920 0909 2009 0a20 2009 0920 0920 200a . .. .. .. . .
00000090: 2009 0920 0909 0920 0a20 0920 0909 0909 .. ... . . ....
000000a0: 090a 2009 0909 2009 2020 0a20 2009 0920 .. ... . . ..
000000b0: 2009 090a 2009 0920 2020 2009 0a20 0909 ... .. .. ..
000000c0: 2020 2009 090a 2009 0920 0920 2020 0a20 ... .. . .
000000d0: 0909 2009 2020 090a 2009 0920 0909 0920 .. . .. .. ...
000000e0: 0a20 0909 2020 0909 090a 2009 2009 0909 . .. .... . ...
000000f0: 0909 0a20 0909 2020 2020 090a 2009 0920 ... .. .. ..
00000100: 0909 0920 0a20 0920 0909 0909 090a 2020 ... . . ......
00000110: 0909 2009 2020 0a20 0909 2009 0909 200a .. . . .. ... .
00000120: 2009 0920 2009 2020 0a20 0909 0920 2009 .. . . ... .
00000130: 200a 2020 0909 2020 2020 0a20 0909 2009 . .. . .. .
00000140: 2020 090a 2009 0920 2009 2020 0a20 0920 .. .. . . .
00000150: 0909 0909 090a 2009 0920 0920 2020 0a20 ...... .. . .
00000160: 2009 0920 2020 200a 2009 0909 2009 0909 .. . ... ...
00000170: 0a20 0920 0909 0909 090a 2009 0909 2009 . . ...... ... .
00000180: 2020 0a20 0909 2009 0909 090a 2009 2009 . .. ..... . .
00000190: 0909 0909 0a20 0909 0920 2020 200a 2009 ..... ... . .
000001a0: 0920 2020 2009 0a20 2009 0920 2020 090a . .. .. ..
000001b0: 2009 0920 0909 0920 0a20 0909 0920 0920 .. ... . ... .
000001c0: 200a 2009 0909 0909 2009 0a . ..... ..
```
- It's not just whitespaces ! there are some values in there !
# Execution
- I went ahead and opened the file with python :
```python
f = open('transmission.txt', 'rb')
for i in f.read():
print(i)
```
- Here's some of what the script outputs :
```
─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:57]
└─[$] <> python3 solve.py | head
32
9
9
9
32
32
9
9
10
32
```
- Interesting, so we know we have mainly 32 and 9 as our values, as the challenge hints, this is binary encoding, where one byte represents the value of 1 and the other represents the value of 0. I wrote this python script to solve the challenge :
```python
f = open('transmission.txt', 'rb')
binary = ''
def decode_binary_string(s):
return ''.join(chr(int(s[i*8:i*8+8],2)) for i in range(len(s)//8))
for i in f.read():
if i == 32:
binary += '0'
elif i == 9:
binary += '1'
print(decode_binary_string(binary))
```
- After running the script, we get the flag :
```
┌─[not1cyyy@0x45] - [~/Desktop/space-heroes-CTF/2023/Bynary_Encoding_FINISHED] - [mar. avril 25, 20:59]
└─[$] <> python3 solve.py
shctf{a_bl1nd_m4n_t3aching_an_4ndr0id_h0w_to_pa1nt}
```
# Flag
shctf{a_bl1nd_m4n_t3aching_an_4ndr0id_h0w_to_pa1nt}
