# Intergalactic Cinema

## Description
```
I was partaking in a fun viewing of some show/movie with my alien bros but couldn't understand anything due to the alien voice dubbing present in the movie.

I've managed to download a script of what was said during the viewing but it seems to be encrypted, can you help me decrypt it?

Follow this link to get started: http://intergalactic.martiansonly.net
```

## Writeup

Looking at the website we can see some encrypted text and some `letter/number mapping`.
These clues should indicate that it is most likely a `substitution cipher`.
Copying the `ciphertext` into an [online-solver](https://planetcalc.com/8047/) returns some plaintext although it isn't able to map everything correctly.
```py
# frqdq wdq 87 s2mr fr38zs ws zr7sfs, bwbq.
# there are no such things as ghosts, babe.
```
Mapping some letters we can make our own script to decrypt the whole thing.
```py
known_chars = {
'f': 't',
'r': 'h',
'q': 'e',
'd': 'r',
'w': 'a',
'8': 'n',
'7': 'o',
's': 's',
'2': 'u',
'm': 'c',
'r': 'h',
'3': 'i',
'z': 'g',
'b': 'b',
}

# frqdq wdq 87 s2mr fr38zs ws zr7sfs, bwbq.
# there are no such things as ghosts, babe.

with open('ct.txt', 'r') as file:
out = ''
for line in file:
for char in line:
if char in known_chars.keys():
out += known_chars[char]
elif char == ' ':
out += ' '
elif char in ['!', ',', ':', '.', "'", '"', '#', '?', '-', '_', '{', '}']:
out += char
elif char == '\n':
out += '\n'
else:
out += '$'

print(out)
```

Executing the script, we can see certain words which are only missing single letters.
Applying this method we can get the correct `mapping` letter by letter.
```
### line $$$$ ###
in a strange galaxy.

### line $$$$ ###
$aybe right no$

### line $$$$ ###
she's settling in $or the long na$...
```

Mapping all correct letters reveals the flag.
The numbers can be mapped via the lines `### line $$$$ ###` (starts off with 1).
```py
known_chars = {
'f': 't',
'r': 'h',
'q': 'e',
'd': 'r',
'w': 'a',
'8': 'n',
'7': 'o',
's': 's',
'2': 'u',
'm': 'c',
'r': 'h',
'3': 'i',
'z': 'g',
'b': 'b',
'6': 'l',
'o': 'x',
'1': 'y',
'y': 'f',
'v': 'm',
'i': 'w',
'k': 't',
'9': 'l',
'a': 'd',
'4': 'p',
'c': '1',
'p': '2',
'g': '3',
'h': '4',
't': '5',
'0': '6',
'5': '7',
'n': '8',
'l': '9',
'x': '0',
'e': 'z',
'j': 'q',
}

# frqdq wdq 87 s2mr fr38zs ws zr7sfs, bwbq.
# there are no such things as ghosts, babe.

with open('ct.txt', 'r') as file:
for line in file:
line_out = ''
for char in line:
if char in known_chars.keys():
line_out += known_chars[char]
elif char == ' ':
line_out += ' '
elif char in ['!', ',', ':', '.', "'", '"', '#', '?', '-', '_', '{', '}']:
line_out += char
elif char == '\n':
line_out += '\n'
else:
line_out += '$'

if line_out.startswith('shctf{'):
print(line_out)
```

Executing the script reveals the flag which concludes this writeup.
```sh
$ python3 solver.py
shctf{d0_n0t_g0_g3ntle_into_that_g0od_n1ght}
```

Original writeup (https://github.com/Aryt3/writeups/tree/main/jeopardy_ctfs/2024/space_heroes_ctf_2024/intergalactic_cinema).