# Use Alphabet!
- Category: Crypto- Rating: Hard
In this challenge we are given a png that was supposedly sent by a German friend
![ADFDVX](task.png)
This is clearly an [ADFGVX cipher](https://en.wikipedia.org/wiki/ADFGVX_cipher), as those are the only letters that show up.The encryption process is in two stages- First, a 6x6 polybius square is used as a simple substitution cipher![Imgur](https://i.imgur.com/b83VO1J.png)- Second, the resulting AGFGVX pairs are put into columns and scrambled using a keyword![Imgur](https://i.imgur.com/MytOhMj.png)
Now, if take a look again at the `task.png`, we can eliminate it being related to the polybius square because it's only 6x5. So It's probably meant to be the table from the second stage. i.e. The keyword is 6 letters.
Another hint is the name of the challenge `Use Alphabet!`. This obviously refers to the polybius square since it's not the right length to be the keyword.
Let's try it.
# Solution 
This is our polybius square for the subtitution:
|   | A | D | F | G | V | X ||---|---|---|---|---|---|---|| A | A | B | C | D | E | F || D | G | H | I | J | K | L || F | M | N | O | P | Q | R || G | S | T | U | V | W | X || V | Y | Z | 0 | 1 | 2 | 3 || X | 4 | 5 | 6 | 7 | 8 | 9 |
![ADFGVX](task.png)
| B | E | I | L | N | R ||---|---|---|---|---|---|| D | R | G | N |   | A || N | A |   | H | O | C || S | T |   | N | 1 | E || 9 | 4 |   |   |   | 4 |
And now we can see that the keyword is obviously supposed to be `BERLIN`
| B | E | R | L | I | N ||---|---|---|---|---|---|| D | R | A | N | G |   || N | A | C | H |   | O || S | T | E | N |   | 1 || 9 | 4 | 4 |   |   |   |
`DRANG NACH OSTEN 1944`
# Flag
`InnoCTF{drang_nach_osten_1944}`