> Welcome to crypto. We start with the classics! Deliver the decrypted
> form of the message to the guard on your journey.
>
> nc crypto.chal.csaw.io 5004
Another challenge where you're completely reliant on hints to get
going:
- Length of keys is 25, which (cleaned-up) ramblings would fit that?
- If near-perfect ramblings are used to encrypt in order, how should it be decrypted?
- The guard want's a legible message they can read in!
Searching for the challenge name it turns out the Bifid cipher is a
thing. It's a variation of the polybius one, which has the specialty
of arranging keys into a 5x5 square and using an alphabet where i and
j are represented by the same letter. Going by the first hint I
cleaned up the `ramblings` file by deleting all characters except that
25-letter alphabet and obtained 20 keys. The second hint suggests the
plaintext has been encrypted several times using different keys,
decryption would use the keys in reverse order. Knowing that I
visited <
https://www.dcode.fr/bifid-cipher> and copy-pasted a bunch of
times, resulting in the following decryption:
xustxsomexunnecessaryxtextxthatxholdsxabsolutelyxnoxmeaningxwhatsoeverxandxbearsxnoxsignificancextoxyouxinxanyxway
The third hint suggests turning that into a legible message for the service:
$ nc crypto.chal.csaw.io 5004 <<< 'just some unnecessary text that holds absolutely no meaning whatsoever and bears no significance to you in any way'
> Alright messenger, what did the boss tell you to tell me? Better be right or you're not getting in!
flag{t0ld_y4_1t_w4s_3z}
