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Crypto

This section showcases some of the more challenging and interesting crypto challenges we encountered during the competition.

The writeups below are written by @xbowery.

Julius' ancient script (100 pts)#

I found this Ancient Roman papyrus. Could you decypher it for me?

Not another Caesar cipher......

For this challenge, we are provided with a text file โ€“ flag.txt

Here's what the file contained:

Cipher

Seems daunting on first glance, huh?

To solve this challenge, I worked backwards.

Knowing that the flag will be starting in the form of dctf{...},

I got this cipher for a start:

d -> rc -> qt -> 7f -> t

I noticed that this was a spin-off of the ROT rotation (good old Caesar cipher), given that c is next to d in normal alphabetical form, and in the cipher, the similar output is observed as q is next to r.

Noticing that there was capital letters in the encoded flag as well, I proceeded to include them accordingly in my own handwritten cipher.

Here's a peek at my cipher (pardon my handwriting):

Handwritten notes

Using this cipher, we get the following flag:

Handwritten flag

Flag: dctf{th3_d13_h4s_b33n_c4st}

This one is really basic (300 pts)#

description
The answer to life, the universe, and everything.

For this challenge, we are provided with a text file โ€“ cipher.txt

Here's a snippet of it:

Huge chunk of text

Many, many long lines of words indeed.

Something unusual with this file was that it has a repeated โ€œVmโ€. I took the first few characters of the file and ran a quick Google Search, and realised that it was basically a prefix-invariant base64 encoded string.

Putting the file onto Cyberchef and after numerous repeated rounds of Base 64 decoding (41 times in total actually), we get the following result:

Flag decoded

Flag: dctf{This_l00ks_4_lot_sm4ll3r_th4n_1t_did}