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th3jackers 2015 CTF crypto100 writeup

24 January 2015 No Comment

OHSHIT (crypto100)

Description: Decrypt the cipher using the encryption program

And attachment is supplied: challenge.7z

It contains an encryption program and crypto.txt containing

Name: Automated Crypter
Description:
Decrypt this:
019t-0-080-3-1b-19t-25z-080-03f-8j-1b-12n-12n
Using this program.
(Note: the – is just a separator)
Hint: Not all letters chars are crypted

I didn’t have to solve this using a python script as usual.
I tried it manually:

./crypto abcdefghijklmnopqrstuvwxyz
Crypted text: 1b3f4g5i8j12n13o16q19t22u25z

./crypto ABCDEFGHIJKLMNOPQRSTUVWXYZ
Crypted text: 01b03f04g05i08j012n013o016q019t022u025z

I notice something, the character is encrypted to number+character after it
The encryption of “s” is “19t”
So just manually i know “019t” is 0+s.
And with trial and error i get:
0s-0-080-3-a-s-y-080-0c-h-a-l-l

But the 0’s don’t feel right.
So i take another look, and turns out the capital letters add that 0 before the character.
So it should be:
S-0-080-3-a-s-y-080-C-h-a-l-l
And 080 looked like a separator?

So i tried characters and “.” turns out to be the 080

flag{S0.3asy.Chall}

And it works!

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