RC3 CTF Writeup – Salad(Crypto 100)

Being a part of bi0s, this was the first time I was able to decode a cipher while the CTF was still going on and the feeling was amazing! I feel extremely lucky to be a part of bi0s with such great seniors who are experienced exploit experts and mentor like Vipin Sir!

Its been more than a month I have joined bi0s, and I am pretty sure that Cryptography will the field I am going to focus upon in Computer Science for the rest of my life, I mean for my Masters and if possible then PhD. My blog from now will primarily focus upon the new ideas that emerge in the field of Cryptography, along with the topics which I learn.

Of Course, I won’t stop writing critical thoughts and ideas of mine!

The question stated:

Salad

Category: Cryptography              Points: 100

Description:

“The fault, dear Brutus, is not in our stars, but in ourselves.” (I.ii.141) Julius Caesar in William Shakespeare’s Julius Caesar

Cipher Text: 7sj-ighm-742q3w4t

At the first glance, the question looks like one encoded with Caesar(Shift) cipher but it is kinda not so. It involves applying Substitution cipher along with Caesar to get the flag.

So here we go,

Since each flag in the CTF had to start with RC3-2016, it struck me at a point of time if even the encoded message contains RC3-2016, but in encoded form! This forced me to find a relation between the letters and the digits using the first 7 encoded digits and RC3-2016. The pattern that I could make out was this:

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ
KLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJ 

7sj-ighm-742q3w4t
RC3-2016-ROMANGOD

Here is the flag: RC3-2016-ROMANGOD

Looking forward to exploring and solving more questions in Cryptography!

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