# How to find/determine the Polybius Square Key and Additive Key, if only the Cipher Text is known, in Nihilist Cipher?

I was solving a Crypto Challenge in a CTF, and I got a Nihilist Cipher Encoded String. Luckily the additive key was given as a hint, and for the polybius Square key, I kinda bruteforced or I just got lucky somehow and found the word with which the polybius square was made able to decrypt the cipher text.

But what if only the Cipher text is given in a challenge, or what if I was not able to guess the right polybius Square key?

How should I approach the Nihilist Cipher to crack the Cipher text?

The Challenge cipher text for the reference:

42 24 44 30 28 70 110 66 43 34 48 26 90 100 63 55 64

• Here's an ancient scroll describing the method of solving a Nihilist cipher: SOLVING CIPHER SECRETS, Edited by M. E. Ohaver – Shifrin Mar 21 '20 at 11:13
• @Shifrin Please do not use the answer field for comments. You can turn above into an answer by creating an excerpt of the document and include that. Note that on StackExchange, answers should be as much self contained as possible and not rely on external sources (especially since links can go dead). For that reason, please also try and include the title of the document you point to so we can find it again when required. – Maarten Bodewes Mar 21 '20 at 21:23