# Decryption Problem

The following message was posted in our math department and I wouldn't mind some help into getting started at cracking it:

gectl atnoy danwm etaim oroni snair ohass wveno faome nceto kils

Any ideas?

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This is called a "ciphertext only" attack, which usually includes "security by obscurity" (in other words, the proposer is withholding both the key and the method used to encrypt the plaintext).

In order to even approach such a problem, you have to know (or guess) what domain the underlying plaintext comes from. English text? JPG? MP3? .exe? Swahili??

If the underlying plaintext has full entropy (ie, it's essentially random bits), the problem is unsolvable since all plaintexts would be, by definition, equally likely.

Suppose we know (or suspect) the plaintext is English. Then we have to start guessing at the encryption method. I would suggest you start with the simplest ones like Caesar, substitution, Vigenere, etc. If frequencies in the plaintext are reflected in the ciphertext (as they are with simplistic ciphers like Caesar) you can use statistical measures to correlate with known statistics on English. In English, the most common letters are ETAOIN, SHRDLU...

After this you can measure diphthong and triphthong frequencies, index of coincidence, etc. You can also look at word-length statistics, except that here it appears the lengths have been obscured by (the common trick of) blocking characters into 5's.

Note: This area of cryptography is purely recreational/historical. Modern cryptographers rarely spend any time thinking about this stuff unless they are bored, cornered at a party, or responding to various online forum posts.

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Well strictly speaking, cryptographers never spend any time thinking about this stuff. They leave it to the cryptanalysts :) – Brandon Carter Apr 30 '11 at 22:54
@Brandon: Hah! Ok, you got me. Cryptographers create, cryptanalysts break, and cryptologists do both. The main conference in our field is called Advances in Cryptology, but I rarely use that term it forums like this one because it's not known to lay audiences. But yeah, cryptanalysts was the proper word. – Fixee May 1 '11 at 3:04

It's a transposition cipher. Write the ciphertext as 3 rows of 18, then read it vertically instead of horizontally.

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+1 This is attributed to the leading Rothschild. – Henry May 18 '11 at 1:59