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How do I see if the following ciphertext eaeairtntrnaeemtve is a transposition cipher, using letter frequency?
This article suggest how to detect by using letter frequency detection and cryptanalysis but I am unsure of how to do this. Any suggestions?
It might be problematic to use letter frequency when you have short ciphertext.
Anyway, this is transposition cipher! so your plaintext is probably some permutation of the ciphertext.
You have $18 = 3 \cdot 6$ chars. Try to write them as a rectangle of $3 \times 6$ or $6 \times 3$ and read it in different ways (spiral, zigzag, etc.).
It is impossible to do cryptanalysis on such a short message. It could be anything, encrypted with anything. Even if you somehow knew that it's a substitution cypher, it could be any 18-character message where the right characters match (there must be seven different letters; characters 1, 3, 13, 14 and 18 have to be the same; characters 1 and 2 must be different, etc.).
If you had a longer cyphertext (say a few hundred characters), you'd just compute the frequency of the characters appearing in the cyphertext and compare that to the frequency of the letters in the language you think the message was written in. The frequencies in English are: e 12.7%, t 9.1%, a 8.2%, o 7.5%, i 7.0%, n 6.9%, etc. If a decently long cyphertext comes decently close to that distribution, you'd expect a transposition cypher. If you only have that the most common characters occur about 13%, 9%, 8%, 8%, 7%, 7%... of the time, you'd suspect a substitution cypher and you'd start guessing the letters based on their frequency. Note that the process is not automatic and you need to be flexible: for example, if the message is about the Zulus playing zithers in zoos in Zanzibar, the letter frequencies may be skewed quite some distance from the average over all English text.
