Set #1 starts out with a simpler problem than Pigpen, and then a problem that's just barely harder than Pigpen, but that requires actual code and whose solution will also help you break repeated-nonce AES-CTR.
For anyone else looking for another one along the same lines, this is a good one too: http://overthewire.org/wargames/natas/natas0.html
Key: ABCABCABCABCA... Plaintext: MYNAMEISAARON... Ciphertext: MZPANGITCASQN...
At first glance, frequency analysis would be useless against this ciphertext. But if you can work out the key length (in this case, 3), you can group every 3rd character together and then perform frequency analysis on each group, as we know each group as been encrypted with the same key, making frequency analysis relevant again.
it was close. but the trick is the frequencies on short messages only give you probabilities that a certain mapping is true.
but those probabilities are enough to shorten the potential mappings significantly (i.e. e is probably e or t, g is probably e or t). Combine that with a word match (does the result contain stop words such as - it not the etc)
and bam, game over.
Like, imagine a cipher where you shift each letter once (modulo 'Z') for each character of the text thus far (e.g. "HELLO WORLD" becomes "HFNOS BUYTM"). I'm sure someone has a name for this that I don't know. This seems immune to frequency analysis (unless you have such a large ciphertext that you begin to notice recurring strings with 1/26th the probability of a monoalphabetic cipher), but still trivial enough that an experienced cryptanalyist should crack it immediately. My question is, how would they do it? What's the "next step" if frequency analysis fails?