Square CTF WriteUp - Crypt 50pt -
Square CTFのCrypt 50ptの問題として、以下の暗号文が出題。
The androids’ plans for domination include securing ancient artifacts relating to the animal world to be used for nefarious means. We managed to infiltrate one of their digsites and intercepted this ancient scientific tome, but it's encrypted! We think it relates to Julius Caesar's time in Rome, for he was a great friend and benefactor to the feline community. We could use your cryptanalysis skills to determine the contents of this tome and whether it will give us any leverage against our enemies.
Kyv ufdvjkzt trk (Wvczj jzcmvjkizj trklj fi Wvczj trklj) zj r jdrcc, kpgztrccp wliip, triezmfiflj drddrc. Kyvp riv fwkve trccvu yfljv trkj nyve bvgk rj zeuffi gvkj fi jzdgcp trkj nyve kyviv zj ef evvu kf uzjkzexlzjy kyvd wifd fkyvi wvczuj reu wvczevj. Trkj riv fwkve mrclvu sp yldrej wfi tfdgrezfejyzg reu wfi kyvzi rszczkp kf ylek mvidze. Kyviv riv dfiv kyre 70 trk sivvuj, kyflxy uzwwvivek rjjftzrkzfej giftcrzd uzwwvivek eldsvij rttfiuzex kf kyvzi jkreuriuj. Kyv wcrx zj kyv gyirjv nzky urjyvj: wcrx nyrk zj r ufdvjkzt trk.
まず暗号文を見て直感的に感じたのは、ROT13ではないかとのこと。
（以前にも同じような文章を見たことがあったため）
さらに出題文を読むと「Julius Caesar」の文言があることから、シーザー暗号を使っていると確信。
そのため、シフト数を適当に変更しながら復号化を行ったところ、シフト数9で以下の有意な文章に変換成功。
The domestic cat (Felis silvestris catus or Felis catus) is a small, typically furry, carnivorous mammal. They are often called house cats when kept as indoor pets or simply cats when there is no need to distinguish them from other felids and felines. Cats are often valued by humans for companionship and for their ability to hunt vermin. There are more than 70 cat breeds, though different associations proclaim different numbers according to their standards. The flag is the phrase with dashes: flag what is a domestic cat.
フラグは
The flag is the phrase with dashes: flag what is a domestic cat.
とのことだったので、答えは以下になります。
flag-what-is-a-domestic-cat
