i春秋第二届春秋欢乐赛 Substituted writeup

第一步：审题

题目中给出了一个文本文档：

Lw!

Gyzvecy ke WvyVKT!

W’zz by reso dsbdkwksky tzjq teo kly ujr. Teo keujr, gy joy dksurwmq bjdwv vorakeqojalr jmu wkd jaazwvjkwemd. Vorakeqojalr ljd j zemq lwdkeor, jzklesql gwkl kly juxymk et vecaskyod wk ljd qekkym oyjzzr vecazwvjkyu. Decy dwcazy ezu vwalyod joy kly Vjydjo vwalyo, kly Xwqymyoy vwalyo, kly dsbdkwkskwem vwalyo, glwvl wd klwd emy, jmu de em. Jzcedk jzz et klydy vwalyod joy yjdwzr boeiym keujr gwkl kly lyza et vecaskyod. Decy myg ymvorakwem cykleud joy JYD, kly vsooymk dkjmujou teo ymvorakwem, jzemq gwkl ODJ. Vorakeqojalr wd j xjdk twyzu jmu wd xyor wmkyoydkwmq klesql. De iwvi bjvi, oyju sa em decy veez vwalyod jmu ljxy tsm!

El jmu teo reso oyveoud cr mjcy wd WvyVKT{jzgjrd_zwdkym_ke_reso_dsbdkwksky_tzjqd}.

第二步：试探

这里我用了凯撒密码和栅栏密码发现都不行后，利用词频分析，网站为：https://quipqiup.com/

得到结果：
Hi! Welcome to IceCTF! I’ll be your substitute flag for the day. For today, we are studying basic cryptography and its applications. Cryptography has a long history, although with the ad?ent of computers it has gotten really complicated. Some simple old ciphers are the Caesar cipher, the ?igenere cipher, the substitution cipher, which is this one, and so on. Almost all of these ciphers are easily bro?en today with the help of computers. Some new encryption methods are AES, the current standard for encryption, along with RSA. Cryptography is a ?ast field and is ?ery interesting though. So ?ic? bac?, read up on some cool ciphers and ha?e fun! Oh and for your records my name is IceCTF{always_listen_to_your_substitute_flags}.

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