Try using English to write the solution down.
However, my English is not so good.
I hope to improve it through this method. :)
The problem gave us a bzip2 file. We get the cipher in twenty.txt after extracting it.
fvoxoxfvwdepagxmwxfpukleofxhwevefuygzepfvexwfvufgeyfryedojhwffoyhxcwgmlxeylawfxfurwfvoxecfezfvwbecpfpeejuygoyfefvwxfpwwfxojumwuxfuffvwawuxflecaazubwjwoyfvwyepfvwuxfhwfjlopwckaohvfjlzopwoaahevupgwpfvuywjoywjdwyfufjupouvbuaajwuaoupkecygjwoyfvwuxxdofvyeacmwbvuzoyhlecpwzcbroyhdofvfvwgcgwdveheffvwrwlxfelecpxuzwuygfvexwfvufbuyfgempoyhxcofxbplfelecpcybawxujfexwffawgoxkcfwxfvechvflecgfubrawfvoxdofvuaoffawjepwfubfmcffvwyuhuoyzcghwkubrwpxogeyfryediubroxvwgufwupwswplfojwofvoyrezaorxuyhmcfxvofjuyfvwlpwubepkepufoeyuygojukwpxeyozobufoeyezzpwwgejzepuaaleczoaagebrwfxaorwfvufxubeybwkfzepwohyfeluaadvoawaudlwpxjcggldufwpuygfpexxfuaaecfezmcxoywxxoxiuoazepjwuyglecpwxcoyhjwbosoaalwnvomoffvoxoyfvwbecpfpeejheeygeofogupwlecbeyhpufcaufoeyxfvwzauhoxxoybwywdbplkfejohvfvuswyxumubrgeepxocxweagbplkfe
I didn't notice this text is encrypted by replacing at first because there are not blanks or other symbols. I had no idea to solve it until jeffxx tell me fvox = this.
The usual way to break Substitution cipher is find some repeat patterns in cipher. The patterns usually present a word in natual language, such as the,this, in ... etc. The more pattern we can find, the more character we get.
However, this cipher does not have any blank or other symbols. It is hard to identify which pattern is the word. Therefore, I used a regular expression dictionary to assist me find a word that match the pattern.
For example, after we substituted 'fvoxw' to thise, we could get a part of plain as below: (The capital letters were substituted.)
THISISTHEdepagSmESTpukleITShEeHeTuygzepTHeSETHuTgeyTryedIjhETTIyhScEgmlSeylaETSTurETHISecTe
zTHEbecpTpeejuygIyTeTHESTpEETSIjumEuSTuTTHEaEuSTlecaazubEjEIyTHEyepTHEuSThETjlIpEckaIhHTjlzI
pEIaaheHupgEpTHuyEjIyEjdEyTuTjupIuHbuaajEuaIupkecygjEIyTHEuSSdITHyeacmEbHuzIyhlecpEzcbrIyhdI
....
Observing the text, we could find some duplicate patterns, like THeSE, STpEET. I guessed that means those, street. If not sure which words match the pattern, we can use dictionary to search possible words. We could get o and r, and substitute them to cipher again.
We could use google to search sentences after some characters were substiuted. Finally, I found the plaintext was a lyrics of rap from youtube ([https://www.youtube.com/watch?v=9iUvuaChDEg]). And the last sentence is CONGRATULATIONSTHEFLAGISSINCENEWCRYPTOMIGHTHAVENSABACKDOORSIUSEOLDCRYPTO.
flag: sincenewcryptomighthavensabackdoorsiuseoldcrypto