PicoCTF_2017: Substitute

Category: Cryptography Points: 40 Description:

A wizard (he seemed kinda odd...) handed me this. Can you figure out what it says?

Hint:

There are tools that make this easy this.

Write-up

An example of a classical substitution cipher, where every letter is most likely not the actual letter. There are indeed many tools that make this easily solvable but my personal top favourite is Quipuip.

On the first try of attempting to solve with Quipuip, you would probably get a paragraph of text that starts to make more sense.

THE FLAG IS IFONLYMODERNCRYPTO?ASLI?ETHIS. GENERATED BY MAR?OV CHAIN OF
THE ?I?IPEDIA PAGE ON CALVIN AND HOBBES. BE ?ERE LONG, THE FLA?S ON A
2005 PRESENT TIMES THAN STAMINA OR A ?EE?LY SUNDAY, DECEMBER 21, 1989
1990, THE DRE? EDITORS ?AS UPROOTED EVERY ?ORLD CAN BE FOUND THE
CONTINUED TO ?OR? AT A FAITHFUL REPLIES ?HERE HOBBES, STYLE AIM CALVIN,
ATTAC? BOTH SIDES TO MAR?S, "?HAT DO CONTROLLING AN ACTUAL BOARD BOXES
...

This seems very promising, so let's do a letter for letter comparison with the original paragraph.

MIT YSAU OL OYGFSBDGRTKFEKBHMGCALSOQTMIOL. UTFTKAMTR ZB DAKQGX EIAOF GY
THE FLAG IS IFONLYMODERNCRYPTO?ASLI?ETHIS. GENERATED BY MAR?OV CHAIN OF

MIT COQOHTROA HAUT GF EASXOF AFR IGZZTL. ZT CTKT SGFU, MIT YSACL GF A
THE ?I?IPEDIA PAGE ON CALVIN AND HOBBES. BE ?ERE LONG, THE FLA?S ON A

2005 HKTLTFM MODTL MIAF LMADOFA GK A CTTQSB LWFRAB, RTETDZTK 21,
2005 PRESENT TIMES THAN STAMINA OR A ?EE?LY SUNDAY, DECEMBER 21,

We are getting close. Now we just need to add clues to the quipuip machine. For one, MIT=THE and YSAU=FLAG. Additionally, COQOHTROA is most likely WIKIPEDIA. So we add clues like these,

MIT=THE
YSAU=FLAG
COQOHTROA=WIKIPEDIA

Viola, we got the flag!

THE FLAG IS IFONLYMODERNCRYPTOWASLIKETHIS. GENERATED BY MARKOV CHAIN OF
THE WIKIPEDIA PAGE ON CALVIN AND HOBBES. BE WERE LONG, THE FLAWS ON A
2005 PRESENT TIMES THAN STAMINA OR A WEEKLY SUNDAY, DECEMBER 21,
...

Therefore, the flag is IFONLYMODERNCRYPTOWASLIKETHIS.

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