Substitution Cipher
6 minutes to read
We are given this encrypted text:
MIT YSAU OL OYGFSBDGRTKFEKBHMGCALSOQTMIOL. UTFTKAMTR ZB DAKQGX EIAOF GY MIT COQOHTROA HAUT GF EASXOF AFR IGZZTL. ZT CTKT SGFU, MIT YSACL GF A 2005 HKTLTFM MODTL MIAF LMADOFA GK A CTTQSB LWFRAB, RTETDZTK 21, 1989 1990, MIT RKTC TROMGKL CAL WHKGGMTR TXTKB CGKSR EAF ZT YGWFR MIT EGFMOFWTR MG CGKQ AM A YAOMIYWS KTHSOTL CITKT IGZZTL, LMBST AOD EASXOF, AMMAEQ ZGMI LORTL MG DAKQL, "CIAM RG EGFMKGSSOFU AF AEMWAS ZGAKR ZGVTL OF MIT HKTHAKTFML FADT, OL ODHWSLOXT KADHAUTL OF CIOEI ASCABL KTYTKTFETL MIT HALLCGKR, CIOEI DGFTB, AFR MITB IAR SOMMST YKGFM BAKR IOL YKWLMKAMTR EGSGK WFOJWT AZOSOMB COMI AFR OFROLHTFLAMT YGK MTAEI GMITK LMWROTL, AKT ACAKRL ZARUTL, HWZSOLITR ZTYGKT CTSS AL A YOKT UKGLL HSAFL CTKT GKOUOFASSB EIAKAEMTKL OF MIT LMKOH MG CIOEI LTTD MG OM CITF MTDHTKTR OF AFR IASSGCOFU MITB'KT LODHSB RKACOFU OF UOXTL GF" HKOFEOHAS LHOMMST ROLMGKM, KTARTKL EGDOEL AKT WLT, CAMMTKLGF MGGQ MCG 16-DGFMIL AYMTK KTLOLMAQTL A DGKT EKTAM RTAS MG EASXOF GYMTF IGZZTL MG ARDOML "LSODB, "ZWM OM'L FADTR A FOUIM GWM LIT OL HGOFM GY FGM LTTF IGZZTL MIT ZGGQL AM MIAM O KTDAOFOFU ZGGQ IADLMTK IWTB AKT AHHTAKAFET: RTETDZTK 6, 1995 DGD'L YKADTL GY EASXOF UOXTF A CAUGF, LGDTMODTL MIAM LG OM'L YAMITKT'L YADOSB FG EAFETSSAMOGFLIOH CAL HKTLTFML YKGD FGXTDZTK 21, 1985 SALM AHHTAK AZLTFET OF AFGMITKCOLT OM IAHHB MG KWF OM YGK MIOL RAR AL "A SOMMST MG MGSTKAMT EASXOF'L YADOSB RKACF ASDGLM EGDDTFRTR WH ZTOFU HTGHST OFLMAFET, UTM DAKKOTR ZB A RAFET EASXOF'L GWMSAFROLOFU MIT FTCLHAHTK GK MAZSGOR FTCLHAHTK ZWLOFTLL LIGC OL GF!" AFR LHKOFML GY EIOSRKTF'L RAR'L YKWLMKAMTR ZB MWKF IWDGK, CAL HWZSOE ROASGU MITKT'L FGM DWEI AL "'94 DGRTKFOLD" CAMMTKLGF IAL RTSOUIML GY YAFMALB SOYT CAMMTKLGF LABL LTKXTL AL AF AKMOLML OL RTLMKWEMOGF ZWLOFTLL, LHAETYAKTK GY MIT GHHGKMWFOMOTL BGW ZGMI A MGHOE YGK IOL IGDT MGFUWT-OF-EITTQ HGHWSAK MIAM OM CAL "IGF" AFR JWAKMTK HAUT DGKT LHAEOGWL EAFETSSAMOGF MIT HAOK AKT ESTAKSB OF HLBEIOE MKAFLDGUKOYOTK'L "NAH" LGWFR TYYTEM BGW MIOFQTK CAMMTKLGF ASLG UKTC OFEKTROZST LHAET ZWBL OF EGDDGFSB CIOST GMITKCOLT OM'L FADT OL FGMAZST LMGKBSOFT UAXT MIT GHHGKMWFOMOTL BGW EAFETSSAMOGF MIT "EASXOF GYYTK MG DAQT IOD OFEGKKTEM AFLCTKL CAMMTK AKMCGKQ GMITK GYMTF CIOEI OL TXORTFM MG GMITK LMKOH OL MG MITOK WLT GY KWSTL MIAM LIGCF GF LAFROYTK, CIG WLTL A EKGCJWOSS ZT LTTF "USWTR" MG MIT GFSB HTKL AFR IOL YAMITK LWHHGKM OL SWFEISOFT UAXT MITLT MIOF A BTAK OF DWSMODAMTKOAS AFR GZMAOF GF LAFMALB, IOL WLT, CAMMTKL ROASGUWT OL AF "AKMOLM'L LMAMWL AL "A ROD XOTC OF MIT TLLTFMOASSB MG DAQT IOD LTTD MG OFESWRTR MIAM EASXOF OL AF GRR ROASGUWT DGLM GY MIT ESWZ IAL TVHKTLLOGF GWMLORT AXAOSAZST MG
From the name of the challenge, we know that it is encrypted using a substitution cipher. We can try ROT13, but it won’t decrypt. Therefore, we must guess the character substitution that is used. I will be using CyberChef.
First, we notice that the text starts with MIT and that there are a lot of words MIT. Since the code is likely to be in English, the most probable word that matches is THE:
So, M -> T, I -> H and T -> E.
Now, we see that there are a lot of A as a single word. This means that A -> A because there is no single-letter word in English but A:
Now we see that there are some apostrophes and usually they match with 'S, so L -> S:
After that, there are a lot of TG (where the T is already substituted), and this means that G -> O because there are no more two-letter words that start with T but TO:
A similar procedure will allow us to guess that OS will be IS and OT will be IT (again, S and T are already substituted), so O -> I:
Now we are able to see a recognizable sentence: ATTAEQ ZOTH SIRES TO DAKQS. This is likely to be something like: ATTACK BOTH SIDES TO .... Here we have E -> C, Q -> K, Z -> B and R -> D:
Now we have another recognizable word: CIKIHEDIA, which is WIKIPEDIA, obviously. So C -> W and H -> P:
Looking for more recognizable words, we find WEKE, WOKK, DKEW and EDITOKS. This means that K -> R:
We see some AFD (where A and D are already substituted), so F -> N:
Then we see some months and days and more recognizable words, so we have W -> U, B -> Y, D -> M and X -> V:
Three more words: UENERATED, WEEKSY and SIMPSY. Hence, U -> G and S -> L:
Again, using the same procedure we can guess that Y -> F:
Although the flag can be read, let’s finish the substitution. We see some more words to find out that J -> Q, N -> N, V -> X and P -> J (there is no P and J is the remaining character):
And now we have finished:
The decrypted text is:
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, 1989 1990, THE DREW EDITORS WAS UPROOTED EVERY WORLD CAN BE FOUND THE CONTINUED TO WORK AT A FAITHFUL REPLIES WHERE HOBBES, STYLE AIM CALVIN, ATTACK BOTH SIDES TO MARKS, "WHAT DO CONTROLLING AN ACTUAL BOARD BOXES IN THE PREPARENTS NAME, IS IMPULSIVE RAMPAGES IN WHICH ALWAYS REFERENCES THE PASSWORD, WHICH MONEY, AND THEY HAD LITTLE FRONT YARD HIS FRUSTRATED COLOR UNIQUE ABILITY WITH AND INDISPENSATE FOR TEACH OTHER STUDIES, ARE AWARDS BADGES, PUBLISHED BEFORE WELL AS A FIRE GROSS PLANS WERE ORIGINALLY CHARACTERS IN THE STRIP TO WHICH SEEM TO IT WHEN TEMPERED IN AND HALLOWING THEY'RE SIMPLY DRAWING IN GIVES ON" PRINCIPAL SPITTLE DISTORT, READERS COMICS ARE USE, WATTERSON TOOK TWO 16-MONTHS AFTER RESISTAKES A MORE CREAT DEAL TO CALVIN OFTEN HOBBES TO ADMITS "SLIMY, "BUT IT'S NAMED A NIGHT OUT SHE IS POINT OF NOT SEEN HOBBES THE BOOKS AT THAT I REMAINING BOOK HAMSTER HUEY ARE APPEARANCE: DECEMBER 6, 1995 MOM'S FRAMES OF CALVIN GIVEN A WAGON, SOMETIMES THAT SO IT'S FATHERE'S FAMILY NO CANCELLATIONSHIP WAS PRESENTS FROM NOVEMBER 21, 1985 LAST APPEAR ABSENCE IN ANOTHERWISE IT HAPPY TO RUN IT FOR THIS DAD AS "A LITTLE TO TOLERATE CALVIN'S FAMILY DRAWN ALMOST COMMENDED UP BEING PEOPLE INSTANCE, GET MARRIED BY A DANCE CALVIN'S OUTLANDISING THE NEWSPAPER OR TABLOID NEWSPAPER BUSINESS SHOW IS ON!" AND SPRINTS OF CHILDREN'S DAD'S FRUSTRATED BY TURN HUMOR, WAS PUBLIC DIALOG THERE'S NOT MUCH AS "'94 MODERNISM" WATTERSON HAS DELIGHTS OF FANTASY LIFE WATTERSON SAYS SERVES AS AN ARTISTS IS DESTRUCTION BUSINESS, SPACEFARER OF THE OPPORTUNITIES YOU BOTH A TOPIC FOR HIS HOME TONGUE-IN-CHEEK POPULAR THAT IT WAS "HON" AND QUARTER PAGE MORE SPACIOUS CANCELLATION THE PAIR ARE CLEARLY IN PSYCHIC TRANSMOGRIFIER'S "NAP" SOUND EFFECT YOU THINKER WATTERSON ALSO GREW INCREDIBLE SPACE BUYS IN COMMONLY WHILE OTHERWISE IT'S NAME IS NOTABLE STORYLINE GAVE THE OPPORTUNITIES YOU CANCELLATION THE "CALVIN OFFER TO MAKE HIM INCORRECT ANSWERS WATTER ARTWORK OTHER OFTEN WHICH IS EVIDENT TO OTHER STRIP IS TO THEIR USE OF RULES THAT SHOWN ON SANDIFER, WHO USES A CROWQUILL BE SEEN "GLUED" TO THE ONLY PERS AND HIS FATHER SUPPORT IS LUNCHLINE GAVE THESE THIN A YEAR IN MULTIMATERIAL AND OBTAIN ON SANTASY, HIS USE, WATTERS DIALOGUE IS AN "ARTIST'S STATUS AS "A DIM VIEW IN THE ESSENTIALLY TO MAKE HIM SEEM TO INCLUDED THAT CALVIN IS AN ODD DIALOGUE MOST OF THE CLUB HAS EXPRESSION OUTSIDE AVAILABLE TO
And the flag is:
CTFlearn{IFONLYMODERNCRYPTOWASLIKETHIS}