Description: It seems that another encrypted message has been intercepted. The encryptor seems to have learned their lesson though and now there isn’t any punctuation! Can you still crack the cipher?
Difficulty: Medium
Author: Will Hong
Summary
This challenge provides a long ciphertext with no spacing or punctuation, making frequency analysis slightly harder than usual. Since the title is substitution2, this strongly suggests another monoalphabetic substitution cipher, just without word boundaries. By leveraging automated tools and frequency patterns, we can still recover the plaintext and extract the flag hidden at the end.
Analysis
We are given the file message.txt :
$ file message.txtmessage.txt: ASCII text, with very long lines (1288), with no line terminatorsAnd its content is a massive block of text with no spaces and no punctuation:
fnjdjjzqsfsjpjdxwmfnjdcjwwjsfxhwqsnjynqensknmmwkmuvafjdsjkadqftkmuvjfqfqmgsqgkwayqgekthjdvxfdqmfxgyaskthjdknxwwjgejfnjsjkmuvjfqfqmgslmkasvdquxdqwtmgstsfjusxyuqgqsfdxfqmglagyxujgfxwscnqknxdjpjdtasjlawxgyuxdojfxhwjsoqwwsnmcjpjdcjhjwqjpjfnjvdmvjdvadvmsjmlxnqensknmmwkmuvafjdsjkadqftkmuvjfqfqmgqsgmfmgwtfmfjxknpxwaxhwjsoqwwshafxwsmfmejfsfayjgfsqgfjdjsfjyqgxgyjzkqfjyxhmafkmuvafjdskqjgkjyjljgsqpjkmuvjfqfqmgsxdjmlfjgwxhmdqmasxllxqdsxgykmujymcgfmdaggqgeknjkowqsfsxgyjzjkafqgekmglqeskdqvfsmlljgsjmgfnjmfnjdnxgyqsnjxpqwtlmkasjymgjzvwmdxfqmgxgyquvdmpqsxfqmgxgymlfjgnxsjwjujgfsmlvwxtcjhjwqjpjxkmuvjfqfqmgfmaknqgemgfnjmlljgsqpjjwjujgfsmlkmuvafjdsjkadqftqsfnjdjlmdjxhjffjdpjnqkwjlmdfjknjpxgejwqsufmsfayjgfsqgxujdqkxgnqensknmmwsladfnjdcjhjwqjpjfnxfxgagyjdsfxgyqgemlmlljgsqpjfjkngqiajsqsjssjgfqxwlmdumagfqgexgjlljkfqpjyjljgsjxgyfnxffnjfmmwsxgykmglqeadxfqmglmkasjgkmagfjdjyqgyjljgsqpjkmuvjfqfqmgsymjsgmfwjxysfayjgfsfmogmcfnjqdjgjutxsjlljkfqpjwtxsfjxknqgefnjufmxkfqpjwtfnqgowqojxgxffxkojdvqkmkflqsxgmlljgsqpjwtmdqjgfjynqensknmmwkmuvafjdsjkadqftkmuvjfqfqmgfnxfsjjosfmejgjdxfjqgfjdjsfqgkmuvafjdskqjgkjxumgenqensknmmwjdsfjxknqgefnjujgmaenxhmafkmuvafjdsjkadqftfmvqiajfnjqdkadqmsqftumfqpxfqgefnjufmjzvwmdjmgfnjqdmcgxgyjgxhwqgefnjufmhjffjdyjljgyfnjqduxknqgjsfnjlwxeqsvqkmKFL{G6D4U_4G41T515_15_73Y10A5_42JX1770}The absence of whitespace is deliberate: it makes pattern spotting harder, but it’s still just a substitution cipher, as implied by the challenge title.
Solution
To decode this efficiently, we load the ciphertext into: Mono-alphabetic Substitution
The tool performs frequency analysis and heuristic reconstruction to rebuild natural English from the continuous letter stream.
After processing, we obtain the fully decrypted plaintext:
THEREEXISTSEVERALOTHERWELLESTABLISHEDHIGHSCHOOLCOMPUTERSECURITYCOMPETITIONSINCLUDINGCYBERPATRIOTANDUSCYBERCHALLENGETHESECOMPETITIONSFOCUSPRIMARILYONSYSTEMSADMINISTRATIONFUNDAMENTALSWHICHAREVERYUSEFULANDMARKETABLESKILLSHOWEVERWEBELIEVETHEPROPERPURPOSEOFAHIGHSCHOOLCOMPUTERSECURITYCOMPETITIONISNOTONLYTOTEACHVALUABLESKILLSBUTALSOTOGETSTUDENTSINTERESTEDINANDEXCITEDABOUTCOMPUTERSCIENCEDEFENSIVECOMPETITIONSAREOFTENLABORIOUSAFFAIRSANDCOMEDOWNTORUNNINGCHECKLISTSANDEXECUTINGCONFIGSCRIPTSOFFENSEONTHEOTHERHANDISHEAVILYFOCUSEDONEXPLORATIONANDIMPROVISATIONANDOFTENHASELEMENTSOFPLAYWEBELIEVEACOMPETITIONTOUCHINGONTHEOFFENSIVEELEMENTSOFCOMPUTERSECURITYISTHEREFOREABETTERVEHICLEFORTECHEVANGELISMTOSTUDENTSINAMERICANHIGHSCHOOLSFURTHERWEBELIEVETHATANUNDERSTANDINGOFOFFENSIVETECHNIQUESISESSENTIALFORMOUNTINGANEFFECTIVEDEFENSEANDTHATTHETOOLSANDCONFIGURATIONFOCUSENCOUNTEREDINDEFENSIVECOMPETITIONSDOESNOTLEADSTUDENTSTOKNOWTHEIRENEMYASEFFECTIVELYASTEACHINGTHEMTOACTIVELYTHINKLIKEANATTACKERPICOCTFISANOFFENSIVELYORIENTEDHIGHSCHOOLCOMPUTERSECURITYCOMPETITIONTHATSEEKSTOGENERATEINTERESTINCOMPUTERSCIENCEAMONGHIGHSCHOOLERSTEACHINGTHEMENOUGHABOUTCOMPUTERSECURITYTOPIQUETHEIRCURIOSITYMOTIVATINGTHEMTOEXPLOREONTHEIROWNANDENABLINGTHEMTOBETTERDEFENDTHEIRMACHINESTHEFLAGISPICOCTF{N6R4M_4N41Y515_15_73D10U5_42EA1770}
From here, at the very end, we can see the flag already.
⚡ Raikiri🎉 Flag pwned!
THE FLAG IS PICOCTF{N6R4M_4N41Y515_15_73D10U5_42EA1770}
💡 TL;DR / Lesson LearnedThe file contains a giant block of text encrypted via monoalphabetic substitution.
Removing punctuation and spaces increases difficulty slightly but doesn’t change the cipher.
Feeding the ciphertext into DCODE’s substitution solver reconstructs readable English.
Even when obfuscated with no word boundaries, substitution ciphers crumble under frequency analysis.