PicoCTF Write-ups

substitution1-2

2 min read

Task desc

substitution1

As in substitution0, we need to decrypt the given text:

LKOb (bwvek ove lgqkhej kwj osgx) gej g kyqj vo lvrqhkje bjlhetky lvrqjktktvu. Lvukjbkgukb gej qejbjukjz dtkw g bjk vo lwgssjuxjb dwtlw kjbk kwjte lejgktftky, kjlwutlgs (guz xvvxstux) bitssb, guz qevmsjr-bvsftux gmtstky. Lwgssjuxjb hbhgssy lvfje g uhrmje vo lgkjxvetjb, guz dwju bvsfjz, jglw ytjszb g bketux (lgssjz g osgx) dwtlw tb bhmrtkkjz kv gu vustuj blvetux bjeftlj. LKOb gej g xejgk dgy kv sjgeu g dtzj geegy vo lvrqhkje bjlhetky bitssb tu g bgoj, sjxgs juftevurjuk, guz gej wvbkjz guz qsgyjz my rguy bjlhetky xevhqb gevhuz kwj dvesz ove ohu guz qeglktlj. Ove kwtb qevmsjr, kwj osgx tb: qtlvLKO{OE3AH3ULY_4774LI5_4E3_L001_6J0659OM}

At first I found a Python module for decryption purposes and wrote a script to decryption:

from cipher_solver.simple import SimpleSolver
 
with open("message.txt", "r") as f:
    ciphertext = f.read()
 
solver = SimpleSolver(ciphertext)
 
solver.solve()
 
print("Decrypted text:")
print(solver.plaintext())
 
print("\nDecryption key:")
print(solver.decryption_key())

The above given script searches for the key and decodes the text correctly, but makes a mistake in one of the symbols on the flag, which makes it unsuitable:

Decrypted text:
CTFs (short for capture the flag) are a type of computer security competition. Contestants are presented with a set of challenges which test their creativity, technical (and googling) skills, and problem-solving ability. Challenges usually cover a number of categories, and when solved, each yields a string (called a flag) which is submitted to an online scoring service. CTFs are a great way to learn a wide array of computer security skills in a safe, legal environment, and are hosted and played by many security groups around the world for fun and practice. For this problem, the flag is: picoCTF{FR3XU3NCY_4774CK5_4R3_C001_6E0659FB}

Decryption key:
gmlzjoxwtcisruvqnebkhfdayp

Therefore, let’s use the tool to recheck key : quipquip:

image_2

Let’s compare the flags from the script and the online tool:

image_3

Flag extracted by quipquip is suitable:

picoCTF{FR3QU3NCY_4774CK5_4R3_C001_6E0659FB}

substitution2

image_4

This time, the text is without spaces.

fnjdjjzqsfsjpjdxwmfnjdcjwwjsfxhwqsnjynqensknmmwkmuvafjdsjkadqftkmuvjfqfqmgsqgkwayqgekthjdvxfdqmfxgyaskthjdknxwwjgejfnjsjkmuvjfqfqmgslmkasvdquxdqwtmgstsfjusxyuqgqsfdxfqmglagyxujgfxwscnqknxdjpjdtasjlawxgyuxdojfxhwjsoqwwsnmcjpjdcjhjwqjpjfnjvdmvjdvadvmsjmlxnqensknmmwkmuvafjdsjkadqftkmuvjfqfqmgqsgmfmgwtfmfjxknpxwaxhwjsoqwwshafxwsmfmejfsfayjgfsqgfjdjsfjyqgxgyjzkqfjyxhmafkmuvafjdskqjgkjyjljgsqpjkmuvjfqfqmgsxdjmlfjgwxhmdqmasxllxqdsxgykmujymcgfmdaggqgeknjkowqsfsxgyjzjkafqgekmglqeskdqvfsmlljgsjmgfnjmfnjdnxgyqsnjxpqwtlmkasjymgjzvwmdxfqmgxgyquvdmpqsxfqmgxgymlfjgnxsjwjujgfsmlvwxtcjhjwqjpjxkmuvjfqfqmgfmaknqgemgfnjmlljgsqpjjwjujgfsmlkmuvafjdsjkadqftqsfnjdjlmdjxhjffjdpjnqkwjlmdfjknjpxgejwqsufmsfayjgfsqgxujdqkxgnqensknmmwsladfnjdcjhjwqjpjfnxfxgagyjdsfxgyqgemlmlljgsqpjfjkngqiajsqsjssjgfqxwlmdumagfqgexgjlljkfqpjyjljgsjxgyfnxffnjfmmwsxgykmglqeadxfqmglmkasjgkmagfjdjyqgyjljgsqpjkmuvjfqfqmgsymjsgmfwjxysfayjgfsfmogmcfnjqdjgjutxsjlljkfqpjwtxsfjxknqgefnjufmxkfqpjwtfnqgowqojxgxffxkojdvqkmkflqsxgmlljgsqpjwtmdqjgfjynqensknmmwkmuvafjdsjkadqftkmuvjfqfqmgfnxfsjjosfmejgjdxfjqgfjdjsfqgkmuvafjdskqjgkjxumgenqensknmmwjdsfjxknqgefnjujgmaenxhmafkmuvafjdsjkadqftfmvqiajfnjqdkadqmsqftumfqpxfqgefnjufmjzvwmdjmgfnjqdmcgxgyjgxhwqgefnjufmhjffjdyjljgyfnjqduxknqgjsfnjlwxeqsvqkmKFL{G6D4U_4G41T515_15_73Y10A5_42JX1770}

TIP

Put message.txt and script in one directory. This script as well as others I left here: click

# Module used: https://pypi.org/project/cipher-solver/
 
from cipher_solver.simple import SimpleSolver
 
with open("message.txt", "r") as f:
    ciphertext = f.read()
 
solver = SimpleSolver(ciphertext)
 
solver.solve()
 
print("Decrypted text:")
print(solver.plaintext())
 
print("\nDecryption key:")
print(solver.decryption_key())

Above given Python script works on this ciphertext:

Decrypted text:
thereexistseveralotherwellestablishedhighschoolcomputersecuritycompetitionsincludingcyberpatriotanduscyberchallengethesecompetitionsfocusprimarilyonsystemsadministrationfundamentalswhichareveryusefulandmarketableskillshoweverwebelievetheproperpurposeofahighschoolcomputersecuritycompetitionisnotonlytoteachvaluableskillsbutalsotogetstudentsinterestedinandexcitedaboutcomputersciencedefensivecompetitionsareoftenlaboriousaffairsandcomedowntorunningchecklistsandexecutingconfigscriptsoffenseontheotherhandisheavilyfocusedonexplorationandimprovisationandoftenhaselementsofplaywebelieveacompetitiontouchingontheoffensiveelementsofcomputersecurityisthereforeabettervehiclefortechevangelismtostudentsinamericanhighschoolsfurtherwebelievethatanunderstandingofoffensivetechnijuesisessentialformountinganeffectivedefenseandthatthetoolsandconfigurationfocusencounteredindefensivecompetitionsdoesnotleadstudentstoknowtheirenemyaseffectivelyasteachingthemtoactivelythinklikeanattackerpicoctfisanoffensivelyorientedhighschoolcomputersecuritycompetitionthatseekstogenerateinterestincomputerscienceamonghighschoolersteachingthemenoughaboutcomputersecuritytopijuetheircuriositymotivatingthemtoexploreontheirownandenablingthemtobetterdefendtheirmachinestheflagispicoCTF{N6R4M_4N41Y515_15_73D10U5_42EA1770}

Decryption key:
xhkyjlenqiowugmvbdsfapcztr

picoCTF{N6R4M_4N41Y515_15_73D10U5_42EA1770}

Graph View