French

This is a writeup of the crypto challenge french from the Security Valley CTF

Premise

Haha, tu n’y arriveras jamais… this old french men thinks he can troll us. He sends messages and messages again. Show him that you are awesome by breaking the message. Hint: Flag content should be lowercase.

Link: https://github.com/SecurityValley/PublicCTFChallenges/tree/master/crypto/french

Challenge ciphertext:

VIERSZPUCWGBTZPWIQVLGUNQETXVEVGDHGPZGGJIVOKFXZSAQJVFNIIISHVIUHBTYKPLKRIGGFVJCFVIOGVXMEHVUTWHRMSGMAIXQFRXSCZRVLGANYXVVJKXJOOQSTONFIQTQJEUPLDEVHYJWYMCUXJSTWIVHLVVCRRKIUIECXKCAMEJWGQTRSQFPCWUKTRWALXFXUGWOOYQTCEAGXQTAFFFSJJMNSGMITSAIVGGFTJKLRTIRIOQMTIAFPGGFQCUIOCXGGGMMJEYCVOWALGYEVPSHSIJRKWGJIUICWIDIPJEPQRQPFVUCWUSPWIKPLFMUDNYGYIQVAQXRIMBRVILVGGMIXYNTHKOAXSWTRCGGOAINLWGKGGWAYLVKNNEZMGTWVXGNIVVRHSEJYKGVWGNWRTRTMQRBKGZZVNACFEJFVPFREESFMMGWFVVKYVSKWVBOEJWQIIEFNUIJOIJAFRGJIKFSNVJXIKGVCEDEXEVPWVHUJIMMYIENOPYMTIZRMTSLTYTSHNHWGRFRROVPWMMJFPBIECWHZNLOVICKROWAIXYEGASWBRJHKSSQVOOGYLVJYCKDSSTVVWHDQKHGNRXHHTMPUGMISEGVPGFRGICWCKIUANSEXIQVSUHRFPJIPTIVDYFRJXBVLGSZUMIIFWPVWZFXVARCTQBGMIUINVLUHNWEEEEOSTSQXTRGRUXCHVTRNMGJIPCHLLGSJGVVCQJWKVBAEPSAYMIICNEPSGUYIWHGHDMGMIVQCKVGGFNRZWGGVCURSXJTEKRESFXPVMNTEESFMSDINDSCFQMIIWGCVUVVUGLWGQHKOATJKLRUXQZRSTCEAUXJOGHEEWNXIJSEUIFTYGEPREJWKSEGJTSRISDXBVLGUNQEOC

Solution

Initially, we go to dcodes cipher identifier and enter the ciphertext. The top result is Vigenere, who also happens to be a 16th-century French cryptographer so we investigate this further. Using the site boxentriq, we can bruteforce our way into finding the key required to decipher the text with the Autosolve (without key) button. After finding the key, we can decode the text, which contains the line: YOUCOULDUSE[REDACTED]ASFLAGKEEPINMINDTHATYOUNEEDTOFORMATTHEFLAGBEFORESUBMITTING

Which we then format so we get the actual flag: SecVal{[REDACTED]}