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write-ups-challenges-2019-2020/DecryptThis
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Robbe De Greef be3cd0d84b initial commit
2022-11-24 22:43:03 +01:00
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README.md

CHALLENGE DecryptThis

A moderate encryption-challange.

Description

I made an awesome encryption tool to send secret messages to my mother, but forgot to make a decryption tool... Anyway, now I'm stuck with these messages without knowing what they mean.

  • >tmGUYjG<V1Y=S6P>QZtc|;6y?tFE<V)HxFH8t@Gd6>')IB5ucG7H,JT
  • Bx1inr2m[u"J_u:TBU3MQb(QZX7S8@iz7/Q$hXW3d{eC:-5AgN8\H:d9LS
  • rJ6nrv4oLfDlMcMgFYHbizEn/M8ImT9eZ4EIH5j]2m9Af^KL6WHG@]~99SNt

Flag

IG{DeWielenVanDeBusGaanRondEnRond_RondEnRond}

Given files

decrypt.php Should be uploaded to a server first.

How the challenge works

The encryption is a variant of Vignere, where the codeword is 7-14 characters and is hidden in the message. EG. if the coded message is xxxxxxxxxx and the codeword ABC, the code will be xAxBxCxxxxxxCx. The second last character represents the lenth of the codeword (here C = 3). The best way to solve would be to try a few strings containing only one character like AAAAAAAAAAAA to figure out how the encryption works. As the encrypted flags (this ia 3 times the same flag) are quiet long, quickly programming an encryption tool will save a lot of time.