Difficulty : Easy
Unzip the challenge file to get getkey.txt and a password protected zip TRYME.zip.
To get the password of the zip you solve a simple RSA problem by this exploit.
This is also known as low exponent attack.
With the password you can unzip TRYME.zip and find the chall file.
The flag is divided into chunks of four and each chunk is XORed with next of the next chunk (chunk + 2)th chunk.
As flag is in usual format, you already know first 2 chunks dark and CON{ and the XORed ciphertext chunks are also given.
By XOR property, if chunk1 ^ chunk3 = ct1 then chunk3 = ct1 ^ chunk1. In this way all chunks can be recovered.
The exploit is provided here.
darkCON{n0T_Th@t_haRd_r1Ght}