haXXor

you either know it or not take this and get your flag 5552415c2b3525105a4657071b3e0b5f494b034515

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As the name suggests, it's XOR.

We know the plaintext starts with darkCTF{ so using this as the key as so you'll get the actual key outputted- see here

Therefore using the key 1337hack gives us the flag- see here

Flag:

darkCTF{kud0s_h4xx0r}