Text from ctf:

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I present to you my new encryption scheme. It again works with base64 so supports arbitrary data and is extremely simple. First the plaintext payload is encoded with the usual A-Za-z0-9+/= charset b64 but then a random substitution is applied to all characters bar padding,  for example "Hello World!" would in step become "SGVsbG8gV29ybGQ=" then "t5qTk5DfqJCNk5v=" with an atbash like substitution of "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" to "/+9876543210zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA".

Unlucky for you though the substitution mapping used here is completely random but because I'm confident in the encryption's resilience I'll let you know that it only carries capital letters and spaces.

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

""""

The result of this when combined with the flag format of DCTF{SOMETHING} will give the MD5 of 906bdf61bf235276bb65b230bff996d8
Credit to https://www.reddit.com/user/YefimShifrin/ for the original idea for the challenge