Challenge

We receive a block of text that looks like Base64 but doesn’t decode cleanly to ASCII.

Approach

  1. Base64-decode the ciphertext — the result is still non-printable.
  2. Recognize the output as Base91 (note the character set and ~ terminator).
  3. Base91-decode the intermediate result to get the flag.
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import base64, base91  # pip install pybase91

ct = "..."  # given ciphertext
step1 = base64.b64decode(ct)
flag  = base91.decode(step1.decode())
print(flag)

Flag

boroCTF{B@5ics_0f_B@si6s}