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Hidden in Plain Sight

Challenge​

Challenge link (login required):

https://utctf.live/challenges#Hidden%20%F3%A0%81%B5%F3%A0%81%B4%F3%A0%81%A6%F3%A0%81%AC%F3%A0%81%A1%F3%A0%81%A7%F3%A0%81%BB%F3%A0%80%B1%F3%A0%81%AE%F3%A0%81%B6%F3%A0%80%B1%F3%A0%81%B3%F3%A0%80%B1%F3%A0%81%A2%F3%A0%81%AC%F3%A0%80%B3%F3%A0%81%9F%F3%A0%81%B5%F3%A0%81%AE%F3%A0%80%B1%F3%A0%81%A3%F3%A0%80%B0%F3%A0%81%A4%F3%A0%80%B3%F3%A0%81%BDin%20Plain%20Sight-25

The challenge page is behind login, but the useful artifact is the weird Unicode sequence embedded directly in the challenge title/URL.

Solution​

The encoded characters are Unicode tag characters (in the U+E0000 range), commonly used to hide plain ASCII text.

Decode logic:

  • URL decode the challenge title fragment.
  • Keep only characters with codepoint between 0xE0000 and 0xE007F.
  • For each such character, compute chr(ord(c) - 0xE0000).
import urllib.parse

s = "Hidden%20%F3%A0%81%B5%F3%A0%81%B4%F3%A0%81%A6%F3%A0%81%AC%F3%A0%81%A1%F3%A0%81%A7%F3%A0%81%BB%F3%A0%80%B1%F3%A0%81%AE%F3%A0%81%B6%F3%A0%80%B1%F3%A0%81%B3%F3%A0%80%B1%F3%A0%81%A2%F3%A0%81%AC%F3%A0%80%B3%F3%A0%81%9F%F3%A0%81%B5%F3%A0%81%AE%F3%A0%80%B1%F3%A0%81%A3%F3%A0%80%B0%F3%A0%81%A4%F3%A0%80%B3%F3%A0%81%BDin%20Plain%20Sight-25"
text = urllib.parse.unquote(s)

hidden = "".join(
    chr(ord(c) - 0xE0000)
    for c in text
    if 0xE0000 <= ord(c) <= 0xE007F
)

print(hidden)

Output:

utflag{1nv1s1bl3_un1c0d3}

Flag:

utflag{1nv1s1bl3_un1c0d3}