Print X without X (cop's thread)
Python 3, Score: 81, Cracked
X =  abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!&quot;#$%&amp;'*,./:;&lt;=&gt;?@[]^_`{|}~
Not terribly interesting since it has such a high score, but this should be somewhat difficult to crack...
Clarification: This is in fact a full Python 3 program, running it locally with python3 print_x_out.py produces the above output for me. Likewise, the below verifier script produces no output:
with open(&quot;print_x_out.py&quot;) as file:
    source = file.read()

output = &quot;&quot;&quot; abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!&quot;#$%&amp;'*,./:;&lt;=&gt;?@[\]^_`{|}~&quot;&quot;&quot;

for c in output:
    if c in source:
        print(f&quot;Failed verifier check for '{c}'&quot;)

Edit: Well, that was faster than expected...