Python 3, 73 bytes, cracked by pxeger
Hopefully this time there are no loopholes.
s=input()
assert not{*s}&{*'()[]{}'}
eval(s,{'__builtins__':{}})(**{0:0})
Just as before, the password is inputted via STDIN, the flag is (nothing) and should be printed to STDERR. That means no output to STDERR.