# [Python 2], 5 bytes ([cracked by tsh][1])

<!-- language-all: lang-python -->

    bin()

[Try it online!][TIO-kh5kilxu]

[Python 2]: https://docs.python.org/2/
[TIO-kh5kilxu]: https://tio.run/##K6gsycjPM/r/PykzT0Pz/38A "Python 2 – Try It Online"

Banned: all characters *except* alphanumeric characters

    abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789

So, you're allowed the above characters and nothing else (in addition to `bin()` itself).

(This idea isn't mine, I remember it from an earlier challenge, but I don't remember who deserves credit for it.)


  [1]: https://codegolf.stackexchange.com/a/214817/20260