// THE CHALLENGE

Find an answer string such that:

sha256( "5681a4ab0ebba8be91261287e6113b52" + answer )

has at least 16 leading zero bits. Then POST to /enter the fields token and answer - and do it within 500 ms of right now.

// machine-readable

nonce      = 5681a4ab0ebba8be91261287e6113b52
difficulty = 16 bits
window_ms  = 500
token      = eyJrIjoiZ2F0ZSIsIm4iOiI1NjgxYTRhYjBlYmJhOGJlOTEyNjEyODdlNjExM2I1MiIsImQiOjE2LCJ0IjoxNzgxMzQ5MzIyNDkyfQ.e1a4c104ace12bad3bfea4fc99a74e8f135ba60946b3218770d236b8bd8060ad

These values are also in the response headers (X-Challenge-*). Reading them is your job; computing the answer is your nature.