A book. Or any other medium of expression protected by 1st amendment, such as t-shirt [1], poetry [2], music [3]. And more broadly, any way of encoding a number in an artform via steganography [4].

[1] https://www.wired.com/2000/08/court-to-address-decss-t-shirt... [2] https://en.wikipedia.org/wiki/DeCSS_haiku [3] https://www.gamedev.net/forums/topic/138581-decss-midi/ [4] https://en.wikipedia.org/wiki/Steganography