Jelly, length 12 - cracked
cracked by Dennis
^[^ḊḢṚUṖṪ]*$ Python regex (although I think all flavours are valid for this pattern).
That's right, none of those lovely array manipulation atoms!
No:
Ḋequeue;Ḣead;Ṛeversal;Upend;Ṗop; orṪail;
For an input of 4 mine outputs:
1 2 3 4 8 7 6 5 9 10 11 12 16 15 14 13 ...because I formatted a list of lists as a grid with G.