Jelly, 2116 bytes, A000042 (Cracked)
(uses Jelly codepage) Byte Set:
%%1@JJOOiv¤ØØȷȷḌṁị“”€%1@JOiv¤ØȷḌṁị“”€
(The two quotes are different: one is opening and the other is closing).
This yields the unary representation of the input.(uses Jelly codepage)
DifficultyThe sequence is: Only 1a(n)
is the unary representation of n
.6 quadrillion possibilities!