05AB1E, 28 bytes (Cracked)
#D>D>D>D>*##*#D<*#+*#n#n#**# a(1) = 1
a(2) = 8
a(3) = 42
Behaviour for zero is unspecified.
Cracked:
DD>D>D>D>****rD<*6+*6n2n5**/ There is no need for an explanation I think as the crack was almost 100% spot on. The only difference is I used r to reverse stack instead of swapping top two elements.