Cops and Robbers: Redacted Primality (Cops' Thread)
8086 DOS COM, 87 bytes, score 19/87 ~= 0.2183

Cracked by NieDzejkob

1└╣██1█╛ü ¼&lt;█t&lt;&lt; u≈¼&lt;█t█,0|/&lt;██+ô≈ßô☺├δδâ√█|█╞█S█Y╣██9┘t█ë╪1╥≈±○╥t█Aδ∩╞█S█N┤█║S█═!├A
$


This is a COM program; expects number as a command line argument, outputs Y or N. Limit: 65535 because 16 bit processor (sizeof(int) would be 2). Newline is 0x0D 0x0A on this platform. Yes you count 20 █ instead of 19 █. One of them's a real █ and has not been substituted. Muhahaha.

The space in position 10 is actually a NUL byte. The symbol for NUL is the same as space in the old VGA font.