How can I break REDSHIRT / REDSHIRT2 encryption?
I'm not going to explain this but after looking at redshirt.c; the core part of the program is that after skipping the header (which is "REDSHIRT" (9 bytes includes an ending null byte)), it has a loop which does:

int j;
j = (j &amp; 0x7F) + (~j &amp; 0x80);


Which, if you don't recognize it, maps 128-255 to 0-127 and 0-127 to 128-255. (I think this might be the same as swapping the nibbles of a byte.) and is the same as j = (j + 128) % 256.