[difficulty: 6][protection: int1, hash]
I'm 14 again ruining any chance of being awake tomorrow at school due to a VX zine that I'm nowhere near comprehending. The 16-bit world lives on with ZeroCoder's 425 bytes of frustration that he calls his v10.0 crackme.
It sets up an int1 handler and uses pushf/popf to set the TF. A simple NOP invokes the handler which retrieves "instructions" from an area in the crackme and executes them. Yea it's like a VM, but there are only four instructions. Pretty damn cool so far.
Enter the pain. A simple hash calculation involving add/xor is used with inputs and outputs all over the place (from the crackme, from a supplied password, from the calculations of previous hashes, to the goodboy message, to temporary areas). You don't know what the goodboy is, what characters are in the goodboy or the password, or even what the length of the password is (except that it's capped at 10).
Anyways, it is solvable, although with a little brute-force and guesswork. Like many before it, I could never admit how much time was spent on this thing :P
SIDE NOTES: It was pretty cool using debug.exe and recognizing some of its influence in WinDBG. For a cool description of why interleaving add/xor is so powerful, check out the paper "Block Ciphers and Cryptanalysis" by Fauzan Mirza.