[difficulty: 2][protection: code protected with xor]
This was recently posted to crackmes.de. It is another crackme where your serial is used to decrypt some code that will eventually execute.
Specifically here, a word is made by summing your serial characters with strlen(serial). This word then is xor'd sequentially (no feedback) across 10 words.
Before execution reached the decrypted code, a one byte comparison is made for validity, revealing half of the xor key.
To get the other half, I used z0mbie's XDE engine on trial decryptions (incrementing trial keys) to see which key produced the most normal disassembly; normal meaning it consisted of typical instructions: push, pop, call, etc.
UPDATE: crackme was removed from crackmes.de since it is preferred that the real author submit - a copy is in this site's crackmes folder