Encryption algorithm using CTR mode
It looks like you are XORing a mildly modified key with the plaintext, then mildly modifying the key every 10 blocks.  As @SimonForsberg has noted, such simplistic schemes just are amateur-grade ciphers, and certainly not cryptographically secure.  (Cryptanalysis would be beyond the scope of Code Review, though.)

Only a few experts are capable of producing cryptographically secure algorithms — and even then, they submit their schemes to merciless peer review.  There's always someone out there who is smarter than you are.  Attacks only get better, never worse.  If you want a cryptographically secure scheme, don't roll your own crypto.