How secure is a lagged fibonacci sequence for encrypting brief messages?
Your key only seems to consist of n, which should be easily brute forced. So not very secure is the only answer that can be given. n could be very large (a 128 bit number or so), but in that case this scheme seems very inefficient.

Please note the remarks on this kind of generator here and here. So even if it is secure, it may be very tricky to show that it is secure (for a large n). There are plenty of better stream ciphers to choose from.