> Are there any UDFs out there that provide encryption support?

Probably, I haven't checked but it wouldn't be too hard to refactor a
standard implementation into the dll format required by Firebird.

> How secure
> would this be though since I am using an embedded FB database?
>

No less secure than what you are doing now. The problem is that you
must have the private key either compiled into your binary or
available through some other way. The security concern must come from
the private key being discovered. You can have the most sophisticated
security door with the best practice methods for keeping it closed,
but until you do something about the chicken wire fence along the
back, security is still a problem for you.