Implicit declaration of function 'sqlite3_key'?
If you are using SQLCipher, you need to define SQLITE_HAS_CODEC in your application's C Flags. Thats all.
Yes, that is the reason you are getting that compiler warning. The function sqlite3_key() is not defined in the version of libsqlite3 included with iOS. Adding in a function declaration isn't going to help-- it would fix that compiler warning, but it would just mean you'll get a linker error since the function isn't defined anywhere.
If you purchased SEE you could probably build your own copy of SQLite, embed it in your app, and just not use the system's libsqlite3. That this would mean you'd have to say "yes" when the app store submission process asks if your app includes encryption, meaning extra paperwork and time before you could submit the app. I'm not certain whether there's any clear indication of whether Apple would accept it even then-- probably they would, but they've been known to surprise people.