This PR resolves #6

It sounds like, quoting the Postgres docs:

https://www.postgresql.org/docs/10/extend-extensions.html#EXTEND-EXTENSIONS-RELOCATION

If any prerequisite extensions are listed in requires in the control file, their target schemas are appended to the initial setting of search_path. This allows their objects to be visible to the new extension's script file.

pgcrypto is listed in the control file so it should be available within this extension via @extschema@. And indeed, this fixes the issue for me.

Thanks!