Ideally, you want to use constructor injection to inject the dependencies a class needs. An extension method however, is just a static method on a static class and you can't do constructor injection on static classes.
Because of this you are currently falling back to the Service Locator pattern, which is far from ideal and should be avoided whenever possible.
So you will either have to promote your class to a non-static class and your extension method to a non-static method to allow it to be injected as usual, or you will have to pass in the encryptor into the method.