To your first, yes and no. Most BGP implementations are configured in such a way to protect consumption of prefixes that are larger than normal. A large prefix would be a /30 for instance in IPv4. This generally is a very specific route and is considered a bad thing in the global BGP table. Why? Because you don't advertise 2 reputable addresses at a time, you go for smaller prefixes to make the table manageable to make routing decisions on. So the no part is that generally all configurations of BGP will prevent the more specific routes like this. However it's just a numbers game of advertising a prefix one bit larger than the real advertised by, say splitting that network in half.
As for the second part of the question, no. There's no signing of any owned AS announcements. At best you can have a digest to validate your peer. But peering configurations in BGP are generally very specific, as in your peer is a host route, generally reachable directly via the transport provided by, say, a purchased circuit. So - is it trivial to swing routes on improperly configured downstream? Sure. You have to find a broken subset of routing configuration at a very critical point in the network though which would indicate a core router at a large telecom hotel is comprised or, an administrator is in cahoots with the redirect operation.
There's a lot more with regard to possibilities - but just a high level take away.
As for the second part of the question, no. There's no signing of any owned AS announcements. At best you can have a digest to validate your peer. But peering configurations in BGP are generally very specific, as in your peer is a host route, generally reachable directly via the transport provided by, say, a purchased circuit. So - is it trivial to swing routes on improperly configured downstream? Sure. You have to find a broken subset of routing configuration at a very critical point in the network though which would indicate a core router at a large telecom hotel is comprised or, an administrator is in cahoots with the redirect operation.
There's a lot more with regard to possibilities - but just a high level take away.