I would like to publish a website. I would like the website to run over HTTPS. I have read about the requirehttpsattribute: http://msdn.microsoft.com/en-us/library/system.web.mvc.requirehttpsattribute.aspx
Is https done at the application level or does the hosting provider need to support this as well?
HTTPS (or HTTP with SSL) is regular HTTP, but there’s an extra layer in the OSI model. Before the HTTP-layer there’s the SSL layer. In reality, before you get access to the HTTP protocol you have to decrypt the SSL protocol. Since .NET works against the HTTP protocol (kind of..) someone has to decrypt the SSL protocol first, and that someone is your web server.
So yes, your hosting provider has to support SSL, and configure SSL for your application, amongst other you will probably need a dedicated ip address apart from a certificate.
No, HTTPS does not have a negative impact on SEO.