To say nothing of SSOs, where you might not want to carry your login across, but you’d like to have the option of saying “yes, I’d like to use this account here too” and then authenticate.
Domains that want to collaborate together can still do so.
In some ways that will be less strict than Firefox's implementation. A CDN might have a certificate with hundreds of unrelated sites in it.
In some ways that will be more strict than Firefox's implementation. A site might want to serve large static files on static.foo.com but a secure login page on login.foo.com , and want to hand over the private key for static.foo.com to a CDN but not hand over the private key for login.foo.com .
Or maybe encrypting cookies using the site certificate, which would still allow cookies to be shared with domains having a different certificate, but the server needs the correct key for decryption.
Also it'll likely break javascript access to the cookies.
Adding random padding makes it harder to get a signal, but with a high enough sample size, it's still possible to get some information. If you always pad to a fixed size, then there's probably no useful information.
At the moment, I think I have enough to do, but I'll probably be looking for something new in the next few weeks. Got any novel single player video game recommendations?
That's are really big word in that sentence. You have NO idea, like 0, what is stored in an encrypted cookie. To even think you do is just pure folly.
I will provide a very simple counterpoint to your argument: say I log in, log out and log in again, dumping my two encrypted session cookies in between. If they are perfectly identical, then knowing nothing else about the environment nor the encryption used, I already know that there's no timestamp value in the cookie acting as a time barrier, nor a challenge-response check, nor a nonce value.
Knowing that, I can focus my attention on stealing session cookies from users for trying replay attacks.
The problem with that is domains are a poor way of representing ownership that can be trusted. If the web was rebuilt from scratch, a better approach might be to allow cookies to be shared between secure sites using the same certificate. But that adds more complexity that I'm not sure is worthwhile. The web can absolutely get away with not having shared cookies.