[JoshuaDavid]

If the website uses SSL, would it be possible to prove that the server signed the particular sequence of bytes you received? That doesn't prove that nobody modified the data but it does prove that anyone who did was able to sign things with a key that nobody else should have access to.

[drdeca]

My understanding is that with usual SSL, it seems that one cannot make a proof based on your interaction with the server that you could use to convince a skeptic at some undetermined later time, However, others here have mentioned the tlsnotary idea which, if you are interacting with the person you want to prove it to live while you get the info from the server, then you can. And, if the server supports the TLS-N extension, then you can instead make a proof that should convince arbitrary people later.