There was a recent question about trusting (or not) a website showing an SHA-1 certificate. I would like to know whether I understand the actual issue correctly.
The dangers are (a) that someone manages to redirect me to a different site, produces a fake certificate, and as a result I get a totally secure connection to a website run by evil hackers. And (b) that I connect to the site I want to connect to, but evil hackers decode what I send and receive.
Is it correct that hackers could at a huge but not impossible cost create a fake SHA-1 certificate for some small website, and at the same cost create a fake SHA-1 certificate for www.amazon.com? Since they would redirect me to their site, the security of the actual website would be completely irrelevant. That small website, and amazon's website, could have unbreakable security, which doesn't help since I never actually manage to contact them. In other words, if I see an SHA-1 certificate for Amazon, then it is most likely fake.
On the other hand, if that small website, or www.amazon.com, genuinely used an SHA-1 certificate, would that make it easier to decrypt the messages that I exchange with the site? Could a hacker somehow "break" the certificates and then easily decrypt all messages exchanged with amazon? Or is the communication just as safely encrypted independent of the certificate?
Thanks, Gordon, for answering the question: So using a low quality certificate on your site doesn't make your site any less secure. The problem is indirect: Low quality certificates could be forged, therefore browsers (and users) should reject low quality certificates, but as long as many sites use low quality certificates, browsers can't do that.