NOTE: Google has since revised its plan to enable a more gradual migration to SHA256, this post is no longer accurate.
For the last few weeks there has been an ongoing discussion on the Chromium security-dev mailing list on how Google intends to implement a user interface change to warn users that a SHA1 certificate is in use.
I wont talk to the reasoning behind this change or to the current and future security properties of SHA1 in this post but I thought some folks might be interested in what this might ultimately look like. I say might because right now there is only a mail thread and who knows how things will evolve and what the copy would be in such user interfaces.
With that said the thread does describe what affordances they intend to use when a site has a certificate where it or the corresponding certificate chain has SHA1 based signature in it (excluding the root) that expires after 2016/1/1 the user interface may be “degraded” for these sessions.
At this time it seems the “red x” that is used for mixed content will be used; if so this will look something like this:
Again for SHA1 certificates that expire after 2017/1/1 if the page contains passive content (such as images) that is served over a SSL session with such a certificate it will not be loaded unless the user chooses to do so and the lock will get a yellow arrow, which will look something like this:
Which combinations of these things one will see would be dependent on the specific combination of conditions but this will give you some idea on what these changes may look like.