Google recently announced the gradual sunsetting of the SHA-1 cryptographic hash algorithm, and we're expecting other browsers to follow suit shortly.
Cryptographic algorithms are used to encrypt messages that flow between your browser and web servers when you are connected over an https connection. However, not all https connections are the same because there are different types of cryptographic algorithms available, and most modern browser support more than one of them.
Google have made the decision to start sunsetting SHA-1 because it's simply not as has been known to be considerably weaker than it was designed to be since at least 2005.
As such, we (and many other web service providers) have begun to migrate all of our services to exclusively use the newer and much stronger SHA-256 cryptographic hash algorithm. This transition has already begun, and will conclude shortly.
Because we can see what types and versions of browser are being used to access our services, we are confident that there will be no interruption to end users by this transition. But if you do find yourself getting https certificate errors over the next few weeks, please do get in touch and we can work through it with you.
For more information on SHA-1 deprecation and how it might affect you, it's worth reading this detailed article from the security experts at Qualys.