What is happening, and what was happening?
The SSL Industry and the CA/B Forum have planned for the "sunsetting" (depreciation) of the SHA-1 signing algorithm for quite some time. However, their plan was mainly formed around Microsoft's desires to phase it out in 2017, alongside the end-of-life for Windows XP. This was widely understood to be the approved plan for CAs to follow, and the preparation for moving from SHA-1 to its successor, SHA-2, wouldn't be necessary for many months from now.
However, Google recently made an announcement, in stark contrast to Microsoft's plan, that they are implementing their own SHA-1 sunsetting timeline, which will begin on September 26th 2014.
This timeline has three distinct stages, which will result in degraded visual indicators in Google Chrome (padlock, green-bar) for SHA-1 signed certificates meeting specific criteria (this is discussed in the section "What certificates are affected?" below).
This means it is now necessary to educate and assist our partners and customers on how to make the transition away from SHA-1.
First, let's understand what SHA-1 does. Both SHA-1 and its successor, SHA-2, are specific types of signing algorithms. Signing algorithms are used as part of the identity validation role that SSL certificates perform. They are mathematical functions (referred to as a "hash") which, when performed, should calculate a persistent and unique value for each file. So, for instance, the Word doc this text is stored in has a unique SHA-1 hash value. If I change a single part of this file – add an extra period somewhere, change a letter, etc. – it will produce a different SHA-1 hash value.
When a certificate is downloaded from a server to the client's browser, a hash is taken of it. The type of hash taken (SHA-1, SHA-2, MD5, etc.) depends on how the certificate is signed. The hash calculated by the browser is compared to the hash value provided by the server, which has been verified by the Certificate Authority (CA) at the time of issuance. If they match, the identity of the certificate and server are verified.
When is this happening?
Google's policy involves three distinct steps, the first beginning on September 26th. On this date, only customers with SHA-1 signed certificates expiring in 2017 are affected. However, the amount of affected certificates will expand in November, and again in Q1 2015. The full details on what certificates are affected is in the below section, "The Nitty Gritty."
What certificates are affected?