The SHA-1 hash function has long been used for cryptographic and data integrity purposes. Unfortunately in recent years the algorithm has been identified as possessing sufficient vulnerability to attack that users should replace it with a newer protocol (SHA-256). Companies have been working towards adopting a replacement for several years, however one problem is dealing with the millions of devices using the deprecated function. If companies mandate a more secure protocol, they risk denying access to those without the ability to update their system. If companies do nothing, users remain exposed and may not feel motivated to update or look for alternatives.
Facebook’s Alex Stamos reports that his company has identified between 3%-7% of their users are using browsers that do not support the newer protocol. They have implemented technology to use SHA-256 where available, and only use SHA-1 as a last resort. Twitter reports a similar number of their users are also using older devices limited to SHA-1.
In collaboration with CloudFlare, the three companies have issued an amended proposal to the CA/Browser Forum (Certification Authority Browser Forum) to provide a transition that addresses the needs of the users limited to SHA-1:
- The proposal ensures the continued migration to SHA-256 for all mainstream devices.
- It only allows legacy validated SHA-1 certificates when a domain also provides SHA-256 support.
- Legacy validated SHA-1 certificates are only available per specific requirements and will still sunset in March, 2019.
- Increased randomization of serial numbers in legacy validated certificates results in less probable SHA-1 collisions.
- In the event an attack is discovered that forces the improper use of SHA-1 certificates, the owners would terminate use of these legacy validated certificates.
Of course despite this proposed allowance, it is important for affected users to upgrade their browser and/or device as soon as possible as policy considerations aside, they are still affected by SHA-1’s weaknesses.
