(credit: Michael Rivera)

Tens of millions of Internet users will be cut off from encrypted webpages in the coming months unless sites are permitted to continue using SHA1, a cryptographic hashing function that’s being retired because it’s increasingly vulnerable to real-world forgery attacks, Facebook and Web security company CloudFlare have warned.

Facebook said as many as seven percent of the world’s browsers are unable to support the SHA256 function that serves as the new minimum requirement starting at the beginning of 2016. That translates into tens of millions of end users, and a disproportionate number of them are from developing countries still struggling to get online or protect themselves against repressive governments. CloudFlare, meanwhile, estimated that more than 37 million people won’t be able to access encrypted sites that rely on certificates signed with the new algorithm.

Both companies went on to unveil a controversial fallback mechanism that uses SHA1-based certificates to deliver HTTPS-encrypted webpages to people who still rely on outdated browsers. The remaining, much larger percentage of end users with modern browsers would be served HTTPS pages secured with SHA256 or an even stronger function. The mechanisms, which both companies are making available as open-source software, will allow websites to provide weaker HTTPS protection to older browsers while giving newer ones the added benefits of SHA256. Facebook is deploying the plan on most or all of the sites it operates, while CloudFlare will enable it by default for all of its customers. CloudFlare said other sites, including those run by Chinese portal Alibaba, are also implementing it.

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SHA1 sunset will block millions from encrypted net, Facebook warns