A person opening a document should not have to become a cryptographer to assess the service delivering it. On February 23, 2017, Google and CWI researchers announced two PDFs with different contents but the same SHA-1 digest. Their constructed collision exposed a weakness in a method systems use to distinguish files; it did not demonstrate that every existing document could be forged. [announcing-first-sha1-collision]

Google said it had added detection for this PDF collision technique to Gmail and GSuite, and offered a public detector. Those were specific protective measures claimed by the vendor, not independently tested guarantees. They locate one practical responsibility with the service handling the attachment, rather than merely telling its recipient that an old algorithm has become unsafe. [announcing-first-sha1-collision]

Browser protection addressed another point of contact. Google’s November 16, 2016 plan assigned removal of trust for public-CA SHA-1 certificates to Chrome 56’s release sequence, not a simultaneous calendar switch. Website operators were urged to check their certificates and request SHA-256 replacements from their certification authorities. That is work for operators and providers, not a repair visitors can make to someone else’s website. [sha-1-certificates-in-chrome]

These complementary accounts share Google’s institutional origin. Our public-interest judgment is to ask providers which protection they supply: detecting this crafted PDF technique, replacing a website certificate, or migrating other SHA-1 uses. The earlier browser plan does not establish what every installed client did by February 23. A useful answer names the service-level action and its scope, rather than passing a general security warning down to the person opening a file. [announcing-first-sha1-collision][sha-1-certificates-in-chrome]

Source documents

  1. Announcing the first SHA1 collisionhttps://security.googleblog.com/2017/02/announcing-first-sha1-collision.html
  2. SHA-1 Certificates in Chromehttps://security.googleblog.com/2016/11/sha-1-certificates-in-chrome.html