![]() ![]() ![]() The U2F device also concatenates the counter value on to the hash of the client data before signing so that the origin can strongly verify that the counter value was not tampered with (by the browser). The U2F device sends the actual counter value back to the browser which relays it to the origin after every signing operation. The U2F device remembers a count of the number of signature operations it has performed - either per key pair (if it has sufficient memory) or globally (if it has a memory constraint, this leaks some privacy across keys) or even something in between (e.g., buckets of keys sharing a counter, with a bit less privacy leakage). The U2F device protocol incorporates a usage counter to allow the origin to detect problems in some circumstances. When you sign in, 2-Step verification helps make sure your personal information stays private, safe and secure. It’s already supported in Chrome, Firefox, and Opera for Google, Facebook, Dropbox, and GitHub accounts. ![]() These tokens can use USB, NFC, or Bluetooth to provide two-factor authentication across a variety of services. I would be more concerned about the privacy of the usage counter: U2F is a new standard for universal two-factor authentication tokens. Click Signing in to Google on the My Account page, and then click 2-Step Verificationor just click here to head straight to that page. Click the profile picture in the upper-right corner of any Google page and select My Account to view information about your account. Anyway, it's the same problem as with client authentication in TLS. Head to and sign in with your Google account. Can they? Yes - by sending multiple user handles of the same origin during the key exchange and seeing which are signed back.
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