Re:Facebook hacker beat my 2FA, bricked my Oculus, and hit t...
by tialaramex on Saturday August 21, 2021 @05:34AM
(#28255552)
> The browser tells the Yubikey: "sign this: 'logging in to site.com at 12:34PM'"
It's even a little bit cleverer than that. During enrollment (to say, Facebook.com) your Yubikey provides a random looking "identifier" to Facebook.com, and it promises that it can sign future logins if Facebook.com shows it the same identifier. The identifier is bound to the DNS name!
So a phishing site has a few choices, none of which help the bad guys even a tiny bit:
* It claims to be Facebook.com, but it isn't, so the web browser just doesn't even show the UI for Security Keys. There's a behind the scenes Javascript error basically, "What? You aren't Facebook.com fool".
* It admits its real DNS name, and makes up a random identifier. The browser gives the random identifier and the real DNS name to your Yubikey. But, it has never heard of this combination, so, it blanks the entire authentication figuring this must be for a different Security Key plugged in on another port or something.
* It gets that identifier code for your login from Facebook, and then admits its real name to your browser and provides the identifier taken from Facebook. This still doesn't match, and the Yubikey again assumes it must be for some other Security Key on your system.
Behind the scenes this is actually done with AEAD cryptography, maybe with AES keys baked inside your Yubikey. The "identifier" is actually something like a private key (likely elliptic curve parameters) that has been encrypted using an onboard secret AES key in an AEAD mode, with the DNS name (well, a hash derived from it) as a factor.
As a result, your Yubikey can't even decrypt the "identifier" correctly in order to log you in without the matching DNS name. This means goofs in the implementation fail safe - e.g. one brand of cheap Security Keys can fail to sign in once every 256 tries on average due to a logic bug. But they'd never sign in where they shouldn't because of mathematics, to do that they'd need to "accidentally" completely break the mathematical foundations of the cryptography!
It's even a little bit cleverer than that. During enrollment (to say, Facebook.com) your Yubikey provides a random looking "identifier" to Facebook.com, and it promises that it can sign future logins if Facebook.com shows it the same identifier. The identifier is bound to the DNS name!
So a phishing site has a few choices, none of which help the bad guys even a tiny bit:
* It claims to be Facebook.com, but it isn't, so the web browser just doesn't even show the UI for Security Keys. There's a behind the scenes Javascript error basically, "What? You aren't Facebook.com fool".
* It admits its real DNS name, and makes up a random identifier. The browser gives the random identifier and the real DNS name to your Yubikey. But, it has never heard of this combination, so, it blanks the entire authentication figuring this must be for a different Security Key plugged in on another port or something.
* It gets that identifier code for your login from Facebook, and then admits its real name to your browser and provides the identifier taken from Facebook. This still doesn't match, and the Yubikey again assumes it must be for some other Security Key on your system.
Behind the scenes this is actually done with AEAD cryptography, maybe with AES keys baked inside your Yubikey. The "identifier" is actually something like a private key (likely elliptic curve parameters) that has been encrypted using an onboard secret AES key in an AEAD mode, with the DNS name (well, a hash derived from it) as a factor.
As a result, your Yubikey can't even decrypt the "identifier" correctly in order to log you in without the matching DNS name. This means goofs in the implementation fail safe - e.g. one brand of cheap Security Keys can fail to sign in once every 256 tries on average due to a logic bug. But they'd never sign in where they shouldn't because of mathematics, to do that they'd need to "accidentally" completely break the mathematical foundations of the cryptography!