This question came up today... For added security it seems reasonable (imperative?) to implement a two factor authentication (2FA) mechanism. Google, Microsoft, etc. have been out some time with their protocols. There are third party products like RSA, Yubikey that also can be used. From the point of view of minimizing one's online profile, does the use of these mechanisms leave a "fingerprint" that would associate the user to the various services in which 2FA was employed? Is one service superior in this regard (i.e. no fingerprints)? Don't know how this works and thought someone here would have some insight. Thanks.