The below paragraph mentioning multiple devices for authentication jumps to cross-site auth without explaining the connection between the two (and I'm confused as to what you're trying to get at). It then leaps to cross-site CAPTCHA, again without explaining the link.
In this context we note that it has become common for users to access various on line services through multiple devices such as desktop and mobile computers, smart phones, tablets, and wearables such as smart watches. This proliferation provides online services multiple vectors for simple and effective user authentication, including persons with disabilities [auth-mult]. We note that several major service providers (such as Facebook) now support cross-site user authentication. As of this writing however, it appears only Google's V. 3 reCAPTCHA API provides cross-site CAPTCHA services, i.e. a service to distinguish human from robot users without actually passing specific identifying data.
The below paragraph mentioning multiple devices for authentication jumps to cross-site auth without explaining the connection between the two (and I'm confused as to what you're trying to get at). It then leaps to cross-site CAPTCHA, again without explaining the link.