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Can you see any obvious challenges to requiring 2+ users to confirm authentication requests using an association such as a friendship type of relationship.

A practical application would be an Enterprise SAAS application that requires another user in the same company to authorise his login request within a short period of time? It would then be up the the users company to decide how many authorisation confirmations they require. (Usability vs Security)

And edge case would be if a company only had a single user, they could simply register for 2 accounts, ideally using 2 seperate machines on different networks (LTE & ADSL).

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I see two problems with the proposal: normal users will not like it at all, and attackers will not find it hard to work around.

Usability is decreased in that any user must have his colleagues/friends around when he wants to log in. Multi-user approval is a valid model for initial registration (old-style clubs work that way: you can enter only if one or two existing members approve it) or when the system is especially critical (e.g. in a Titan II launch complex, two officers had to turn their "launch keys" simultaneously to trigger the ICBM lift-off), but for everyday tasks users will find it overly cumbersome. When users are employees, they will grumble and be unproductive; when users are customers, they will wander away.

Security is not much increased for the reason that you state at the end: even if the server sees the requests as coming from different users, it cannot check whether they come from different humans. Attackers who succeed in stealing some credentials can often steal several sets of credentials at the same time; for instance, if the attacker could plant a keylogger on a desktop computer, then he will grab all the passwords typed on that computer. If multi-login is enforced, then it is probable that in many cases, the two involved users will type their passwords on the same computer ("Hey, Bob, while you're here, would you type in your password ? I must update sales report 6X42.").

This contrasts with two-factor authentication in which the user login is controlled by two distinct types of credentials. The extra security comes from this type diversity; requiring authentication from two distinct users does not provide this type diversity.

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The principal difference of multi-user login vs. single-user login that I can see immediately is separating phases of initiating request for access and granting access after another user's authorization. In terms of security, this difference immediately demands special attention to protect integrity of first user's login session. Basically, the authenticating system must ensure that person logging in after authorization is still the same person who initiated the login. Otherwise, it would be vulnerable to session hijacking attacks.

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