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What should be the mechanism for login in a shared smartphone or tablet? Should it be username + password as it is in PCs or should be enough using a password / PIN?

I'm interested on both security and UX but mostly in the balanced tradeoff between both of them.

EDIT: I'm thinking about the concept (not an actual product) of a shared device (same as shared workstation).

Scenario 1: A company has a pool device with data (mostly) in the cloud and decides to control who access the device.

Shall go to traditional username + password login or is there something else that can be done? Of course username / password would work but trying to improve UX (simplify).

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what are we logging into? The operating system on the tablet it self? An app? A web site? – DA01 Jan 16 '13 at 4:36
System (I've updated original question for including more information about a possible scenario) – OnaBai Jan 16 '13 at 9:08

Android pattern lock is quite user friendly, where user is asked to connect the dots on the screen without lifting his finger up. As long as the pattern is suggested by the system when you create the user and is unique, it should be secure too.

Biometrics based authentication like voice recognition, fingerprints etc could be another secure and user friendly method of login in shared mobile devices.

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As far as I see it, pattern lock replaces pin/password but does not solve the question of multiple users accessing the device. – OnaBai Jan 16 '13 at 9:09
@OnaBai Why not? Each pattern (which will be given by the system to user) will identify the user. – ripu1581 Jan 16 '13 at 9:38
Oops! I missed the critical suggested by the system Sorry! Now another question: is a password suggested by the system a good UX (might be hard to remember)? – OnaBai Jan 16 '13 at 9:44
@OnaBai I guess system has to only generate a sequence of numbers between 0-9 upto 8-10 digits. As long as system is keeping in mind that in a 3X3 matrix (1,2,3 \n 4,5,6 \n 7,8,9) 9 cannot be reached from 1 and generate passwords accordingly, usability should not be a problem – ripu1581 Jan 16 '13 at 9:47
Do you easily remember a sequence of digits? Since it is a password provided by the system, you should remember it just after seeing and typing it for less than a minute. The very first thing that I (personally) do when I get a pin is changing it (and I don't share a single password between accounts) – OnaBai Jan 16 '13 at 9:52

The only solution not involving any biometric check with some form of security is to have a username + password/pin. The moment there is more than one user, you need to know which user you are logging in.

The username doesn't have to be text in a traditional sense. You could use an image if you prefer, and that would work fine.

If you are open to biometrics then either a fingerprint or a retina scan would work well, but I don't know of any mobile devices that have that feature.

For a more detailed discussion, see Computer login with password only, without username

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If the question is having a password unique enough (as fingerprint and retina scan) would be enough having a password long enough or lets say... a signature? – OnaBai Jan 20 '13 at 23:15

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