When we meet our friends, family or colleagues, we recognise them on the basis of what they look, talk or move like; we know who they are and need no further authentication. New people introduce themselves to us and next time we see them we, hopefully, remember them. This interpersonal network also provides reputational measures of authentication. One example of this is when young people apply for a British passport for the first time, they need the ‘countersignatures’ of who has known them personally for at least two years to endorse the passport application form and passport photographs. A second example comes from Facebook which asks you - if your account has been hacked - to recognise three pictures of your ‘friends’ to prove that your profile is yours.
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