Of course, the most practical solution might be to have a chip
embedded in your arm, as they do with many pets these days.

A similar idea was the subject of a SciFi book I read many years
ago. Unfortunately I cannot remember the author, but the idea was that
everyone wore a 'watch' which represented their identity. All
transactions were done using this 'watch'. Entering buildings like
theatres, using a taxi, train or bus etc automatically debited your
balance. If for any reason your balance went negative or your 'watch'
became detached from your wrist, you became a non-person.