That's the principle of two-factor authentication, which is getting big in the. It works alongside usernames and passwords as a second level of security, helping to ensure that a user of a service such as e-banking is the person they say they are.
The second level of security can be anything--from hardware that generates a single-use number to a series of pre-selected security questions. Typically, though, it is the--tokens and password/PIN generators--that are most commonly discussed.
It's early days for the tech, but it already has its critics. Back in 2005, security guru Bruce Schneier gave it a vote of no confidence, saying it is "designed to solve the issues from 10 years ago."
Natasha Lomas of Silicon.com reported from London.