In 2009, the Indian government embarked on an ambitious task to register all of the country's billion-plus citizens with a unique digital ID.

Aadhaar - which means foundation in many Indian languages - became the world's largest ever biometrics project.

It allowed millions of people to open bank accounts or access a mobile connection for the very first time.

But the project also attracted considerable opposition from privacy advocates and civil rights groups, who brought a case that went all the way to India's Supreme Court.

Dan Hardoon speaks to Nandan Nilekani, who chaired the Aadhaar project.

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(Photo: Aadhaar system. Credit: Getty Images)

How a billion Indians got a digital ID

Witness History