India hopes to assign each of its 1.2 billion people a unique 12-digit ID number based on digital fingerprints and iris scans, in what many specialists consider the most technologically and logistically complex national identification effort ever attempted.

The sign-up effort is already under way in a handful of districts. In one early registration drive in Nagaram, dozens of people streamed into a government office one recent afternoon to have their fingerprints taken and irises scanned. Sanjit Das/Panos for The Wall Street...