If someone can’t read or write how can they be asked to sign an data consent form?

In Uganda 26% of male adults and 46% of female adults meet the UN’s definition of illiterate. During a recent street survey in Kampala we offered respondents a pen (for signature) or an ink pad (for thumb print). 114 of the 115 respondents signed opted for a pen.

What is a digital equivalent of a thumb print signature? What social stigmas would be associated with signing with a digital thumb print signature?