Blind Signatures

A blind signature is a special kind of digital signature created so that the signer cannot see what they are signing. First proposed by David Chaum, this process involves disguising (or “blinding”) a message before it’s sent for signing. When the signer adds their signature, they do so without knowing the actual content. Later, the blind can be removed, and anyone can verify that the signature matches the original, unhidden message—just like with a standard digital signature.

Blind signatures play a key role in applications where privacy is essential. In digital cash systems, for example, they let someone prove they own a token or “coin” without letting the bank know its details, preventing tracking of individual payments. In online voting, blind signatures can be used so an election authority can confirm a ballot comes from a registered voter, but without learning how that person voted.

To illustrate, imagine Alice wants to cast a secure online vote. She blinds her ballot and sends it to the voting authority, Bob. Bob checks that Alice is eligible but cannot see her vote’s content. After confirming her identity, Bob signs the blinded ballot. Alice then removes the blinding, revealing a signed ballot that proves it’s authentic, but still keeps her chosen vote private when submitting it.