Blind Signature Schemes
Blind signature schemes exist for many public key signing protocols. Some examples are provided below. In each example, the message to be signed is contained in the value m. m is considered to be some legitimate input to the signature function. As an analogy, consider that Alice has a letter which should be signed by an authority (say Bob), but Alice does not want to reveal the content of the letter to Bob. She can place the letter in an envelope lined with carbon paper and send it to Bob. Bob will sign the outside of the carbon envelope without opening it and then send it back to Alice. Alice can then open it to find the letter signed by Bob, but without Bob having seen its contents.
More formally a blind signature scheme is a cryptographic protocol that involves two parties, a user Alice that wants to obtain signatures on her messages, and a signer Bob that is in possession of his secret signing key. At the end of the protocol Alice obtains a signature on m without Bob learning anything about the message. This intuition of not learning anything is hard to capture in mathematical terms. The usual approach is to show that for every (adversarial) signer, there exists a simulator that can output the same information as the signer. This is similar to the way zero-knowledge is defined in zero-knowledge proof systems.
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