A Guide To Blind Signature And Its Importance

By Prashant Jha

Some of the most widely used cryptographic algorithms are often public-key in nature, for example, the RSA Algorithm. This, and some other algorithms use something known as the blind signature scheme. Here, we are discussing a basic guide to blind signature and why it is important and useful in the first place.

What is Blind Signature?

David Chaum came up with the concept of blind signature in cryptography. It is basically a kind of a digital signature where the person signing it cannot view the message at the time of signing. In more concise terms, the message is blinded or covered up before it can be signed. The blind signature that is attached to the message can eventually be publicly matched with the original plaintext, just as you would do with a normal digital signature. Blind signature schemes are commonly employed in situations where privacy protocols have the message sender and the signer as entirely different entities. Some of the most common uses of these schemes include digital cash schemes of even cryptographic election systems.

In real-life, for example, the action of an election official signing a voter’s ballot paper without peeking into the vote cast by him, just by seeing the voter’s identity details, would mimic the use of a blind digital signature. Later, the officials responsible for counting votes can be sure that the ballot paper is a legitimate one, even though the signer did not even see the contents of it.

Blind signature helps promote anonymity by providing for unlinkability, where the signing entity cannot connect the blinded message to its decrypted plaintext version. While RSA is one of the algorithms that use blind signature scheme, it isn’t the only one. One other example is the DSA, one of the oldest cryptographic algorithms there is. Before signing, the message is processed with a randomly generated blinding factor and covered up. Then the signer receives it and uses the usual signature algorithm for signing it. The signer’s public key later serves as the means of verifying the validity of it. While it is a rather effective means of securing sensitive data, the blinding attack on the RSA algorithm, which is one of the methods used in attacking RSA, can trick the signer into revealing the message plaintext by blind signing another message.

How does Blind Signatures work?

For example, if a person X wants to send a message to Y, with a signature from person A. However, X does not want A to view the contents of the message. So message m is blinded using random blinding factor b. Then A signs it with his key a without reading the contents, resulting in message mb(a). Then X unblinds the message by using b again, reducing it to m(a).

Why is Blind Signature Important?

Blind Signature is important in ascribing a degree of anonymity and privacy to sensitive information involving transactions that the parties would like to keep private. For digital cash use, blind signature schemes are frequently used to protect the anonymity needs of senders and recipients while still making sure the transaction is valid.

In cryptographic voting systems too, just like in the ballot paper voting example, blind signature schemes serve a similar purpose by making sure votes cast are kept anonymous yet authentic.