Father of online anonymity David Chaum has brought in the revolutionary change in cryptography by introducing the concept of Blind signature. Among many of his notable contribution such as digi cash, new digital signature, anonymous communication, trust worthy voting system the ground concept lingers around maintaining the anonymity. Blind signature is that very concept which serve the premise of researches and security protocol which aims to maintain anonymity of the send message.
Blind signature is technique of signing message after blinding it with blinding factor. The message is typically blinded with a random number and then sent to the authority (say B) to sign this message.
Suppose A wants the message M to be signed by B but does not want B to read the message M. B will blind the message M with some factor D. The message becomes blind (M, D). B will receive blind(M, D) and sign it with its privacy key K the message become sign(K,blind(M, D)). Sender A will now decrypt the message with blinding factor D and will left with sign(K,M). Blind signature finds it application in privacy related protocols where sender and signer are different parties.
Blind signature is typically employed where sender’s anonymity is a great deal. This is useful in cryptographic election and digital cash flow.
Blind Signature has four phase…
Blinding phase, signing phase, Unblinding phase, Verification phase
Blinding phase: the sender needs a blinding factor to blind his message so that the signer could not see his message.
Signing Phase: when the signer gets encrypted message, he signed the message with his key.
Unblinding phase: The sender uses the blind factor to recover signer’s digital signature.
Verification: anyone can verify the signer’s signature through its public key.
How good is blind signature?
Blind signature scheme is only save to use if certain properties are secure. The parameters which could measures security of a blind signature are
a) Strong masking factor/blinding factor: the blinding factor must be unable to extract.
b) Powerful electronic signature: the electronic signature must be incompetent so that only signing authority can sign the message
c) Blindness: most important factor which guaranteed that the variable that signing authority receives during protocol development and the resulting signature are independent
d) Indistinct: to maintain the anonymity. Inability to determine the data which is provided in the message
Attacks on blind signature are
a) Attacks on selected message: This attack is gone for getting the secret key. In any convention to get a secret signature key depends on the numerical issues to an extensive computational.
b) Attack on the basis of predicted variable: this attack is based on predicting the random variable which has been used to blind the message
c) Signature morphing: this attack is replacing the user can replace the masked signed message.
How Blind Signature find its application?
Blind signature in electronic voting system
In electronic voting system ballot are signed by election authority before it can be accepted for counting. This allow authority to ensure that only certified voters have voted and have submitted only one vote. Simultaneously, authority must not know the voter’s selection.
Blind signature for untraceable payment in an untraceable electronic cash protocol based on blind signatures, an identified customer can withdraw a blinded electronic cash from the bank and the unblinding operation is adopted by the customer to transform the blinded electronic cash into a valid one. Before performing the operation, the blinded electronic cash is protected well since attackers cannot convert it into a valid electronic cash without the blinding factor corresponding to the operation.
Blind signature cryptosystem improved auditability compared to current system and at the same time hides the payment details.
