Blinded Signature

A blinded signature is a form of digital signature where the message is first obfuscated, or 'blinded,' by the requester before it is sent to the signer. The signer applies their private key to this blinded data, producing a blind signature. The requester can then 'unblind' this signature, resulting in a valid, standard digital signature on the original, clear message. This process ensures the signer cannot link the final, unblinded signature back to the specific signing session, providing unlinkability.

The core mechanism relies on a mathematical blinding factor. The requester combines their original message with this random factor using a one-way function. After the blinded message is signed, the requester removes the blinding factor, which also removes its influence from the signature. The resulting signature is cryptographically identical to one created directly on the original message. This property is crucial for systems requiring anonymity, such as anonymous digital cash or privacy-preserving voting protocols.

David Chaum first introduced the concept in 1982 as the foundation for digital cash systems like eCash. In such a system, a bank can sign a blinded token representing a digital coin without knowing its serial number. The user can later spend the unblinded, signed coin, and the bank can verify its validity but cannot trace the coin back to the specific withdrawal transaction. This prevents the bank from profiling a user's spending habits.

Beyond digital cash, blinded signatures are fundamental to privacy-enhancing technologies. They are used in anonymous credential systems, certain voting protocols to ensure ballot secrecy, and even in some blockchain designs for private transactions. The RSA and Schnorr signature schemes have well-established blind variants, such as the RSA blind signature and Blind Schnorr Signature, which form the basis for many implementations.

It is important to distinguish blinded signatures from ring signatures or zero-knowledge proofs. While all enhance privacy, a blinded signature specifically protects the link between the signing act and the later use of the signature. The signer's role is limited to attesting to a message's validity without knowledge of its content, making it a key building block for systems that require both authorization and anonymity from the authorizing party.

A blinded signature is a form of digital signature where the message is first obfuscated, or "blinded," by the requester before it is sent to the signer. The signer applies their private key to this blinded data, creating a blind signature. Crucially, the signer cannot view or link the blinded data to the original, unblinded message. The requester then performs an unblinding operation on the signed, blinded data, which yields a valid, standard digital signature on the original, clear message. This final signature is cryptographically verifiable by anyone with the signer's public key, just like a normal signature.

The core mechanism relies on a mathematical blinding factor, a random value known only to the requester. This factor is used to transform the original message hash. The process ensures that the signer's output is a function of the blinded input, and the unblinding step correctly removes the blinding factor's influence, leaving a signature that matches the original hash. Common implementations use cryptographic schemes like RSA Blind Signatures or those based on elliptic curves, such as those used in privacy coins like Monero for its ring signatures.

The primary use case for blinded signatures is in systems requiring unlinkable authorization. The classic example is a digital cash or e-voting system. A user can obtain a signature from a bank on a coin (the message) without revealing which specific coin it is. Later, when the user spends that coin, the merchant can verify the bank's signature, but the bank cannot trace the spent coin back to the initial withdrawal request. This provides payer anonymity while maintaining the integrity and unforgeability guaranteed by the bank's digital signature.

Beyond digital cash, blinded signatures are foundational for privacy-enhancing technologies. They are used in anonymous credential systems, certain Zero-Knowledge Proof constructions, and protocols designed to prevent tracking and profiling. By decoupling the act of authorization from the identification of the requester, they solve a critical problem in designing systems that are both secure and respectful of user privacy, where authentication does not necessitate full disclosure of transactional metadata.

It is important to distinguish blinded signatures from other privacy techniques. Unlike ring signatures (which hide the signer among a group) or zero-knowledge proofs (which prove knowledge without revealing the data), a blinded signature specifically hides the content of the message from the signer during the signing act. The signature itself, once unblinded, is perfectly normal and does not conceal the signer's identity; the privacy lies in the inability to link the signing event to the eventual use of the signature.

The blinded signature flow is a multi-party cryptographic protocol initiated by a requester (or user) who wishes to obtain a valid signature on a sensitive message from a signer (or issuer) without revealing the message itself. The core mechanism relies on a mathematical blinding factor, which the requester uses to obfuscate, or 'blind,' the original message before sending it to the signer. This transformed data appears random to the signer, who then applies their private key to create a signature on the blinded message.

Once the signer returns the blind signature, the requester performs the crucial unblinding operation. Using the inverse of the original blinding factor, the requester removes the cryptographic mask. The result is a standard, verifiable digital signature on the original, unblinded message, as if the signer had signed it directly. The signature is cryptographically valid under the signer's public key, yet the signer has no record or knowledge of the specific message they endorsed.

This flow is fundamental to privacy-preserving systems. In digital cash (e.g., Chaumian e-cash), a bank signs a blinded token representing money, preventing it from linking the withdrawn token to the one later spent. For anonymous credentials, an issuer can sign a user's blinded attributes, allowing the user to later prove possession of a valid credential without revealing the transaction link back to the issuance. The security rests on the blindness property, which guarantees the signer cannot correlate the blinded message they saw with the final, unblinded signed message.

Common implementations use the RSA blind signature scheme or schemes based on elliptic curve cryptography. The protocol's integrity requires that the blinding function is a perfect trapdoor—easy to compute in one direction with the secret blinding factor, but infeasible to reverse without it. This ensures the signer cannot derive the original message from the blinded data, fulfilling the core promise of content anonymity while maintaining the non-repudiation of a standard digital signature.