Definition
A blinded (or blind) signature is a cryptographic construction, introduced by David Chaum in 1982, in which a signer authenticates a message whose contents have been disguised, or "blinded," by the requester. The signer applies their secret key without ever seeing the underlying data; the requester then removes the blinding to obtain a valid signature on the original message. Crucially, the signer cannot later connect that finished signature to the specific request that produced it.
Blind, sign, unblind
The flow has three steps. First the requester multiplies the message by a secret random "blinding factor" and sends the result. Second, the signer signs the blinded value with their private key and returns it. Third, the requester divides out the blinding factor, leaving a normal signature that verifies against the signer's public key. Because the random factor is known only to the requester, the signer's view of the session reveals nothing about the message they actually authorized.
Why it matters for Bitcoin
Blinded signatures are the engine of Chaumian ecash. A mint signs token serial numbers blindly, guaranteeing two properties at once: unforgeability, because only the mint holds the key, and untraceability, because the mint cannot tell which token it signed or who holds it. Even if a mint and a merchant collude, they cannot reconstruct who spent a given note.
This primitive underpins both single-operator Cashu mints and federated Fedimint guardians, making it foundational reading for anyone studying private digital money.
In Simple Terms
A blinded (or blind) signature is a cryptographic construction, introduced by David Chaum in 1982, in which a signer authenticates a message whose contents have…