Self-Blinding: Chaumian Tokens

In the context of Chaumian tokens, self-blinding refers to a cryptographic technique that allows a user to re-randomize or alter the representation of a token such that:

The issuer or mint cannot link the new version of the token to the original one.

The value and validity of the token remain intact.

It preserves user privacy even in repeated use.

Context: What Are Chaumian Tokens?

Chaumian tokens are a type of privacy-preserving digital cash or voucher system, inspired by the ideas of David Chaum (a pioneer in cryptographic anonymity). They use blind signatures so that a mint (or central issuer) can sign tokens without seeing their contents.

What Is Self-Blinding?

Self-blinding is a property that allows a user to take a valid token and transform it into a new-looking token (with a new identifier or blinding factor), such that:

It’s cryptographically unlinkable to the previous form.

The new token can be used without revealing any link to the original issuance.

This transformation is done entirely by the user, without needing to go back to the mint.

Why Self-Blinding Matters

1. Privacy: Even if a user spends the same token multiple times (e.g., across different services), no one—including the mint—can link the activity.

2. Untraceability: The issuer can't build a spending history based on token serial numbers or identifiers.

3. User Control: The user can maintain privacy without involving a third party or mint during re-randomization.

Example (Simplified):

1. A user receives a Chaumian token signed by the mint.

2. Before using it, the user applies a self-blinding transformation.

3. The token looks entirely different, but it's still valid and will be accepted as genuine by any verifier.