# Blind RSA signatures

Author-blinded RSASSA-PSS RSAE signatures.

This is an implementation of the [RSA Blind Signatures](https://chris-wood.github.io/draft-wood-cfrg-blind-signatures/draft-wood-cfrg-rsa-blind-signatures.html) proposal, based on [the Zig implementation](https://github.com/jedisct1/zig-rsa-blind-signatures).

## Protocol overview

A client asks a server to sign a message. The server receives the message, and returns the signature.

Using that `(message, signature)` pair, the client can locally compute a second, valid `(message', signature')` pair.

Anyone can verify that `(message', signature')` is valid for the server's public key, even though the server didn't see that pair before.
But no one besides the client can link `(message', signature')` to `(message, signature)`.

Using that scheme, a server can issue a token and verify that a client has a valid token, without being able to link both actions to the same client.

1. The client creates a random message, and blinds it with a random, secret factor.
2. The server receives the blind message, signs it and returns a blind signature.
3. From the blind signature, and knowing the secret factor, the client can locally compute a `(message, signature)` pair that can be verified using the server's public key.
4. Anyone, including the server, can thus later verify that `(message, signature)` is valid, without knowing when step 2 occurred.

The scheme was designed by David Chaum, and was originally implemented for anonymizing DigiCash transactions.

## Usage

```rust
let kp = KeyPair::generate(2048)?;
let (pk, sk) = (kp.pk, kp.sk);

let msg = b"test";
let blinding_result = pk.blind(msg)?;
let blind_sig = sk.blind_sign(&blinding_result.blind_msg)?;
let sig = pk.finalize(&blind_sig, &blinding_result.secret, &msg)?;
sig.verify(&pk, msg)?;
```