Blind RSA signatures

Author-blinded RSASSA-PSS RSAE signatures.

This is an implementation of the RSA Blind Signatures (RFC 9474), ported from the Zig implementation.

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

use blind_rsa_signatures::{KeyPair, Sha384, PSS, Randomized, DefaultRng};

// [SERVER]: Generate a RSA-2048 key pair
let kp = KeyPair::<Sha384, PSS, Randomized>::generate(&mut DefaultRng, 2048)?;
let (pk, sk) = (kp.pk, kp.sk);

// [CLIENT]: create a random message and blind it for the server whose public key is `pk`.
// The client must store the message and the secret.
let msg = b"test";
let blinding_result = pk.blind(&mut DefaultRng, msg)?;

// [SERVER]: compute a signature for a blind message, to be sent to the client.
// The client secret should not be sent to the server.
let blind_sig = sk.blind_sign(&blinding_result.blind_message)?;

// [CLIENT]: later, when the client wants to redeem a signed blind message,
// using the blinding secret, it can locally compute the signature of the
// original message.
// The client then owns a new valid (message, signature) pair, and the
// server cannot link it to a previous (blinded message, blind signature) pair.
// Note that the finalization function also verifies that the new signature
// is correct for the server public key.
let sig = pk.finalize(&blind_sig, &blinding_result, msg)?;

// [SERVER]: a non-blind signature can be verified using the server's public key.
pk.verify(&sig, blinding_result.msg_randomizer, msg)?;

Configuration options

The key types take three compile-time parameters:

• Hash algorithm: Sha256, Sha384, Sha512
• Salt mode: PSS (with salt), PSSZero (without salt)
• Message preparation: Randomized, Deterministic

Pre-defined type aliases are available for all SHA-384 configurations:

Variant	KeyPair	PublicKey	SecretKey
PSS + Randomized (recommended)	KeyPairSha384PSSRandomized	PublicKeySha384PSSRandomized	SecretKeySha384PSSRandomized
PSSZero + Randomized	KeyPairSha384PSSZeroRandomized	PublicKeySha384PSSZeroRandomized	SecretKeySha384PSSZeroRandomized
PSS + Deterministic	KeyPairSha384PSSDeterministic	PublicKeySha384PSSDeterministic	SecretKeySha384PSSDeterministic
PSSZero + Deterministic	KeyPairSha384PSSZeroDeterministic	PublicKeySha384PSSZeroDeterministic	SecretKeySha384PSSZeroDeterministic

For SHA-256 or SHA-512, use the generic types directly:

use blind_rsa_signatures::{KeyPair, Sha512, PSS, Randomized, DefaultRng};

let kp = KeyPair::<Sha512, PSS, Randomized>::generate(&mut DefaultRng, 2048)?;

Key import/export

Keys can be imported and exported in DER, PEM, and SPKI formats:

use blind_rsa_signatures::PublicKeySha384PSSRandomized;

// Export
let der = pk.to_der()?;
let pem = pk.to_pem()?;
let spki = pk.to_spki()?;

// Import
let pk = PublicKeySha384PSSRandomized::from_der(&der)?;
let pk = PublicKeySha384PSSRandomized::from_pem(&pem)?;
let pk = PublicKeySha384PSSRandomized::from_spki(&spki)?;

Partially Blind RSA Signatures

The pbrsa module implements Partially Blind RSA Signatures (IRTF CFRG draft), which allow a signer to bind signatures to public metadata while keeping the message content blind.

use blind_rsa_signatures::pbrsa::{PartiallyBlindKeyPair, DefaultRng};
use blind_rsa_signatures::{Sha384, PSS, Randomized};

// [SERVER]: Generate a key pair with safe primes (required for PBRSA)
let kp = PartiallyBlindKeyPair::<Sha384, PSS, Randomized>::generate(&mut DefaultRng, 2048)?;

// [SERVER]: Derive a key pair for specific metadata
let metadata = b"2024-01-15";
let derived_kp = kp.derive_key_pair_for_metadata(metadata)?;

// [CLIENT]: Blind a message with metadata
let msg = b"token-12345";
let blinding_result = derived_kp.pk.blind(&mut DefaultRng, msg, Some(metadata))?;

// [SERVER]: Sign the blinded message
let blind_sig = derived_kp.sk.blind_sign(&blinding_result.blind_message)?;

// [CLIENT]: Finalize to get the actual signature
let sig = derived_kp.pk.finalize(&blind_sig, &blinding_result, msg, Some(metadata))?;

// [ANYONE]: Verify the signature with metadata
derived_kp.pk.verify(&sig, blinding_result.msg_randomizer, msg, Some(metadata))?;

Key differences from standard blind RSA:

• Keys must use safe primes (p and q where (p-1)/2 and (q-1)/2 are also prime)
• Signatures are bound to public metadata
• Key derivation generates per-metadata key pairs

For other languages

• Zig
• C
• Go
• TypeScript