Blind RSA signatures

Author-blinded RSASSA-PSS RSAE signatures.

This is an implementation of the RSA Blind Signatures RFC, based on 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.

Random noise must be added to messages that don't include enough entropy. An optional "Message Randomizer" can be used for that purpose.

Dependencies

This implementation requires OpenSSL (1.1.x or 3.x.y) or BoringSSL.

Usage

    #include <blind_rsa.h>

    // Initialize a context with the default parameters
    BRSAContext context;
    brsa_context_init_default(&context);

    // [SERVER]: Generate a RSA-2048 key pair
    BRSASecretKey sk;
    BRSAPublicKey pk;
    assert(brsa_keypair_generate(&sk, &pk, 2048) == 0);

    // Noise is not required if the message is random.
    // If it is not NULL, it will be automatically filled by brsa_blind_sign().
    BRSAMessageRandomizer *msg_randomizer = NULL;

    // [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.
    uint8_t            msg[32];
    const size_t       msg_len = sizeof msg;
    BRSABlindMessage   blind_msg;
    BRSABlindingSecret client_secret;
    assert(brsa_blind_message_generate(&context, &blind_msg, msg, msg_len, &client_secret, &pk) ==
           0);

    // [SERVER]: compute a signature for a blind message, to be sent to the client.
    // The client secret should not be sent to the server.
    BRSABlindSignature blind_sig;
    assert(brsa_blind_sign(&context, &blind_sig, &sk, &blind_msg) == 0);
    brsa_blind_message_deinit(&blind_msg);

    // [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 signature is
    // correct for the server public key.
    BRSASignature sig;
    assert(brsa_finalize(
               &context, &sig, &blind_sig, &client_secret, msg_randomizer, &pk, msg, msg_len) == 0);
    brsa_blind_signature_deinit(&blind_sig);
    brsa_blinding_secret_deinit(&client_secret);

    // [SERVER]: a non-blind signature can be verified using the server's public key.
    assert(brsa_verify(&context, &sig, &pk, msg_randomizer, msg, msg_len) == 0);
    brsa_signature_deinit(&sig);

    brsa_secretkey_deinit(&sk);
    brsa_publickey_deinit(&pk);

Deterministic padding is also supported, by creating a context with brsa_context_init_deterministic():

    // Initialize a context to use deterministic padding
    BRSAContext context;
    brsa_context_init_deterministic(&context);

Most applications should use the default (probabilistic) mode instead.

A custom hash function and salt length can also be specified with brsa_context_init_custom():

    // Initialize a context with SHA-256 as a Hash and MGF function,
    // and a 48 byte salt.
    BRSAContext context;
    brsa_context_init_custom(&context, BRSA_SHA256, 48);

Some additional helper functions for key management are included:

    // Get a key identifier
    uint8_t key_id[4];
    assert(brsa_publickey_id(&context, key_id, sizeof key_id, &pk) == 0);

    // Key serialization
    BRSASerializedKey sk_der, pk_der;
    assert(brsa_secretkey_export(&sk_der, &sk) == 0);
    assert(brsa_publickey_export(&pk_der, &pk) == 0);

    // Store the SubjectPublicKeyInfo in DER format
    BRSASerializedKey spki_der;
    assert(brsa_publickey_export_spki(&context, &spki_der, &pk) == 0);

    // Free key resources
    brsa_secretkey_deinit(&sk);
    brsa_publickey_deinit(&pk);

    // Key deserialization
    assert(brsa_secretkey_import(&sk, sk_der.bytes, sk_der.bytes_len) == 0);
    assert(brsa_publickey_import(&pk, pk_der.bytes, pk_der.bytes_len) == 0);
    brsa_serializedkey_deinit(&sk_der);
    brsa_serializedkey_deinit(&pk_der);

All these functions return 0 on success and -1 on error.

For other languages

• Zig
• Rust
• Go