authenticated-pseudonyms

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authenticated-pseudonyms

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An implementation of authenticated pseudonyms using BBS and Dodis-Yampolskiy.

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# Authenticated Pseudonyms

This project is intended for demonstration purposes only. It is not intended
for use in a production environment. It currently contains unaudited,
experimental cryptography which is not suited for production environments.

[![Rust](https://github.com/SamuelSchlesinger/authenticated-pseudonyms/actions/workflows/rust.yml/badge.svg)](https://github.com/SamuelSchlesinger/authenticated-pseudonyms/actions/workflows/rust.yml)

## Motivation

In many circumstances, one wants to authenticate in an anonymous or
pseudonymous way. For this purpose, we can think of anonymity as a single use
pseudonym. In that sense, authenticated pseudonyms can be a powerful primitive
for anonymous authentication and authorization. They can enable anonymous rate
limiting schemes, local pseudonyms for identity, and even anonymous voting
schemes.

## Design

In our work, we rely on the BBS signature scheme, and the Dodis-Yampolskiy VRF.
We utilize both the publicly verifiable and privately verifiable variants of
BBS, yielding approaches with various cryptography assumptions and
capabilities. Our publicly verifiable approaches rely on pairings, and we use
the BLS12-381 pairing in this implementation. Our privately verifiable approach
does not rely on pairings, and we instantiate that with Ristretto.

The basic idea is to use as your credential a BBS signature of a VRF key, which
you obscure before sending to the signer. Then, when you show your BBS
signature, you include the output of the VRF and a zero-knowledge proof of
correctness of this output, connected to the BBS signature proof via a DLEQ
proof.  On top of this, you can also have other fields in the BBS signature and
prove various things about them. In our rate limited approach, we prove
something about the VRF input instead of revealing it, specifically that its
lower order bits are a number between `0` and `RATE_LIMIT - 1`, and the higher
order bits are a specific value representing the current epoch. The
`RATE_LIMIT` must be a power of two, as we use a binary decomposition based
range proof. Effectively, this protocol winds up allowing up to `RATE_LIMIT`
pseudonyms per epoch, or up to `RATE_LIMIT` anonymous proofs.

Our rate limited use case is signing `(k, t)` where `t` is some other value
decided by the issuer and `k` is the VRF key as discussed above. We prove that
`t <= BOUND` for some publicly known bound, as well as rate limiting in the way
I describe above. Thus, each token requires two range proofs. Because we are
signing the input `t` to the `t <= BOUND` range proof, we can use an
approximate approach called SHARP in order to create a more efficient range
proof. There are examples of this approach below.

# Examples

## (Issuer Linkable) Pseudonym

```rust
// globally decided upon amongst all parties
let params = Params::default();

// the issuer's private key
let issuer_private_key = IssuerPrivateKey::random(OsRng);

// the issuer can mint a new credential, generating a new, random VRF key
let cred1 = Credential::mint(&params, &issuer_private_key, OsRng).unwrap();
assert!(bool::from(
    cred1.verify(&params, &issuer_private_key.public())
));

// the relying party's id must be agreed upon in advance by all parties
let relying_party_id = 5;

let pseudonym1 = cred1
    .pseudonym_for(&params, relying_party_id, b"nonce", OsRng)
    .unwrap();

// the relying party would then verify the correctness of the proof and that they
// are the relying party for whom this credential is intended
assert!(bool::from(pseudonym1.verify(
    &params,
    &issuer_private_key.public(),
    b"nonce"
)));
assert_eq!(pseudonym1.relying_party_id(), &relying_party_id);

// the server and the client can both retrieve the VRF key itself for backups
// of the user's accounts
let vrf_key = cred1.vrf_key().clone();

// recovering a credential should produce a new credential with the same pseudonym id
let cred2 = Credential::recover(&params, &issuer_private_key, vrf_key, OsRng).unwrap();
assert!(bool::from(
    cred2.verify(&params, &issuer_private_key.public())
));
let pseudonym2 = cred2
    .pseudonym_for(&params, relying_party_id, b"nonce2", OsRng)
    .unwrap();
assert!(bool::from(pseudonym2.verify(
    &params,
    &issuer_private_key.public(),
    b"nonce2"
)));
assert_eq!(pseudonym1.pseudonym_id(), pseudonym2.pseudonym_id());
```

## Unlinkable Pseudonym

![Image](https://raw.githubusercontent.com/SamuelSchlesinger/authenticated-pseudonyms/dev/pseudonym_sequence_diagram.png)

```rust
// globally decided upon amongst all parties
let params = Params::default();

// the issuer's private key
let issuer_private_key = IssuerPrivateKey::random(OsRng);

// the client's private key
let client_private_key = ClientPrivateKey::random(OsRng);

// computed by the client and sent to the server
let cred_request = client_private_key.request(&params, OsRng);

// computed by the server and returned to the client
let cred_resp = cred_request.respond(&issuer_private_key, &params, OsRng);

// computed by the client
let cred1 = client_private_key
    .create_credential(&cred_request, &cred_resp, &issuer_public_key)
    .unwrap();
assert!(bool::from(
    cred1.verify(&params, &issuer_private_key.public())
));

// the relying party's id must be agreed upon in advance by all parties
let relying_party_id = 5;

let pseudonym1 = cred1
    .pseudonym_for(&params, relying_party_id, b"nonce", OsRng)
    .unwrap();

// recovery requires the client store the VRF key persistently
let client_private_key: ClientPrivateKey = ClientPrivateKey::recover(cred1.vrf_key().clone());

// computed on client, sent to server
let cred_request = client_private_key.request(&params, OsRng);

// computed on server, sent to client
let cred_resp = cred_request.respond(&issuer_private_key, &params, OsRng);

// computed on client
let cred2 = client_private_key
    .create_credential(&cred_request, &cred_resp, &issuer_public_key)
    .unwrap();

let pseudonym2 = cred2
    .pseudonym_for(&params, relying_party_id, b"nonce2", OsRng)
    .unwrap();

assert_eq!(pseudonym1.pseudonym_id(), pseudonym2.pseudonym_id());
```

## Publicly Verifiable Age Range Proof with Rate Limiting

```rust
// globally decided upon amongst all parties
let params = Params::default();

// on issuer
let issuer_private_key = IssuerPrivateKey::random(OsRng);

// on client
let client_private_key = ClientPrivateKey::random(OsRng);
let credential_request = client_private_key.credential_request(&params, OsRng);

// on issuer
let t = 1; // for instance the current minute
let credential_response = credential_request.respond(
    &issuer_private_key,
    &issuer_private_key.public(),
    &params,
    t,
    OsRng).unwrap();

// on client
let credential = client_private_key.create_credential(
    &params,
    &credential_request,
    &credential_response,
    &issuer_public_key).unwrap();
let epoch = 0; // for instance the current day
let bound = 2; // for instance five days in the past
let rate_limit_exponent = 10; // rate limit = 2^10
let i = 0; // anything under 2^10
let proof = credential.prove(
    &params,
    bound,
    epoch,
    OsRng,
    rate_limit_exponent,
    i);

if let Some(_extant) = db.lookup(proof.rate_limiting_token()) {
    panic!("reused i");
}

// on issuer
assert!(proof.verify(
    &params,
    &issuer_public_key)
    && proof.bound() == 2
    && proof.epoch() == 2
    && proof.rate_limit_exponent() == 10
);

db.insert(proof.rate_limiting_token());
```

## Privately Verifiable Age Range Proof with Rate Limiting

```rust
// globally decided upon amongst all parties
let params = Params::default();

// on issuer
let issuer_private_key = IssuerPrivateKey::random(OsRng);

// on client
let client_private_key = ClientPrivateKey::random(OsRng);
let credential_request = client_private_key.credential_request(&params, OsRng);

if let Some(_extant) = db.lookup(proof.rate_limiting_token()) {
    panic!("reused i");
}

// on issuer
assert!(proof.verify(
    &params,
    &issuer_private_key)
    && proof.bound() == 2
    && proof.epoch() == 2
    && proof.rate_limit_exponent() == 10
);

db.insert(proof.rate_limiting_token());
```

This is not an officially supported Google product. This project is not
eligible for the [Google Open Source Software Vulnerability Rewards
Program](https://bughunters.google.com/open-source-security).

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