hyperhive/swarm-nats-auth/src/respond.rs

//! Minting the reply the server expects on `msg.reply`.
//!
//! Both JWTs are hand-built because neither is expressible through
//! `nats-jwt`: its `Claims` has no `aud`, which the `authorization_response`
//! wrapper needs (the asking server's id, so a captured reply cannot be
//! replayed elsewhere in the cluster) and the user token needs (the account
//! name). Its `IntoNatsClaims` also returns a closed
//! `NatsClaims { User, Account }`.
//!
//! ⛔ **`issuer_account` must be ABSENT from the user token.** It is an
//! operator-mode field, and `nix/host-modules/swarm-nats.nix` renders
//! *server-config* mode, where its mere presence makes the server refuse the
//! client — `Error non operator mode account "AUTH": attempted to use
//! issuer_account`, with the responder having answered `granted=true`.
//! `nats_jwt::Token::new_user` always sets it, which is why reaching for that
//! constructor again would reintroduce the bug.
//!
//! Signing follows `nats-jwt`'s own `sign()`: serialise the claims with `jti`
//! **empty**, sha256 that, `BASE32HEX_NOPAD` the digest into `jti`,
//! re-serialise, sign `"<b64url header>.<b64url claims>"` with the account
//! key. ⚠️ `BASE32HEX`, not `BASE32` — one word, and the only symptom is a
//! token the server rejects without saying why.
//!
//! `nats-jwt` is therefore a **dev-dependency, not a runtime one**: it is this
//! module's test oracle. [`tests::hand_built_matches_the_reference`] builds a
//! user token both ways and requires byte equality, so the encoder is checked
//! against the reference on the one shape that crate does model.

use data_encoding::{BASE32HEX_NOPAD, BASE64URL_NOPAD};
use nkeys::KeyPair;
use serde::Serialize;
use sha2::{Digest, Sha256};

/// Header every NATS JWT carries. Byte-identical to `nats-jwt`'s, and the
/// equality test would catch it drifting.
const JWT_HEADER: &str = r#"{"typ":"JWT","alg":"ed25519-nkey"}"#;

/// A NATS JWT claim set, with `aud` — which the callout response needs and
/// `nats-jwt`'s own `Claims` lacks.
#[derive(Serialize)]
struct Claims<T: Serialize> {
    iat: i64,
    iss: String,
    jti: String,
    sub: String,
    name: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    aud: Option<String>,
    nats: T,
}

/// The `nats` object of an `authorization_response`.
///
/// Exactly one of `jwt` / `error` is set. They are `Option`s rather than an
/// enum so the serialised shape matches the wire format directly, and the
/// constructors below are the only way to build one — a caller cannot
/// accidentally produce a response that carries both, which a server would be
/// free to read either way.
#[derive(Serialize)]
struct AuthResponse {
    #[serde(skip_serializing_if = "Option::is_none")]
    jwt: Option<String>,
    #[serde(skip_serializing_if = "Option::is_none")]
    error: Option<String>,
    #[serde(rename = "type")]
    kind: &'static str,
    version: i64,
}

/// Sign a claim set the way `nats-jwt` does. `now` is injected so the test
/// can pin it; production passes the wall clock.
fn sign<T: Serialize>(mut claims: Claims<T>, key: &KeyPair) -> String {
    claims.jti = String::new();
    let unhashed = serde_json::to_string(&claims).expect("claims serialisation cannot fail");
    let digest = Sha256::digest(unhashed.as_bytes());
    claims.jti = BASE32HEX_NOPAD.encode(&digest);

    let body = serde_json::to_string(&claims).expect("claims serialisation cannot fail");
    let half = format!(
        "{}.{}",
        BASE64URL_NOPAD.encode(JWT_HEADER.as_bytes()),
        BASE64URL_NOPAD.encode(body.as_bytes())
    );
    let sig = key
        .sign(half.as_bytes())
        .expect("ed25519 signing cannot fail");
    format!("{half}.{}", BASE64URL_NOPAD.encode(&sig))
}

/// Wrap a decision in an `authorization_response` addressed to the server
/// that asked.
///
/// A denial is a *signed response carrying an error*, never silence: the
/// server cannot tell an absent responder from a refusing one, so staying
/// quiet turns every rejection into a timeout and hides an outage behind
/// what looks like normal denials.
fn response(
    now: i64,
    issuer: &KeyPair,
    server_id: &str,
    user_nkey: &str,
    verdict: Result<String, String>,
) -> String {
    let (jwt, error) = match verdict {
        Ok(user_jwt) => (Some(user_jwt), None),
        Err(reason) => (None, Some(reason)),
    };
    sign(
        Claims {
            iat: now,
            iss: issuer.public_key(),
            jti: String::new(),
            sub: user_nkey.to_owned(),
            name: user_nkey.to_owned(),
            aud: Some(server_id.to_owned()),
            nats: AuthResponse {
                jwt,
                error,
                kind: "authorization_response",
                version: 2,
            },
        },
        issuer,
    )
}

/// Current unix seconds, as the JWT `iat`.
fn now_secs() -> i64 {
    i64::try_from(
        std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .expect("system time is after the unix epoch")
            .as_secs(),
    )
    .expect("seconds since the epoch fit in an i64")
}

/// The `nats` object of a **user** JWT, server-config mode.
///
/// Field-for-field what `nats-jwt` emits **minus `issuer_account`**, whose
/// presence a non-operator server rejects outright (see the module docs). The
/// account is named by the enclosing claims' `aud` instead.
///
/// The `-1`s are NATS' "unlimited": no cap on subscriptions, payload size or
/// total data. Limits belong on the account, which the module owns; putting
/// them here would mean every issued token silently disagreed with the
/// deployed policy the moment that policy changed.
#[derive(Serialize)]
struct UserNats {
    #[serde(rename = "type")]
    kind: &'static str,
    subs: i64,
    data: i64,
    payload: i64,
    bearer_token: bool,
    version: i64,
}

/// Mint the user JWT an admitted client presents.
///
/// `account` is the account **name** from the server's `accounts` block (the
/// module's `clientAccount`), not a public key — in config mode the server
/// resolves `aud` against its own config rather than against a key.
fn user_jwt(now: i64, issuer: &KeyPair, account: &str, user_nkey: &str) -> String {
    sign(
        Claims {
            iat: now,
            iss: issuer.public_key(),
            jti: String::new(),
            sub: user_nkey.to_owned(),
            name: user_nkey.to_owned(),
            aud: Some(account.to_owned()),
            nats: UserNats {
                kind: "user",
                subs: -1,
                data: -1,
                payload: -1,
                bearer_token: false,
                version: 2,
            },
        },
        issuer,
    )
}

/// Grant: mint a user JWT placing the client in `account` and wrap it.
pub fn grant(issuer: &KeyPair, account: &str, server_id: &str, user_nkey: &str) -> String {
    let now = now_secs();
    let jwt = user_jwt(now, issuer, account, user_nkey);
    response(now, issuer, server_id, user_nkey, Ok(jwt))
}

/// Deny, with a reason the server can log.
///
/// The reason is deliberately coarse (`"unauthorized"`), not a description of
/// *why*: this string reaches an unauthenticated peer, and a precise one turns
/// the auth path into an oracle for which tokens exist.
pub fn deny(issuer: &KeyPair, server_id: &str, user_nkey: &str) -> String {
    response(
        now_secs(),
        issuer,
        server_id,
        user_nkey,
        Err("unauthorized".to_owned()),
    )
}

#[cfg(test)]
mod tests {
    use super::*;

    /// Mirror of what `nats-jwt` puts in a user token, **in its field order**.
    ///
    /// Deliberately *not* [`super::UserNats`]: this one carries
    /// `issuer_account`, because the reference does. Production must not, and
    /// keeping the two structs separate is what lets one test pin the encoder
    /// against `nats-jwt` while another asserts production diverges from it in
    /// exactly one field.
    ///
    /// 🩸 The first version of the equality test round-tripped `nats` through
    /// `serde_json::Value` and failed — not because the encoder was wrong, but
    /// because `Value` is a `BTreeMap`, so re-serialising sorted the keys and
    /// changed the bytes. The gate's *method* was the defect. Worth keeping the
    /// scar: `jti` is a hash over serialised claims, so anything that reorders
    /// fields between hashing and signing is a live hazard here, not merely a
    /// test artefact.
    #[derive(serde::Serialize)]
    struct ReferenceUserNats {
        #[serde(rename = "type")]
        kind: &'static str,
        issuer_account: String,
        subs: i64,
        data: i64,
        payload: i64,
        bearer_token: bool,
        version: i64,
    }

    /// The gate that makes the hand-built encoder trustworthy: build a *user*
    /// token with `nats-jwt` and with this module's `sign`, and require the
    /// bytes to be identical.
    ///
    /// It works because `nats-jwt`'s `sign()` derives `iat` from the wall
    /// clock and everything else from its inputs, so signing the same claims
    /// in the same second must produce the same string. If this module's
    /// header, field order, `jti` alphabet or signing input ever drifts from
    /// the reference, this fails — including the `BASE32HEX`-vs-`BASE32`
    /// distinction, which is otherwise invisible until a server refuses a
    /// token.
    #[test]
    fn hand_built_matches_the_reference() {
        let account = KeyPair::new_account();
        let user = KeyPair::new_user();
        let reference =
            nats_jwt::Token::new_user(account.public_key(), user.public_key()).sign(&account);

        // Mirror what `nats-jwt` puts in a user token **as a struct**, in its
        // field order.
        //
        // 🩸 The first version of this test round-tripped `nats` through
        // `serde_json::Value` and failed — not because the encoder was wrong,
        // but because `Value` is a `BTreeMap`, so re-serialising sorted the
        // keys and changed the bytes. The gate's *method* was the defect, and
        // it is worth keeping the scar: `jti` is a hash over serialised
        // claims, so anything that reorders fields between hashing and
        // signing is a live hazard here, not just a test artefact.
        let decoded = decode_claims(&reference);
        let mine = sign(
            Claims {
                iat: decoded["iat"].as_i64().expect("iat"),
                iss: account.public_key(),
                jti: String::new(),
                sub: user.public_key(),
                name: user.public_key(),
                aud: None,
                nats: ReferenceUserNats {
                    kind: "user",
                    issuer_account: account.public_key(),
                    subs: -1,
                    data: -1,
                    payload: -1,
                    bearer_token: false,
                    version: 2,
                },
            },
            &account,
        );
        assert_eq!(mine, reference, "hand-built encoder drifted from nats-jwt");
    }

    #[test]
    fn a_denial_is_signed_and_carries_no_jwt() {
        let account = KeyPair::new_account();
        let token = deny(&account, "NSERVER", "UCLIENT");
        let claims = decode_claims(&token);
        assert_eq!(claims["nats"]["type"], "authorization_response");
        assert_eq!(claims["nats"]["error"], "unauthorized");
        assert!(
            claims["nats"]["jwt"].is_null(),
            "a denial must not carry a user jwt"
        );
        // `aud` binds the reply to the asking server; without it a captured
        // response is replayable at any other server in the cluster.
        assert_eq!(claims["aud"], "NSERVER");
    }

    #[test]
    fn a_grant_carries_a_user_jwt_and_no_error() {
        let account = KeyPair::new_account();
        let claims = decode_claims(&grant(&account, "APP", "NSERVER", "UCLIENT"));
        assert!(claims["nats"]["error"].is_null());
        assert!(
            claims["nats"]["jwt"]
                .as_str()
                .is_some_and(|j| j.contains('.'))
        );
    }

    /// 🩸 THE REGRESSION TEST FOR THE BUG THE UNIT TESTS COULD NOT SEE.
    ///
    /// The first version of this module minted the user JWT with
    /// `nats_jwt::Token::new_user`, which always sets `issuer_account`. Every
    /// test passed, the responder logged `granted=true`, and the server still
    /// refused every client: `Error non operator mode account "AUTH":
    /// attempted to use issuer_account`.
    ///
    /// The lesson is in what this asserts — an **absence**. The suite was full
    /// of assertions about fields that had to be *present*, and a field that
    /// must not exist is invisible to all of them. It took running a real
    /// server to find, so the cheap check goes here to keep it found.
    #[test]
    fn the_issued_user_jwt_names_its_account_by_aud_and_sets_no_issuer_account() {
        let account = KeyPair::new_account();
        let wrapper = decode_claims(&grant(&account, "APP", "NSERVER", "UCLIENT"));
        let user = decode_claims(wrapper["nats"]["jwt"].as_str().expect("a user jwt"));

        // In server-config mode the account is named by `aud`...
        assert_eq!(user["aud"], "APP", "the user jwt must name its account");
        // ...and `issuer_account` is an operator-mode field whose mere
        // presence makes the server reject the client.
        assert!(
            user["nats"]["issuer_account"].is_null(),
            "issuer_account is fatal in non-operator mode"
        );
        assert_eq!(user["nats"]["type"], "user");
        assert_eq!(user["sub"], "UCLIENT");
        assert_eq!(user["iss"], account.public_key());
    }

    fn decode_claims(jwt: &str) -> serde_json::Value {
        let body = jwt.split('.').nth(1).expect("claims segment");
        serde_json::from_slice(&BASE64URL_NOPAD.decode(body.as_bytes()).expect("base64url"))
            .expect("claims json")
    }
}