hyperhive/swarm-secret-client/src/forge.rs

//! The forge agreement: where an agent's own forge token lives in the store,
//! and what the object at that path holds.
//!
//! `swarm-controller` mints the token with the forge's admin API and writes it
//! here; the agent container reads it back under its own certificate
//! (`nix/agent-modules/forge-token.nix`). Neither end is senior, so both halves
//! are stated once, here, beside [`crate::matrix`].

use serde::{Deserialize, Serialize};

use crate::{
    Error,
    path::{Kind, principal_prefix},
};

/// The path holding `agent`'s own forge access token.
///
/// A flat leaf under the agent's prefix, like its controller-minted siblings
/// [`crate::queue::agent_queue_path`] and [`crate::mtls::identity_path`], so
/// the agent's own read stanza ([`crate::policy::render_agent`])
/// already covers it.
///
/// # Errors
/// [`Error::PathSegment`] when `agent` contains anything but `[A-Za-z0-9_-]`,
/// which is what keeps one agent's name from addressing another agent's secret.
pub fn agent_token_path(agent: &str) -> Result<String, Error> {
    let prefix = principal_prefix(Kind::Agent, agent)?;
    Ok(format!("{prefix}/forge-token"))
}

/// What an agent's forge-token path holds.
///
/// No `Debug` derive: `value` is a live forge credential, and a derived
/// `Debug` is one `{:?}` away from a log line.
#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Credential {
    /// The token itself. `forge-token.nix` reads the store with
    /// `bao kv get -field=value`, so this name is load-bearing for a reader
    /// this crate does not control, the same as [`crate::matrix::Credential`]'s.
    pub value: String,

    /// The forge's name for the token — not secret. Stored beside the value
    /// so whoever rotates it can name the token it replaces.
    pub name: String,
}

impl std::fmt::Debug for Credential {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Credential")
            .field("value", &"<redacted>")
            .field("name", &self.name)
            .finish()
    }
}

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

    #[test]
    fn the_token_lands_under_the_agent_prefix() {
        // Spelled out rather than rebuilt from the pieces the code uses: the
        // nix reader spells the same string, and a rename here that moved it
        // would leave that reader fetching nothing.
        assert_eq!(
            agent_token_path("atlas").expect("a plain name is legal"),
            "swarm/agents/atlas/forge-token"
        );
    }

    #[test]
    fn a_traversal_in_the_agent_name_is_refused() {
        for bad in ["../argus", "a/b", "atlas/../argus", "a.b", ""] {
            let e = agent_token_path(bad).expect_err("a traversal is not legal");
            assert!(matches!(e, Error::PathSegment { kind: "agent", .. }), "{e}");
        }
    }

    #[test]
    fn the_legal_charset_is_actually_reachable() {
        // The control for the test above: a guard that refused everything
        // would pass it for the wrong reason.
        assert!(agent_token_path("a-b_C9").is_ok());
    }

    #[test]
    fn the_agents_own_read_grant_covers_the_path() {
        // The agent reads this path under `hive-agent-<agent>`, and that
        // policy is rendered elsewhere. If the path ever moved out from under
        // it, the agent's fetch would 403 at boot, naming neither.
        let path = agent_token_path("atlas").expect("legal");
        let policy = crate::policy::render_agent("atlas").expect("legal");
        let covered = policy.lines().any(|line| {
            line.strip_prefix("path \"")
                .and_then(|rest| rest.split_once("\" {"))
                .and_then(|(p, _)| p.strip_suffix('*'))
                .is_some_and(|prefix| format!("secret/data/{path}").starts_with(prefix))
        });
        assert!(covered, "no stanza in\n{policy}\ncovers {path}");
    }

    #[test]
    fn another_agents_read_grant_does_not_cover_the_path() {
        // Control for the test above: the prefix match must actually be
        // discriminating, or it proves nothing.
        let path = agent_token_path("atlas").expect("legal");
        let policy = crate::policy::render_agent("argus").expect("legal");
        let covered = policy.lines().any(|line| {
            line.strip_prefix("path \"")
                .and_then(|rest| rest.split_once("\" {"))
                .and_then(|(p, _)| p.strip_suffix('*'))
                .is_some_and(|prefix| format!("secret/data/{path}").starts_with(prefix))
        });
        assert!(!covered, "argus's policy must not reach atlas's token");
    }

    #[test]
    fn the_value_field_matches_what_the_nix_reader_asks_for() {
        let json = serde_json::to_string(&Credential {
            value: "t".to_owned(),
            name: "swarm-agent".to_owned(),
        })
        .expect("two Strings serialise");
        assert_eq!(json, r#"{"value":"t","name":"swarm-agent"}"#);
    }

    #[test]
    fn debug_never_prints_the_value() {
        let c = Credential {
            value: "0123456789abcdef".to_owned(),
            name: "swarm-agent".to_owned(),
        };
        let shown = format!("{c:?}");
        assert!(!shown.contains("0123456789abcdef"), "{shown}");
        assert!(shown.contains("swarm-agent"), "{shown}");
    }
}