swarm-bao: the reader's identity is not a store-runner option

The block header says `deploy.bao.*` is what the host RUNNING the store
decides and that none of it means anything on a host that only reads
secrets. Three options in that block are exactly what a read-only host must
set, and their own descriptions say so a screen further down — so reading
the options never corrects the header.

It reads as though an off-host reader were inexpressible. It is already
supported: glue-matrix-bao-token gates on the identity rather than on
`enable`, and module-eval's baoRemoteReader is that deployment.

Also drops a sentence describing what used to be undeclarable.

CLAUDE.md

@@ -147,6 +147,14 @@ hand-maintained per-file tree drifts out of sync with the code.
   unix socket the gateway's nginx proxies to — ⚠️ **the socket's
   directory is its access control**; the constraint that governs it is in
   the crate's README, and a unit test pins the path.
+- **`swarm-secret-client/`** — client for the swarm's secret store, over
+  `vaultrs`. Owns the *agreements* both ends of the store must share rather
+  than the HTTP: the path a credential lives at, the field its bytes are in,
+  and the translation from this deployment's environment into a logged-in
+  client. ⚠️ Reads the **`BAO_`** env spellings explicitly — `vaultrs`'s own
+  defaults look for `VAULT_ADDR`/`VAULT_CLIENT_CERT`/`VAULT_CLIENT_KEY`, which
+  no unit in this tree sets, so falling through to them yields a client with
+  **no identity** and a TLS handshake failure that names no cause.
 - **`swarmctl/`** — swarm-level operator CLI, installed by the
   swarm-controller module on the host that runs the daemon. Runs as
   **root and acts directly** — no socket, no HTTP route, no priv helper;

Cargo.lock

@@ -894,14 +894,38 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "darling"
+version = "0.20.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc7f46116c46ff9ab3eb1597a45688b6715c6e628b5c133e288e709a29bcb4ee"
+dependencies = [
+ "darling_core 0.20.11",
+ "darling_macro 0.20.11",
+]
+
 [[package]]
 name = "darling"
 version = "0.23.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "25ae13da2f202d56bd7f91c25fba009e7717a1e4a1cc98a76d844b65ae912e9d"
 dependencies = [
- "darling_core",
+ "darling_core 0.23.0",
- "darling_macro",
+ "darling_macro 0.23.0",
+]
+
+[[package]]
+name = "darling_core"
+version = "0.20.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0d00b9596d185e565c2207a0b01f8bd1a135483d02d9b7b0a54b11da8d53412e"
+dependencies = [
+ "fnv",
+ "ident_case",
+ "proc-macro2",
+ "quote",
+ "strsim",
+ "syn 2.0.119",
 ]
 
 [[package]]
@@ -917,13 +941,24 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "darling_macro"
+version = "0.20.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc34b93ccb385b40dc71c6fceac4b2ad23662c7eeb248cf10d529b7e055b6ead"
+dependencies = [
+ "darling_core 0.20.11",
+ "quote",
+ "syn 2.0.119",
+]
+
 [[package]]
 name = "darling_macro"
 version = "0.23.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ac3984ec7bd6cfa798e62b4a642426a5be0e68f9401cfc2a01e3fa9ea2fcdb8d"
 dependencies = [
- "darling_core",
+ "darling_core 0.23.0",
  "quote",
  "syn 2.0.119",
 ]
@@ -1020,6 +1055,37 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "derive_builder"
+version = "0.20.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "507dfb09ea8b7fa618fcf76e953f4f5e192547945816d5358edffe39f6f94947"
+dependencies = [
+ "derive_builder_macro",
+]
+
+[[package]]
+name = "derive_builder_core"
+version = "0.20.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2d5bcf7b024d6835cfb3d473887cd966994907effbe9227e8c8219824d06c4e8"
+dependencies = [
+ "darling 0.20.11",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
+
+[[package]]
+name = "derive_builder_macro"
+version = "0.20.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ab63b0e2bf4d5928aff72e83a7dace85d7bba5fe12dcc3c5a572d78caffd3f3c"
+dependencies = [
+ "derive_builder_core",
+ "syn 2.0.119",
+]
+
 [[package]]
 name = "derive_more"
 version = "1.0.0"
@@ -1701,6 +1767,7 @@ dependencies = [
  "serde_json",
  "sha2 0.11.0",
  "swarm-queue-client",
+ "swarm-secret-client",
  "tempfile",
  "tokio",
  "tokio-stream",
@@ -3823,7 +3890,7 @@ version = "2.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "783d787bf21813b285f13019adc49e11af501c658890c1e519f31f937c68b7e3"
 dependencies = [
- "darling",
+ "darling 0.23.0",
  "proc-macro2",
  "quote",
  "serde_json",
@@ -4012,6 +4079,40 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "rustify"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4800ce4c1cc2fec12c559dae2ddbf0e17fcee7569b796e6d75898efef443368b"
+dependencies = [
+ "anyhow",
+ "async-trait",
+ "bytes",
+ "http",
+ "reqwest",
+ "rustify_derive",
+ "serde",
+ "serde_json",
+ "serde_urlencoded",
+ "thiserror 1.0.69",
+ "tracing",
+ "url",
+]
+
+[[package]]
+name = "rustify_derive"
+version = "0.5.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "78ea7fda74240f7410d0198b603a8a2f662acc7d76b6667a49f9b162cd8d9b4f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "regex",
+ "serde_urlencoded",
+ "syn 1.0.109",
+ "synstructure 0.12.6",
+]
+
 [[package]]
 name = "rustix"
 version = "1.1.4"
@@ -4642,6 +4743,17 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "swarm-secret-client"
+version = "0.1.0"
+dependencies = [
+ "reqwest",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.18",
+ "vaultrs",
+]
+
 [[package]]
 name = "swarmctl"
 version = "0.1.0"
@@ -4696,6 +4808,18 @@ dependencies = [
  "futures-core",
 ]
 
+[[package]]
+name = "synstructure"
+version = "0.12.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f36bdaa60a83aca3921b5259d5400cbf5e90fc51931376a9bd4a0eb79aa7210f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+ "unicode-xid",
+]
+
 [[package]]
 name = "synstructure"
 version = "0.13.2"
@@ -5337,6 +5461,25 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ba73ea9cf16a25df0c8caa16c51acb937d5712a8429db78a3ee29d5dcacd3a65"
 
+[[package]]
+name = "vaultrs"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "30ffcc0e81025065dda612ec1e26a3d81bb16ef3062354873d17a35965d68522"
+dependencies = [
+ "async-trait",
+ "derive_builder",
+ "http",
+ "reqwest",
+ "rustify",
+ "rustify_derive",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.18",
+ "tracing",
+ "url",
+]
+
 [[package]]
 name = "vcpkg"
 version = "0.2.15"
@@ -5873,7 +6016,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "syn 2.0.119",
- "synstructure",
+ "synstructure 0.13.2",
 ]
 
 [[package]]
@@ -5914,7 +6057,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "syn 2.0.119",
- "synstructure",
+ "synstructure 0.13.2",
 ]
 
 [[package]]

Cargo.toml

@@ -27,6 +27,7 @@ members = [
   "swarm-controller",
   "swarm-nats-auth",
   "swarm-queue-client",
+  "swarm-secret-client",
   "swarmctl",
 ]
 
@@ -90,7 +91,9 @@ hive-sock-client = { path = "hive-sock-client" }
 hive-types = { path = "hive-types" }
 swarm-authelia-bridge-sock = { path = "swarm-authelia-bridge-sock" }
 swarm-queue-client = { path = "swarm-queue-client" }
+swarm-secret-client = { path = "swarm-secret-client" }
 thiserror = "2"
+vaultrs = "0.8"
 tower-http = { version = "0.7", features = ["fs"] }
 uuid = { version = "1", features = ["v4"] }
 rmcp = { version = "2", default-features = false, features = [

hive-c0re/Cargo.toml

@@ -54,6 +54,7 @@ serde_json.workspace = true
 # the swarm controller reads it with, and `kv` for the same reason: the
 # bucket's name and creation config belong to neither end of it alone.
 swarm-queue-client = { workspace = true, features = ["kv", "notices"] }
+swarm-secret-client.workspace = true
 tokio.workspace = true
 tokio-stream.workspace = true
 tracing.workspace = true

hive-c0re/src/swarm_status.rs

@@ -198,7 +198,24 @@ async fn drain_swarm_events(
             return;
         }
     };
-    tracing::info!(%subject, %deploy_subject, "swarm events: listening");
+    // Also this hive's own, and for a second reason on top of the deploy
+    // subject's: the payload names an agent in *this* hive's state dir, so a
+    // notice for another hive is not merely noise, it is unactionable here.
+    let credential_subject = swarm_queue_client::credential_subject(&hive);
+    let mut credential_sub = match client.subscribe(credential_subject.clone()).await {
+        Ok(sub) => sub,
+        Err(e) => {
+            tracing::warn!(
+                subject = %credential_subject, error = %e,
+                "swarm events: subscribe failed; this hive will not hear credential notices"
+            );
+            return;
+        }
+    };
+    tracing::info!(
+        %subject, %deploy_subject, %credential_subject,
+        "swarm events: listening"
+    );
 
     loop {
         tokio::select! {
@@ -223,6 +240,13 @@ async fn drain_swarm_events(
                 };
                 handle_deploy_request(&coord, &msg.payload).await;
             }
+            msg = credential_sub.next() => {
+                let Some(msg) = msg else {
+                    tracing::warn!(subject = %credential_subject, "swarm events: credential subscription closed");
+                    return;
+                };
+                handle_credential_notice(&hive, &msg.payload).await;
+            }
             _ = shutdown.changed() => {
                 tracing::info!("swarm events: shutdown signal received");
                 return;
@@ -236,6 +260,39 @@ async fn drain_swarm_events(
 ///
 /// A payload that will not decode is worth a `warn`: the controller and this
 /// end share one type, so a decode failure means they disagree about it.
+/// Deliver the credential a [`swarm_queue_client::CredentialNotice`] names.
+///
+/// `hive` doubles as the cert-auth role this hive logs into the store as:
+/// `glue-bao-tls.nix` mints the client certificate with the hive name as its
+/// CN, and a bao cert role matches on CN — so the two share a name by
+/// construction rather than by convention.
+///
+/// ⚠️ Nothing here can log the secret, and that is structural rather than
+/// careful: the notice carries only names, and `deliver` writes the value
+/// without returning it.
+async fn handle_credential_notice(hive: &str, payload: &[u8]) {
+    let notice: swarm_queue_client::CredentialNotice = match serde_json::from_slice(payload) {
+        Ok(notice) => notice,
+        Err(e) => {
+            tracing::warn!(error = %e, "swarm events: undecodable credential notice");
+            return;
+        }
+    };
+    if let Err(e) = crate::workers::credential::deliver(&notice, hive).await {
+        // Warn rather than retry: the controller republishes, and a hive that
+        // spun here would hold the queue task off its other two subjects.
+        tracing::warn!(
+            agent = %notice.agent, account = %notice.account, error = ?e,
+            "swarm events: credential delivery failed"
+        );
+        return;
+    }
+    tracing::info!(
+        agent = %notice.agent, account = %notice.account,
+        "swarm events: credential delivered"
+    );
+}
+
 async fn handle_deploy_request(
     coord: &std::sync::Arc<crate::coordinator::Coordinator>,
     payload: &[u8],

hive-c0re/src/workers/credential.rs

@@ -0,0 +1,144 @@
+//! Delivering an agent's external-account credential from the swarm's secret
+//! store into that agent's own state dir.
+//!
+//! The controller publishes a [`CredentialNotice`] naming an agent and an
+//! account; this reads the value out of the store and writes it where the
+//! agent's matrix daemon already watches for it. Nothing here activates
+//! anything: `nix/agent-modules/matrix.nix` has a `systemd.paths` unit
+//! globbing `/agents/*/state/matrix-token*` that re-fires the daemon when a
+//! token appears, so arrival is the whole trigger.
+//!
+//! 🔑 The notice carries no secret — see [`swarm_queue_client::credential_subject`]
+//! for why that is a requirement rather than a preference. The value is read
+//! from the store under this hive's own identity.
+
+use std::os::unix::fs::PermissionsExt as _;
+use std::path::PathBuf;
+
+use anyhow::{Context, Result};
+use hive_types::Ident;
+use swarm_queue_client::CredentialNotice;
+use swarm_secret_client::{SecretStore, path};
+
+use crate::paths::agent_state_dir;
+
+/// The basename every matrix token must start with.
+///
+/// `nix/agent-modules/matrix.nix` asserts the same prefix on every configured
+/// `tokenFile`, because its path-watcher globs for it. A name written here
+/// that did not match would land a file the daemon never notices — no error,
+/// just a credential that silently never arrives.
+const TOKEN_PREFIX: &str = "matrix-token";
+
+/// Where `agent`'s credential for `account` is written.
+///
+/// Takes an [`Ident`] rather than a `&str` because the name arrives off the
+/// queue: `agent_state_dir` addresses a directory, and an unvalidated name
+/// there is a path-traversal argument. The compiler refusing the `&str` is the
+/// check — nothing here has to remember to perform one.
+#[must_use]
+pub fn token_path(agent: &Ident, account: &str) -> PathBuf {
+    agent_state_dir(agent).join(format!("{TOKEN_PREFIX}-{account}"))
+}
+
+/// Read the credential `notice` names and write it into the agent's state dir.
+///
+/// `cert_role` is the role on the store's `cert` auth mount whose policy scopes
+/// what this hive may read.
+///
+/// # Errors
+/// The store refusing, being unreachable, or holding nothing at that path; a
+/// name that is not a single path segment; or the write failing.
+pub async fn deliver(notice: &CredentialNotice, cert_role: &str) -> Result<()> {
+    // Parsed before anything is read, so a malformed name costs a decode and
+    // not a round trip to the store.
+    let agent = Ident::parse(&notice.agent)
+        .map_err(|e| anyhow::anyhow!("agent name {:?} off the queue: {e}", notice.agent))?;
+    let secret_path = path::matrix_account(&notice.agent, &notice.account)
+        .context("building the credential's path in the store")?;
+
+    let store = SecretStore::from_env(cert_role)
+        .await
+        .context("connecting to the swarm secret store")?;
+    let value = store
+        .read(&secret_path)
+        .await
+        .with_context(|| format!("reading {secret_path} from the store"))?;
+
+    write_token(&token_path(&agent, &notice.account), &value)
+}
+
+/// Write `value` to `dest` at `0600`, atomically.
+///
+/// Atomic because the daemon's path-watcher fires on the file *appearing*: a
+/// token written in place would be visible while still partial, and the daemon
+/// would read a truncated credential exactly once, at the moment it is hardest
+/// to reproduce. The temp name is dot-prefixed so it cannot match the
+/// `matrix-token*` glob on its way past.
+fn write_token(dest: &PathBuf, value: &str) -> Result<()> {
+    let dir = dest
+        .parent()
+        .context("a token path always has a parent state dir")?;
+    let name = dest
+        .file_name()
+        .context("a token path always has a file name")?;
+    let mut tmp = dir.join(".");
+    tmp.as_mut_os_string().push(name);
+    tmp.as_mut_os_string().push(".partial");
+
+    std::fs::write(&tmp, value).with_context(|| format!("writing {}", tmp.display()))?;
+    // Before the rename, so the file is never briefly readable by others under
+    // its final name.
+    std::fs::set_permissions(&tmp, std::fs::Permissions::from_mode(0o600))
+        .with_context(|| format!("restricting {}", tmp.display()))?;
+    std::fs::rename(&tmp, dest).with_context(|| format!("publishing {}", dest.display()))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_written_name_matches_the_glob_the_daemon_watches() {
+        // The other end of this agreement is an assertion in
+        // `nix/agent-modules/matrix.nix` and a `systemd.paths` glob — neither
+        // reachable from a Rust test, so the prefix is pinned here.
+        let p = token_path(&Ident::parse("dmatrix").expect("a legal agent name"), "ccc");
+        let name = p.file_name().unwrap().to_str().unwrap();
+        assert!(name.starts_with("matrix-token"), "got {name}");
+        assert_eq!(name, "matrix-token-ccc");
+    }
+
+    #[test]
+    fn the_partial_file_cannot_match_that_glob() {
+        // A temp name starting with `matrix-token` would be picked up
+        // mid-write; the dot prefix is what stops it.
+        let p = token_path(&Ident::parse("dmatrix").expect("a legal agent name"), "ccc");
+        let name = p.file_name().unwrap().to_str().unwrap();
+        let tmp = format!(".{name}.partial");
+        assert!(!tmp.starts_with("matrix-token"), "got {tmp}");
+    }
+
+    #[test]
+    fn a_name_that_could_address_another_agent_is_refused() {
+        // `path::matrix_account` owns this rule; asserted here because this is
+        // the module that feeds it names off the wire.
+        assert!(path::matrix_account("../argus", "ccc").is_err());
+        assert!(path::matrix_account("dmatrix", "../../etc/x").is_err());
+        assert!(path::matrix_account("dmatrix", "ccc").is_ok());
+    }
+
+    #[test]
+    fn an_agent_name_off_the_queue_must_pass_the_ident_parser_too() {
+        // Two independent refusals, not one restated: `path::matrix_account`
+        // guards the address in the *store*, `Ident` guards the address on
+        // *disk*. `token_path` cannot even be called without the second,
+        // which is why it takes an `Ident` rather than validating internally.
+        for bad in ["../argus", "dmatrix/../argus", "Dmatrix", "d matrix", ""] {
+            assert!(Ident::parse(bad).is_err(), "{bad:?} must be refused");
+        }
+        // The control: without it, a parser that rejected everything would
+        // satisfy the loop above.
+        assert!(Ident::parse("dmatrix").is_ok());
+    }
+}

hive-c0re/src/workers/mod.rs

@@ -8,6 +8,7 @@
 pub mod agent_sockets;
 pub mod auto_update;
 pub mod crash_watch;
+pub mod credential;
 pub mod knowledge;
 pub mod mcp_sockets;
 pub mod scheduled_prompts_worker;

nix/host-modules/hive-c0re/default.nix

@@ -13,6 +13,13 @@
 let
   cfg = config.services.hyperhive.c0re;
 
+  baoDeploy = config.services.hyperhive.deploy.bao;
+  # Held in one place because the LoadCredential below and the `BAO_*`
+  # environment in ./environment.nix have to agree on when they exist: a
+  # credential with no reader is dead weight, and an environment naming a
+  # credential nobody loaded is a daemon that fails at the TLS handshake.
+  baoClientIdentity = baoDeploy.clientCertFile != null && baoDeploy.clientKeyFile != null;
+
   caTrust = import ../lib/hive-ca-trust.nix {
     inherit lib;
     tlsCfg = config.services.hyperhive.deploy.hive-controller.tls;
@@ -309,7 +316,23 @@ in
           # delivers into a container, across a filesystem boundary.)
           lib.optional (
             config.services.hyperhive.swarm.statusPublish.clientSecretFile != null
-          ) "swarm-status-client.secret:${config.services.hyperhive.swarm.statusPublish.clientSecretFile}";
+          ) "swarm-status-client.secret:${config.services.hyperhive.swarm.statusPublish.clientSecretFile}"
+          # The secret store's client identity, on the same reasoning one
+          # paragraph up — with a sharper edge: ./glue-bao-tls.nix mints the
+          # key `0600` inside a `0700` directory, so hive-core cannot read it
+          # at all. Loading it as a credential is what makes the store
+          # reachable from an unprivileged daemon without widening either.
+          # The gate is the identity, never `deploy.bao.enable`: a hive that
+          # reads a store on another machine holds one of these and runs no
+          # store. ./glue-matrix-bao-token.nix gates its own reader the same
+          # way.
+          ++ lib.optionals baoClientIdentity [
+            "bao-client.pem:${baoDeploy.clientCertFile}"
+            "bao-client-key.pem:${baoDeploy.clientKeyFile}"
+          ]
+          ++ lib.optional (
+            baoClientIdentity && baoDeploy.serverCaFile != null
+          ) "bao-ca.pem:${baoDeploy.serverCaFile}";
         # Sandboxing. hive-c0re is unprivileged (runs as hive-core, never
         # setuid), makes HTTP requests to forge/matrix/Anthropic (keeps INET),
         # and delegates all privileged ops to hive-priv via a Unix socket.

nix/host-modules/hive-c0re/environment.nix

@@ -9,6 +9,13 @@
 }:
 let
   cfg = config.services.hyperhive.c0re;
+  baoDeploy = config.services.hyperhive.deploy.bao;
+  baoCfg = config.services.hyperhive.swarm.bao;
+  # The same test ../glue-matrix-bao-token.nix applies, and for the same
+  # reason: a reader is defined by holding a certificate the store accepts,
+  # not by sharing a host with the store. Deriving this from
+  # `deploy.bao.enable` would be the co-location assumption itself.
+  haveBaoClientIdentity = baoDeploy.clientCertFile != null && baoDeploy.clientKeyFile != null;
 in
 {
   # nix (the prebuild `nix build`, flake-check, and meta eval) writes
@@ -263,3 +270,23 @@ in
     # ./default.nix. The daemon reads a path, never a value.
     HIVE_C0RE_OIDC_CLIENT_SECRET_FILE = "%d/swarm-status-client.secret";
   }
+//
+  # Where the swarm's secret store is, and the identity this hive presents to
+  # it (hive-c0re::workers::credential). `swarm_secret_client` reads these
+  # spellings explicitly rather than vaultrs's `VAULT_*` defaults — falling
+  # through to those would build a client with no identity and fail at the TLS
+  # handshake, naming neither.
+  lib.optionalAttrs haveBaoClientIdentity {
+    BAO_ADDR = "https://${baoCfg.domain}:${toString baoCfg.port}";
+    # `%d`, not the paths themselves: the key is `0600` root-owned and this
+    # daemon runs as hive-core, so it never gets read access to the original.
+    # See the LoadCredential in ./default.nix.
+    BAO_CLIENT_CERT = "%d/bao-client.pem";
+    BAO_CLIENT_KEY = "%d/bao-client-key.pem";
+  }
+// lib.optionalAttrs (haveBaoClientIdentity && baoDeploy.serverCaFile != null) {
+  # Absent means the system trust store — right for a deployment with a real
+  # CA, wrong for the self-signed one ../glue-bao-tls.nix mints, which is why
+  # that file names this path rather than leaving it to a default.
+  BAO_CACERT = "%d/bao-ca.pem";
+}

nix/host-modules/swarm-bao.nix

@@ -245,10 +245,17 @@ in
 {
   # One service, two namespaces, and the split decides who may set what.
   #
-  # `deploy.bao.*` is what the host RUNNING the store decides: whether to run
+  # `deploy.bao.*` is mostly what the host RUNNING the store decides: whether
-  # it (`enable`, declared in ./deploy.nix with its siblings), which build,
+  # to run it (`enable`, declared in ./deploy.nix with its siblings), which
-  # how the root key is sealed, what it listens on. None of it means anything
+  # build, how the root key is sealed, what it listens on.
-  # on a host that only reads secrets.
+  #
+  # ⚠️ Except the reader's own identity. `clientCertFile`, `clientKeyFile` and
+  # `serverCaFile` live here too, and they are set by whoever authenticates TO
+  # the store — which includes a host that runs none of it. A reader is defined
+  # by holding a certificate the store accepts, never by sharing a host with
+  # one; ./glue-matrix-bao-token.nix gates on exactly that and not on `enable`.
+  # Reading this block as store-runner-only is what makes an off-host reader
+  # look inexpressible when it is already supported.
   #
   # `swarm.bao.*` below is what every host in the swarm has to agree on — the
   # name the store answers to, its port, its container. A host that is purely
@@ -378,9 +385,7 @@ in
         Certificate a **reader on this machine** presents to the store.
 
         The counterpart to {option}`services.hyperhive.deploy.bao.clientCaFile`,
-        which is the store's side of the same handshake. Only the store's side
+        which is the store's side of the same handshake.
-        was declarable, so a hive that did not run the store had no way to be
-        pointed at a certificate even when one was placed for it.
 
         On a hive that runs the store, a glue module supplies the leaf it minted,
         as a `mkDefault`. Everywhere else this is the credential an operator

nix/module-eval.nix

@@ -483,6 +483,58 @@ let
         in
         s ? swarm-bao-matrix-token && s.swarm-bao-matrix-token.requires == [ "swarm-bao-pki.service" ];
     }
+    {
+      # hive-c0re runs as hive-core and the client key is `0600` root-owned
+      # inside a `0700` directory, so the identity reaches the daemon as a
+      # systemd credential and the environment names `%d` rather than the
+      # file. Both halves are asserted together because either alone is a
+      # daemon that fails at the TLS handshake, naming neither.
+      name = "a reader hands hive-c0re a store identity the daemon cannot open itself";
+      ok =
+        let
+          s = baoRemoteReader.systemd.services;
+        in
+        s ? hive-c0re
+        && (s.hive-c0re.environment.BAO_CLIENT_CERT or null) == "%d/bao-client.pem"
+        && (s.hive-c0re.environment.BAO_CLIENT_KEY or null) == "%d/bao-client-key.pem"
+        && builtins.elem "bao-client.pem:/etc/pki/bao-client.pem" s.hive-c0re.serviceConfig.LoadCredential
+        && builtins.elem "bao-client-key.pem:/etc/pki/bao-client-key.pem" s.hive-c0re.serviceConfig.LoadCredential;
+    }
+    {
+      # Absence arm for the case above. A hive with no client identity gets no
+      # store environment at all — the daemon reports a queue it cannot serve
+      # rather than a handshake it cannot explain.
+      name = "a hive with no client identity gives hive-c0re no store environment";
+      ok =
+        let
+          s = matrixNoBaoIdentity.systemd.services;
+        in
+        s ? hive-c0re
+        && !(s.hive-c0re.environment ? BAO_ADDR)
+        && !(lib.any (c: lib.hasPrefix "bao-" c) s.hive-c0re.serviceConfig.LoadCredential);
+    }
+    {
+      # The CA is its own arm: absent means the system trust store, which is
+      # right for a deployment with a real CA and wrong for a self-signed one.
+      name = "a reader that names no store CA falls through to the system trust store";
+      ok =
+        let
+          s = baoRemoteReader.systemd.services;
+        in
+        s ? hive-c0re && !(s.hive-c0re.environment ? BAO_CACERT);
+    }
+    {
+      # Presence control for the arm above: the CA is conditional, not gone.
+      # Co-located, ./host-modules/glue-bao-tls.nix mints one and names it.
+      name = "a reader beside a self-signed store is given that store's CA";
+      ok =
+        let
+          s = baoWithMatrix.systemd.services;
+        in
+        s ? hive-c0re
+        && (s.hive-c0re.environment.BAO_CACERT or null) == "%d/bao-ca.pem"
+        && lib.any (c: lib.hasPrefix "bao-ca.pem:" c) s.hive-c0re.serviceConfig.LoadCredential;
+    }
     {
       # The name a reader dials has to resolve where the store runs; a
       # multi-host swarm resolves it upstream instead.

swarm-nats-auth/src/policy.rs

@@ -352,6 +352,17 @@ impl Policy {
             // admission). Same failure mode as the knowledge event above: a
             // refused publish reaches the client as a timeout.
             swarm_queue_client::DEPLOY_SUBJECT_WILDCARD.to_owned(),
+            // The credential notices: same per-hive family and same wildcard
+            // reasoning as the deploy events, and the same timeout-not-error
+            // failure if this line is missing.
+            //
+            // 🔑 Worth being explicit that this grant is not a confidentiality
+            // boundary, because it looks like one. `sub` is unrestricted, so
+            // any hive could subscribe to another's notices — which is exactly
+            // why the payload names a credential and never carries one. What
+            // scopes the secret is the store's own policy at read time, under
+            // the reading hive's certificate.
+            swarm_queue_client::CREDENTIAL_SUBJECT_WILDCARD.to_owned(),
             // The wanted-state buckets — one per hive, all created and written
             // by this single client.
             //
@@ -581,6 +592,36 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_reader_may_publish_a_credential_notice_to_any_hive() {
+        let p = policy().permissions("swarm-controller").expect("a reader");
+        assert!(
+            p.publish
+                .contains(&swarm_queue_client::CREDENTIAL_SUBJECT_WILDCARD.to_owned()),
+            "without this grant the controller's publish is refused, and a \
+             refusal arrives as a timeout — a hive that silently never receives \
+             a credential, with nothing in either log saying why"
+        );
+    }
+
+    #[test]
+    fn a_hive_may_not_publish_a_credential_notice() {
+        // A forged notice cannot leak a secret — the payload carries none — but
+        // it can make a hive fetch and overwrite an agent's token file with
+        // whatever the store holds for a name the forger chose.
+        let p = policy()
+            .permissions("hive-alpha")
+            .expect("a hive is admitted");
+        assert!(
+            !p.publish
+                .iter()
+                .any(|s| s == swarm_queue_client::CREDENTIAL_SUBJECT_WILDCARD
+                    || s == &swarm_queue_client::credential_subject("hive-alpha")),
+            "a hive must not publish credential notices, its own included: {:?}",
+            p.publish
+        );
+    }
+
     #[test]
     fn a_hive_may_not_publish_a_deploy_event_to_anyone_including_itself() {
         // Same arm as the knowledge event's, and it matters more here: a forged

swarm-queue-client/src/lib.rs

@@ -212,6 +212,42 @@ pub const DEPLOY_SUBJECT_WILDCARD: &str = "$SWARM.deploy.*";
 /// cannot drift into naming different families.
 const DEPLOY_SUBJECT_PREFIX: &str = "$SWARM.deploy";
 
+/// The subject the controller publishes on to tell `hive` that a credential
+/// for one of its agents is waiting in the secret store. Same per-hive family
+/// as [`deploy_subject`], here for the same three-crate reason.
+///
+/// 🔑 **The message NAMES a credential and never carries one**, and the note
+/// on [`deploy_subject`] is why: the family split buys quiet, not
+/// confidentiality — the auth-callout responder scopes `pub` and leaves `sub`
+/// unrestricted, so any hive that wanted another's messages could subscribe to
+/// them. A secret in this payload would be readable swarm-wide. The hive reads
+/// the value from the store under its own identity instead, where the store's
+/// policy is the thing that actually scopes it.
+#[must_use]
+pub fn credential_subject(hive: &str) -> String {
+    format!("{CREDENTIAL_SUBJECT_PREFIX}.{hive}")
+}
+
+/// The publish grant covering every [`credential_subject`] — a wildcard for
+/// the same no-roster reason as [`DEPLOY_SUBJECT_WILDCARD`].
+pub const CREDENTIAL_SUBJECT_WILDCARD: &str = "$SWARM.credential.*";
+
+/// Shared by [`credential_subject`] and [`CREDENTIAL_SUBJECT_WILDCARD`] so the
+/// two cannot drift into naming different families.
+const CREDENTIAL_SUBJECT_PREFIX: &str = "$SWARM.credential";
+
+/// What a [`credential_subject`] message says: which agent's credential
+/// changed, and which account it belongs to. Deliberately the whole payload —
+/// anything more would be either derivable by the reader or a secret that
+/// must not be on the wire.
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct CredentialNotice {
+    /// The agent whose state dir receives the credential.
+    pub agent: String,
+    /// The external account the credential authenticates as.
+    pub account: String,
+}
+
 /// What a [`deploy_subject`] message carries.
 ///
 /// Only the agent: the subject already names the hive, and repeating it here

swarm-secret-client/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "swarm-secret-client"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+# `Identity` is re-exported by vaultrs, but the identity is built here (from
+# the `BAO_*` files) rather than by its env defaults, so the dependency is
+# direct rather than incidental.
+reqwest.workspace = true
+serde.workspace = true
+thiserror.workspace = true
+vaultrs.workspace = true
+
+[dev-dependencies]
+serde_json.workspace = true
+
+[lints]
+workspace = true

swarm-secret-client/README.md

@@ -0,0 +1,77 @@
+# swarm-secret-client
+
+Reading and writing a swarm credential in the secret store, over `vaultrs`.
+The HTTP is that crate's job. What this one owns is the **agreements** both
+ends of the store have to state identically: where a credential lives, which
+field its bytes are in, and how this deployment's environment becomes a
+logged-in client.
+
+The surrounding picture — which secret is minted where, and why delivery is a
+copy rather than a bind mount — is [`docs/swarm/secrets.md`](../docs/swarm/secrets.md).
+
+## Why a crate and not a module per binary
+
+The store has two Rust ends and they are peers: the swarm controller writes a
+credential, a hive reads it. Neither is senior to the other, so a path
+formatted at each call site is an agreement with no owner — it holds right up
+until one side is edited alone, and then it fails as a missing key rather than
+as a mismatch.
+
+The third end is what settles it. `nix/host-modules/glue-matrix-bao-token.nix`
+reads the store with `bao kv get -field=value`. That reader is a shell line in
+a nix module: it cannot be renamed by the same refactor as a Rust struct, and
+no Rust test reaches it. So the field name is pinned by a test against the
+serialised literal rather than left to the struct definition.
+
+## The identity is a certificate, and the role is the hive's name
+
+Authentication is the store's `cert` auth method.
+`nix/host-modules/glue-bao-tls.nix` mints the client certificate with its CN
+set to the hive's name, because a cert-auth role matches on the CN. So
+`cert_role` is not a free choice for the caller: a hive passes its own name,
+and the policy attached to that role is what scopes what it may read.
+
+The listener's `tls_require_and_verify_client_cert` is a different thing and
+not a substitute. It decides who may open a connection; it says nothing about
+who the connection belongs to, and a store with the option set and no `cert`
+mount configured refuses every login made here. That refusal is what
+`Error::Vault` out of `connect` means.
+
+## Configuration
+
+`Settings::from_env` reads `BAO_ADDR`, `BAO_CLIENT_CERT`, `BAO_CLIENT_KEY`,
+and the optional `BAO_CACERT`.
+
+**The `BAO_` spellings are read explicitly rather than left to `vaultrs`.** Its
+own defaults look for `VAULT_ADDR` / `VAULT_CLIENT_CERT` / `VAULT_CLIENT_KEY`,
+which no unit in this tree sets. Falling through to them builds a client with
+**no identity at all**, and that surfaces as a TLS handshake failure — a place
+that names neither the variable nor the reason.
+
+**Empty is as absent as unset.** systemd renders an unset nix option as
+`Environment=BAO_CACERT=`, so empty is the shape a missing value arrives in.
+
+**The certificate and key are paths, not values**, and are read at connect
+time — same rule as every other credential in this tree, for the same reason:
+a value in a nix expression is rendered into the world-readable store.
+
+Reading the environment is separate from connecting (`Settings::from_lookup`)
+because every one of those failures is a misconfiguration an operator has to
+read an error about, and none of them needs a reachable store to happen.
+
+## Names that arrive from elsewhere
+
+`path::matrix_account` is fallible, which for a string formatter needs saying:
+its segments are an agent name from the topology and an account name from that
+agent's own config. A `/` turns one agent's segment into another agent's
+directory and `..` walks out of the prefix entirely, so the charset it accepts
+is deliberately narrower than what the store would.
+
+## What this crate does not do
+
+It has no opinion on **what** a caller may read. That is the policy attached to
+the cert role, and it lives in the store.
+
+It holds the token minted at login and renews nothing. A handle is built per
+credential, so the login is the cheap part of a rare operation — a caller that
+wanted to keep one alive across a token's lifetime would need more than this.

swarm-secret-client/src/client.rs

@@ -0,0 +1,248 @@
+//! A logged-in handle on the store, built from this deployment's environment.
+
+use serde::{Deserialize, Serialize};
+use vaultrs::client::{Client, VaultClient, VaultClientSettingsBuilder};
+
+use crate::{Error, path::MOUNT};
+
+/// The store's address.
+pub const ENV_ADDR: &str = "BAO_ADDR";
+/// PEM client certificate presented to the store's listener.
+pub const ENV_CLIENT_CERT: &str = "BAO_CLIENT_CERT";
+/// PEM private key for [`ENV_CLIENT_CERT`].
+pub const ENV_CLIENT_KEY: &str = "BAO_CLIENT_KEY";
+/// CA bundle the store's own certificate is verified against. Optional:
+/// absent means the system trust store, which is what a deployment with a
+/// real CA wants and what a self-signed one must not be left with.
+pub const ENV_CACERT: &str = "BAO_CACERT";
+
+/// The auth mount a cert login goes through, unless a caller says otherwise.
+pub const DEFAULT_CERT_MOUNT: &str = "cert";
+
+#[derive(Serialize, Deserialize)]
+struct Credential {
+    /// `glue-matrix-bao-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. Renaming it silently breaks that unit.
+    value: String,
+}
+
+/// Where the store is and which identity we present to it.
+///
+/// Separate from the connect so the environment can be checked without one:
+/// every arm below is a misconfiguration an operator has to read an error
+/// about, and none of them needs a reachable store to happen.
+#[derive(Debug, PartialEq, Eq)]
+pub struct Settings {
+    address: String,
+    cert_path: String,
+    key_path: String,
+    ca_path: Option<String>,
+}
+
+impl Settings {
+    /// Read the `BAO_*` variables from the process environment.
+    ///
+    /// # Errors
+    /// [`Error::MissingEnv`] naming the first variable that is unset or empty.
+    pub fn from_env() -> Result<Self, Error> {
+        Self::from_lookup(|k| std::env::var(k).ok())
+    }
+
+    /// [`Settings::from_env`] against an arbitrary lookup.
+    ///
+    /// [`vaultrs`]'s own defaults are deliberately not used: it looks for
+    /// `VAULT_ADDR` / `VAULT_CLIENT_CERT` / `VAULT_CLIENT_KEY`, and every unit
+    /// in this tree sets the `BAO_` spellings. Falling through to those
+    /// defaults yields a client with **no identity**, which fails at the TLS
+    /// handshake rather than anywhere that names the cause.
+    ///
+    /// # Errors
+    /// [`Error::MissingEnv`] naming the first variable that is unset or empty.
+    pub fn from_lookup(get: impl Fn(&str) -> Option<String>) -> Result<Self, Error> {
+        let required = |var: &'static str| -> Result<String, Error> {
+            get(var)
+                .filter(|v| !v.is_empty())
+                .ok_or(Error::MissingEnv(var))
+        };
+        Ok(Self {
+            address: required(ENV_ADDR)?,
+            cert_path: required(ENV_CLIENT_CERT)?,
+            key_path: required(ENV_CLIENT_KEY)?,
+            ca_path: get(ENV_CACERT).filter(|v| !v.is_empty()),
+        })
+    }
+}
+
+/// A client that has already exchanged its certificate for a token.
+pub struct SecretStore {
+    inner: VaultClient,
+}
+
+fn read_file(var: &'static str, path: &str) -> Result<Vec<u8>, Error> {
+    std::fs::read(path).map_err(|source| Error::Identity {
+        var,
+        path: path.to_owned(),
+        source,
+    })
+}
+
+impl SecretStore {
+    /// Connect using the `BAO_*` environment and log in with the certificate
+    /// auth method, returning a handle that already holds a token.
+    ///
+    /// `cert_role` is the role configured on the store's `cert` mount; it
+    /// selects which policy the returned token carries.
+    ///
+    /// # Errors
+    /// As [`SecretStore::connect`], plus [`Error::MissingEnv`].
+    pub async fn from_env(cert_role: &str) -> Result<Self, Error> {
+        Self::connect(&Settings::from_env()?, cert_role, DEFAULT_CERT_MOUNT).await
+    }
+
+    /// Present `settings`' identity to the store and log in at `cert_mount`.
+    ///
+    /// # Errors
+    /// [`Error::Identity`] when a named file cannot be read, [`Error::Tls`]
+    /// when the cert/key pair does not form a usable identity,
+    /// [`Error::Settings`] when the address will not parse, and
+    /// [`Error::Vault`] when the store refuses the login — which is what an
+    /// unconfigured `cert` mount looks like from here.
+    pub async fn connect(
+        settings: &Settings,
+        cert_role: &str,
+        cert_mount: &str,
+    ) -> Result<Self, Error> {
+        // One PEM blob holding both, which is the shape `Identity::from_pem`
+        // wants; the two stay separate on disk because the key is the half
+        // that gets `0600`.
+        let mut pem = read_file(ENV_CLIENT_CERT, &settings.cert_path)?;
+        pem.push(b'\n');
+        pem.extend_from_slice(&read_file(ENV_CLIENT_KEY, &settings.key_path)?);
+        let identity = reqwest::Identity::from_pem(&pem).map_err(Error::Tls)?;
+
+        let mut builder = VaultClientSettingsBuilder::default();
+        builder
+            .address(settings.address.clone())
+            .identity(Some(identity));
+        if let Some(ca) = &settings.ca_path {
+            builder.ca_certs(vec![ca.clone()]);
+        }
+        let built = builder
+            .build()
+            .map_err(|e| Error::Settings(e.to_string()))?;
+
+        let mut inner = VaultClient::new(built)?;
+        let auth = vaultrs::auth::cert::login(&inner, cert_mount, cert_role).await?;
+        inner.set_token(&auth.client_token);
+        Ok(Self { inner })
+    }
+
+    /// Read the credential stored at `path`.
+    ///
+    /// # Errors
+    /// [`Error::Vault`] when the path does not exist, the token's policy does
+    /// not cover it, or the stored object has no `value` field.
+    pub async fn read(&self, path: &str) -> Result<String, Error> {
+        let c: Credential = vaultrs::kv2::read(&self.inner, MOUNT, path).await?;
+        Ok(c.value)
+    }
+
+    /// Write `value` as the credential at `path`, creating a new version.
+    ///
+    /// # Errors
+    /// [`Error::Vault`] when the token's policy does not cover the path.
+    pub async fn write(&self, path: &str, value: &str) -> Result<(), Error> {
+        let body = Credential {
+            value: value.to_owned(),
+        };
+        vaultrs::kv2::set(&self.inner, MOUNT, path, &body).await?;
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A lookup standing in for a fully-configured unit's environment.
+    fn full(k: &str) -> Option<String> {
+        match k {
+            ENV_ADDR => Some("https://store.invalid:8200".to_owned()),
+            ENV_CLIENT_CERT => Some("/run/c.pem".to_owned()),
+            ENV_CLIENT_KEY => Some("/run/k.pem".to_owned()),
+            _ => None,
+        }
+    }
+
+    #[test]
+    fn a_complete_environment_is_accepted() {
+        // The control: without this, every assertion below could be passing
+        // because `from_lookup` rejects everything.
+        let s = Settings::from_lookup(full).expect("every required variable is set");
+        assert_eq!(s.address, "https://store.invalid:8200");
+        assert_eq!(s.ca_path, None, "an absent CA is the system trust store");
+    }
+
+    #[test]
+    fn each_required_variable_is_named_when_it_is_the_missing_one() {
+        for var in [ENV_ADDR, ENV_CLIENT_CERT, ENV_CLIENT_KEY] {
+            let e = Settings::from_lookup(|k| if k == var { None } else { full(k) })
+                .expect_err("one required variable is absent");
+            assert!(
+                matches!(e, Error::MissingEnv(v) if v == var),
+                "dropping {var} should name {var}, got {e:?}"
+            );
+        }
+    }
+
+    #[test]
+    fn an_empty_variable_is_as_absent_as_an_unset_one() {
+        // systemd writes `Environment=BAO_CACERT=` for an unset nix option, so
+        // empty is the shape this actually arrives in.
+        let e = Settings::from_lookup(|k| {
+            if k == ENV_ADDR {
+                Some(String::new())
+            } else {
+                full(k)
+            }
+        })
+        .expect_err("an empty address is not an address");
+        assert!(matches!(e, Error::MissingEnv(ENV_ADDR)), "got {e:?}");
+
+        let s = Settings::from_lookup(|k| {
+            if k == ENV_CACERT {
+                Some(String::new())
+            } else {
+                full(k)
+            }
+        })
+        .expect("an empty CA is optional, not fatal");
+        assert_eq!(s.ca_path, None);
+    }
+
+    #[test]
+    fn an_unreadable_cert_names_the_file_and_the_variable() {
+        let e = read_file(ENV_CLIENT_CERT, "/nonexistent/cert.pem")
+            .expect_err("the file does not exist");
+        match e {
+            Error::Identity { var, ref path, .. } => {
+                assert_eq!(var, ENV_CLIENT_CERT);
+                assert_eq!(path, "/nonexistent/cert.pem");
+            }
+            other => panic!("wanted an Identity error, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn the_value_field_matches_what_the_nix_reader_asks_for() {
+        // The literal is the point: `bao kv get -field=value` is the other end
+        // of this agreement and lives in a file no Rust test can reach, so the
+        // name is pinned here rather than derived from the struct.
+        let json = serde_json::to_string(&Credential {
+            value: "t".to_owned(),
+        })
+        .expect("a struct of one String serialises");
+        assert_eq!(json, r#"{"value":"t"}"#);
+    }
+}

swarm-secret-client/src/lib.rs

@@ -0,0 +1,68 @@
+//! The swarm's secret-store client: where a credential lives, and how both ends
+//! reach it.
+//!
+//! The HTTP is [`vaultrs`]'s job. What this crate owns is the *agreements* —
+//! the path a credential is written to and read from ([`path`]), the field its
+//! bytes live in, and the translation from this deployment's environment into
+//! a logged-in client ([`client`]). Each of those is a thing the controller and
+//! a hive must say identically, so it is said once here.
+
+pub mod client;
+pub mod path;
+
+pub use client::SecretStore;
+
+/// What can go wrong between "we have a client certificate" and "we have the
+/// credential".
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    /// A name that would have addressed something other than what the caller
+    /// meant. See [`path`].
+    #[error("{kind} name {value:?} is not a single path segment of [A-Za-z0-9_-]")]
+    PathSegment {
+        /// Which name was rejected — `agent` or `account`.
+        kind: &'static str,
+        /// The offending value, quoted in the message because the caller
+        /// usually got it from config and needs to see which one.
+        value: String,
+    },
+
+    /// A variable the store's address or identity comes from is unset or
+    /// empty. Named rather than defaulted: a wrong store address fails much
+    /// later and much less clearly than a missing one.
+    #[error("{0} is unset or empty")]
+    MissingEnv(&'static str),
+
+    /// A client-certificate file named by the environment could not be read.
+    #[error("reading {path} (from {var}): {source}")]
+    Identity {
+        /// The variable that named the file.
+        var: &'static str,
+        /// The path it named.
+        path: String,
+        /// The underlying IO failure.
+        source: std::io::Error,
+    },
+
+    /// The address would not parse into a URL the client can use.
+    #[error("the store's settings are unusable: {0}")]
+    Settings(String),
+
+    /// The store refused us, was unreachable, or answered something we could
+    /// not parse.
+    #[error(transparent)]
+    Vault(#[from] Box<vaultrs::error::ClientError>),
+
+    /// The client certificate and key did not form a usable identity, or the
+    /// CA bundle did not parse.
+    #[error("building the TLS identity: {0}")]
+    Tls(#[source] reqwest::Error),
+}
+
+impl From<vaultrs::error::ClientError> for Error {
+    fn from(e: vaultrs::error::ClientError) -> Self {
+        // Boxed because `ClientError` is large enough that carrying it inline
+        // makes every `Result` in the crate pay for the rare arm.
+        Self::Vault(Box::new(e))
+    }
+}

swarm-secret-client/src/path.rs

@@ -0,0 +1,98 @@
+//! Where a credential lives, for both ends of the store.
+//!
+//! The controller writes and a hive reads, and neither is senior to the other,
+//! so the path they must agree on is built here rather than formatted at each
+//! call site.
+
+use crate::Error;
+
+/// The KV v2 mount every swarm secret lives under.
+///
+/// A literal because the store's existing reader already hardcodes the same
+/// one (`secret/swarm/matrix/registration-token`); an option nobody sets would
+/// be two ways to say one thing.
+pub const MOUNT: &str = "secret";
+
+/// The prefix under [`MOUNT`] owned by per-agent credentials.
+pub const AGENT_PREFIX: &str = "swarm/agents";
+
+/// A path segment that cannot change the path's shape.
+///
+/// The charset is deliberately narrower than what the store accepts: a `/`
+/// turns one agent's segment into another agent's directory, and `..` walks
+/// out of the prefix entirely. Both are names this crate receives from
+/// elsewhere — an agent name from the topology, an account name from an
+/// agent's own config — so neither is trusted to be well-formed here.
+fn checked_segment(kind: &'static str, value: &str) -> Result<(), Error> {
+    if value.is_empty() {
+        return Err(Error::PathSegment {
+            kind,
+            value: value.to_owned(),
+        });
+    }
+    if !value
+        .bytes()
+        .all(|b| b.is_ascii_alphanumeric() || b == b'-' || b == b'_')
+    {
+        return Err(Error::PathSegment {
+            kind,
+            value: value.to_owned(),
+        });
+    }
+    Ok(())
+}
+
+/// The path holding `agent`'s token for the external matrix account `account`.
+///
+/// # Errors
+/// [`Error::PathSegment`] when either name contains anything but
+/// `[A-Za-z0-9_-]`, which is what keeps one agent's name from addressing
+/// another agent's secret.
+pub fn matrix_account(agent: &str, account: &str) -> Result<String, Error> {
+    checked_segment("agent", agent)?;
+    checked_segment("account", account)?;
+    Ok(format!("{AGENT_PREFIX}/{agent}/matrix/{account}"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_well_formed_pair_lands_under_the_agent_prefix() {
+        let p = matrix_account("atlas", "ops-relay").expect("both segments are legal");
+        assert_eq!(p, "swarm/agents/atlas/matrix/ops-relay");
+        assert!(p.starts_with(AGENT_PREFIX));
+    }
+
+    #[test]
+    fn a_segment_cannot_escape_its_own_directory() {
+        // Each of these is a *different* way to address another agent's tree,
+        // and the last two are the ones a charset check catches but a
+        // `contains("..")` check does not.
+        for bad in [
+            "../argus",
+            "atlas/../argus",
+            "atlas/matrix",
+            "a b",
+            "a.b",
+            "",
+        ] {
+            assert!(
+                matrix_account(bad, "ops-relay").is_err(),
+                "agent segment {bad:?} must be refused"
+            );
+            assert!(
+                matrix_account("atlas", bad).is_err(),
+                "account segment {bad:?} must be refused"
+            );
+        }
+    }
+
+    #[test]
+    fn the_legal_charset_is_actually_reachable() {
+        // The control for the test above: if `checked_segment` rejected
+        // everything, the escape cases would pass for the wrong reason.
+        assert!(matrix_account("a-b_C9", "d-e_F0").is_ok());
+    }
+}