move agent_token from swarm-queue-client to swarm-agent-verify

per #5471, verifiers stop depending on the queue client. the module is
store-free and needs only thiserror, so swarm-agent-verify gains one
dependency and loses its swarm-queue-client edge. consumers import it
from swarm_agent_verify; swarm-logs and swarm-metrics, whose only use of
swarm-queue-client was this module, drop that dependency, as does
swarm-logs-auth.

swarm-agent-verify/Cargo.toml

@@ -7,10 +7,9 @@ edition.workspace = true
 anyhow.workspace = true
 # Comparing an agent's presented secret with the stored one.
 subtle.workspace = true
-# The agent-token spelling the agent also uses.
-swarm-queue-client.workspace = true
 # The read of the stored credential the token is checked against.
 swarm-secret-client.workspace = true
+thiserror.workspace = true
 tokio.workspace = true
 tracing.workspace = true
 

swarm-agent-verify/src/agent_token.rs

@@ -0,0 +1,153 @@
+//! The one spelling of the token an agent presents its own queue secret in,
+//! shared by the agent that formats it and the verifiers that parse it back:
+//! [`AGENT_TOKEN_PREFIX`], the agent's name, `.`, the secret.
+//!
+//! The secret itself lives at `swarm/agents/<agent>/queue` in the swarm's
+//! secret store (`swarm_secret_client::queue`). This module knows nothing of
+//! the store.
+
+/// Marks an `auth_token` as an agent's own credential.
+///
+/// An OIDC access token may itself contain `.`, so the `<agent>.<secret>`
+/// shape alone does not tell the two apart. Authelia's access tokens start
+/// `authelia_at_`.
+pub const AGENT_TOKEN_PREFIX: &str = "swarm-agent.";
+
+/// An agent's own credential as presented at the queue.
+///
+/// No `Debug`: `secret` is the credential itself.
+pub struct AgentToken<'a> {
+    /// The agent the presenter claims to be. Unproven until `secret` is
+    /// checked against that agent's stored credential.
+    pub agent: &'a str,
+    /// The presented secret.
+    pub secret: &'a str,
+}
+
+/// A token that is not `<agent>.<secret>` after its prefix, or parts that
+/// would not make one. Carries the reason only: a token holds a secret.
+#[derive(Debug, thiserror::Error)]
+#[error("malformed agent token: {0}")]
+pub struct Malformed(pub &'static str);
+
+/// Spell `agent`'s credential as the token it presents at the queue.
+///
+/// # Errors
+/// [`Malformed`] when `agent` or `secret` is empty or holds anything outside
+/// `[A-Za-z0-9_-]`. Either would make [`parse_agent_token`] split the token
+/// differently than it was joined.
+pub fn format_agent_token(agent: &str, secret: &str) -> Result<String, Malformed> {
+    check_agent(agent)?;
+    check_secret(secret)?;
+    Ok(format!("{AGENT_TOKEN_PREFIX}{agent}.{secret}"))
+}
+
+/// Read a presented token back into an [`AgentToken`].
+///
+/// `None` when `token` does not start with [`AGENT_TOKEN_PREFIX`]: it is some
+/// other kind of token, not a malformed one of these. `Some(Err(_))` when it
+/// does and is not exactly `<agent>.<secret>` after it.
+#[must_use]
+pub fn parse_agent_token(token: &str) -> Option<Result<AgentToken<'_>, Malformed>> {
+    let rest = token.strip_prefix(AGENT_TOKEN_PREFIX)?;
+    Some(
+        rest.split_once('.')
+            .ok_or(Malformed("no `.` between the agent and the secret"))
+            .and_then(|(agent, secret)| {
+                check_agent(agent)?;
+                check_secret(secret)?;
+                Ok(AgentToken { agent, secret })
+            }),
+    )
+}
+
+fn is_segment(s: &str) -> bool {
+    !s.is_empty()
+        && s.bytes()
+            .all(|b| b.is_ascii_alphanumeric() || b == b'-' || b == b'_')
+}
+
+/// The alphabet a store path segment allows, so the name cannot widen a
+/// subject with `.`, `*` or `>`, or address another agent's path.
+fn check_agent(agent: &str) -> Result<(), Malformed> {
+    if is_segment(agent) {
+        Ok(())
+    } else {
+        Err(Malformed("the agent is not a single [A-Za-z0-9_-] segment"))
+    }
+}
+
+/// The alphabet the controller mints secrets in, base64url without padding.
+/// The error never carries the value.
+fn check_secret(secret: &str) -> Result<(), Malformed> {
+    if is_segment(secret) {
+        Ok(())
+    } else {
+        Err(Malformed(
+            "the secret is empty or holds a byte outside [A-Za-z0-9_-]",
+        ))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn an_agent_token_round_trips() {
+        let token = format_agent_token("atlas", "Ab9_-z").expect("legal");
+        assert_eq!(token, "swarm-agent.atlas.Ab9_-z");
+        let parsed = parse_agent_token(&token)
+            .expect("carries the prefix")
+            .expect("well-formed");
+        assert_eq!(parsed.agent, "atlas");
+        assert_eq!(parsed.secret, "Ab9_-z");
+    }
+
+    /// Anything without the prefix belongs to the OIDC path, including a
+    /// token that happens to look like `<agent>.<secret>`.
+    #[test]
+    fn a_token_without_the_prefix_is_not_an_agent_token() {
+        for token in ["authelia_at_abc.def", "atlas.s3cr3t", "", "swarm-agent"] {
+            assert!(parse_agent_token(token).is_none(), "{token:?}");
+        }
+    }
+
+    #[test]
+    fn a_malformed_agent_token_is_refused() {
+        for token in [
+            "swarm-agent.",
+            "swarm-agent.atlas",
+            "swarm-agent.atlas.",
+            "swarm-agent..s3cr3t",
+            "swarm-agent.atlas.s3.cr3t",
+            "swarm-agent.at*las.s3cr3t",
+            "swarm-agent.at>las.s3cr3t",
+            "swarm-agent.at/las.s3cr3t",
+            "swarm-agent.atlas.s3cr3t=",
+            "swarm-agent.atlas.s3 cr3t",
+        ] {
+            assert!(
+                matches!(parse_agent_token(token), Some(Err(_))),
+                "{token:?} must be refused"
+            );
+        }
+    }
+
+    /// What formats always parses back into the same two parts.
+    #[test]
+    fn formatting_refuses_what_parsing_would_split_differently() {
+        assert!(format_agent_token("at.las", "s3cr3t").is_err());
+        assert!(format_agent_token("atlas", "s3.cr3t").is_err());
+        assert!(format_agent_token("atlas", "").is_err());
+        assert!(format_agent_token("", "s3cr3t").is_err());
+    }
+
+    #[test]
+    fn a_malformed_secret_is_not_echoed_in_the_error() {
+        let Some(Err(e)) = parse_agent_token("swarm-agent.atlas.hunter2!") else {
+            panic!("must be refused");
+        };
+        assert!(!e.to_string().contains("hunter2"), "{e}");
+    }
+}

swarm-agent-verify/src/lib.rs

@@ -1,22 +1,27 @@
 //! Checking an agent's own credential against the swarm secret store.
 //!
-//! The token is `swarm_queue_client::agent_token`'s spelling: a prefix, the
+//! The token is [`agent_token`]'s spelling: a prefix, the
 //! agent's name, and its secret. The name is only a claim. It becomes an
 //! identity when the secret equals the one stored at
 //! `swarm/agents/<agent>/queue`, and [`verify`] grants nothing on the name
 //! alone: every path that does not reach that comparison, and every lookup
 //! that fails, is a denial.
 //!
+//! [`agent_token`] also holds the spelling the agents, `swarm-logs` and
+//! `swarm-metrics` format their token in.
+//!
 //! Two verifiers use this: the queue's auth callout (`swarm-nats-auth`) and
 //! the log store's `auth_request` responder (`swarm-logs-auth`). Each reads
 //! the store under its own certificate.
 
+pub mod agent_token;
+
 use std::future::Future;
 use std::time::Duration;
 
+use crate::agent_token::AgentToken;
 use anyhow::Context;
 use subtle::ConstantTimeEq;
-use swarm_queue_client::agent_token::AgentToken;
 use swarm_secret_client::client::{DEFAULT_CERT_MOUNT, Settings};
 use swarm_secret_client::queue::{self, AgentCredential};
 
@@ -118,7 +123,7 @@ pub async fn verify(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use swarm_queue_client::agent_token::parse_agent_token;
+    use crate::agent_token::parse_agent_token;
 
     /// A store holding at most one credential, or failing outright.
     enum Fake {