1 files changed, 205 insertions, 0 deletions
diff --git a/src/access/challenge.rs b/src/access/challenge.rs
new file mode 100644
index 0000000..64cafac
--- /dev/null
+++ b/src/access/challenge.rs
@@ -0,0 +1,205 @@
+//! Logging in by key (`POST /auth/challenge`, `POST /auth/login/key`): a node hands out
+//! a challenge, signed by it, naming it, valid for a while (`issue`); the user signs it
+//! with one of its keys, as `ssh-keygen -Y sign -n continuum-login` does
+//! (`access::sshsig`), and the node takes the signature once, from the user that key is
+//! of in the registry (`registry::User::keys`; `take`). A challenge is checked by its
+//! signature, so a node keeps none it handed out, only those used, until they expire
+//! (`Used`).
+//!
+//! A challenge is `cc1.<payload>.<signature>` (each base64url, unpadded): the payload is
+//! JSON naming the node, a random nonce and when it expires (Unix ms); the signature is
+//! the node's, over `continuum/challenge/1\0` and the payload.
+
+use std::collections::HashMap;
+
+use base64::{Engine, engine::general_purpose::URL_SAFE_NO_PAD as B64URL};
+use serde::{Deserialize, Serialize};
+
+use super::{registry, sshsig};
+use crate::document::ReadDoc;
+use crate::identity::NodeKey;
+
+/// What a user's signature over a challenge is for (`sshsig`).
+pub(crate) const NAMESPACE: &str = "continuum-login";
+/// What every challenge starts with.
+const PREFIX: &str = "cc1";
+/// How long a challenge is valid (ms).
+pub(crate) const LIFETIME_MS: u64 = 5 * 60 * 1000;
+/// Used challenges kept at most: past it, logins by key wait for some to expire.
+const MAX_USED: usize = 10_000;
+
+#[derive(Debug, Serialize, Deserialize)]
+struct Payload {
+    node: String,
+    nonce: String,
+    exp: u64,
+}
+
+fn message(payload: &[u8]) -> Vec<u8> {
+    let mut message = b"continuum/challenge/1\0".to_vec();
+    message.extend_from_slice(payload);
+    message
+}
+
+/// A new challenge by `key`'s node, valid from `now` for `LIFETIME_MS`; with when it
+/// expires.
+pub(crate) fn issue(key: &NodeKey, now: u64) -> (String, u64) {
+    let payload = Payload {
+        node: key.id(),
+        nonce: registry::new_id(),
+        exp: now.saturating_add(LIFETIME_MS),
+    };
+    let bytes = serde_json::to_vec(&payload).expect("a challenge serializes");
+    let signature = key.sign(&message(&bytes));
+    (
+        format!(
+            "{PREFIX}.{}.{}",
+            B64URL.encode(&bytes),
+            B64URL.encode(signature)
+        ),
+        payload.exp,
+    )
+}
+
+/// The challenges used, by nonce, with when each expires.
+#[derive(Debug, Default)]
+pub(crate) struct Used(HashMap<String, u64>);
+
+/// Takes `signature` (armored) over `challenge` as a login of `user`, whose registry
+/// `doc` holds: a challenge of `key`'s node, unexpired at `now`, not used before (then
+/// used, in `used`), signed by one of `user`'s keys, `user` enabled. Why not, if not.
+pub(crate) fn take(
+    doc: &impl ReadDoc,
+    key: &NodeKey,
+    challenge: &str,
+    signature: &str,
+    user: &str,
+    now: u64,
+    used: &mut Used,
+) -> Result<(), String> {
+    let mut parts = challenge.split('.');
+    let (Some(PREFIX), Some(payload), Some(signed), None) =
+        (parts.next(), parts.next(), parts.next(), parts.next())
+    else {
+        return Err("not a challenge (cc1.<payload>.<signature>)".to_owned());
+    };
+    let bytes = B64URL
+        .decode(payload)
+        .map_err(|_| "not a challenge: its payload doesn't decode".to_owned())?;
+    let by_node: [u8; 64] = B64URL
+        .decode(signed)
+        .ok()
+        .and_then(|signed| signed.try_into().ok())
+        .ok_or("not a challenge: its signature doesn't decode")?;
+    if !crate::identity::verify(&key.public(), &message(&bytes), &by_node) {
+        return Err("not a challenge this node handed out".to_owned());
+    }
+    let payload: Payload = serde_json::from_slice(&bytes)
+        .map_err(|_| "not a challenge: its payload isn't one".to_owned())?;
+    if payload.node != key.id() {
+        return Err("a challenge of another node".to_owned());
+    }
+    if payload.exp <= now {
+        return Err("the challenge expired: ask for another".to_owned());
+    }
+    used.0.retain(|_, exp| *exp > now);
+    if used.0.contains_key(&payload.nonce) {
+        return Err("the challenge was used: ask for another".to_owned());
+    }
+    if used.0.len() >= MAX_USED {
+        return Err("too many logins by key at once: try again in a while".to_owned());
+    }
+    let signer = sshsig::verify(signature, NAMESPACE, challenge.as_bytes())?;
+    let entry = registry::user(doc, user).ok_or_else(|| format!("no user {user}"))?;
+    if entry.disabled {
+        return Err(format!("{user} is disabled"));
+    }
+    if !entry.keys.values().any(|known| *known == signer) {
+        return Err(format!("{user} has no key {}", signer.fingerprint()));
+    }
+    used.0.insert(payload.nonce, payload.exp);
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::document::Document;
+    use crate::document::store;
+    use crate::document::testing::change;
+    use ed25519_dalek::SigningKey;
+    use serde_json::json;
+
+    /// A registry with alice, holding `key`'s public key, and carol, disabled, holding
+    /// it too.
+    fn registry_with(key: &SigningKey) -> Document {
+        let line = sshsig::PublicKey(key.verifying_key().to_bytes()).line();
+        let mut doc = Document::new();
+        change(&mut doc, |doc| {
+            let mut alice = registry::entry("root", 1, &[]);
+            alice["keys"] = json!({ "laptop": line });
+            store::write_path(doc, &registry::user_path("alice"), &alice).unwrap();
+            let mut carol = registry::entry("root", 1, &[]);
+            carol["keys"] = json!({ "laptop": line });
+            carol["disabled"] = json!(true);
+            store::write_path(doc, &registry::user_path("carol"), &carol).unwrap();
+            store::write_path(
+                doc,
+                &registry::user_path("bob"),
+                &registry::entry("root", 1, &[]),
+            )
+            .unwrap();
+        });
+        doc
+    }
+
+    /// Signed by one of the user's keys, of this node, unexpired, once: taken; anything
+    /// else refused.
+    #[test]
+    fn what_a_login_by_key_takes() {
+        let node = NodeKey::from_seed([1; 32]);
+        let other_node = NodeKey::from_seed([2; 32]);
+        let key = SigningKey::from_bytes(&[5; 32]);
+        let stranger = SigningKey::from_bytes(&[6; 32]);
+        let doc = registry_with(&key);
+        let mut used = Used::default();
+        let (challenge, exp) = issue(&node, 1000);
+        assert_eq!(exp, 1000 + LIFETIME_MS);
+        let signed = sshsig::sign(&key, NAMESPACE, challenge.as_bytes());
+        // Another user, a disabled one, an unknown one, another key: refused.
+        assert!(take(&doc, &node, &challenge, &signed, "bob", 1001, &mut used).is_err());
+        assert!(take(&doc, &node, &challenge, &signed, "carol", 1001, &mut used).is_err());
+        assert!(take(&doc, &node, &challenge, &signed, "dave", 1001, &mut used).is_err());
+        let foreign = sshsig::sign(&stranger, NAMESPACE, challenge.as_bytes());
+        assert!(take(&doc, &node, &challenge, &foreign, "alice", 1001, &mut used).is_err());
+        // For another purpose: refused.
+        let elsewhere = sshsig::sign(&key, "continuum-value", challenge.as_bytes());
+        assert!(
+            take(
+                &doc, &node, &challenge, &elsewhere, "alice", 1001, &mut used
+            )
+            .is_err()
+        );
+        // Another node's, expired: refused.
+        assert!(
+            take(
+                &doc,
+                &other_node,
+                &challenge,
+                &signed,
+                "alice",
+                1001,
+                &mut used
+            )
+            .is_err()
+        );
+        assert!(take(&doc, &node, &challenge, &signed, "alice", exp, &mut used).is_err());
+        // Taken, once.
+        assert!(take(&doc, &node, &challenge, &signed, "alice", 1001, &mut used).is_ok());
+        assert!(take(&doc, &node, &challenge, &signed, "alice", 1002, &mut used).is_err());
+        // Tampered with.
+        let tampered = challenge.replacen("cc1.", "cc1.e30", 1);
+        let resigned = sshsig::sign(&key, NAMESPACE, tampered.as_bytes());
+        assert!(take(&doc, &node, &tampered, &resigned, "alice", 1001, &mut used).is_err());
+    }
+}