path: root/src/access/challenge.rs

//! 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());
    }
}