aitp-session-bundle 0.8.0

//! [`SessionTrustBundle`] builder (RFC-AITP-0010 §4).
//!
//! Coordinator-side: collect each participant's coordinator-issued TCT
//! (one per bilateral handshake), assemble the bundle body, JCS-sign.

use crate::error::SessionBundleError;
use crate::types::{ParticipantEntry, SessionTrustBundle};
use aitp_core::{jcs, Aid, ExtensionsMap, Timestamp};
use aitp_crypto::AitpSigningKey;
use aitp_tct::TctClaims;
use serde::Serialize;
use sha2::{Digest, Sha256};
use uuid::Uuid;

/// `version` constant for v0.2 bundles.
pub const DEFAULT_BUNDLE_VERSION: &str = "aitp/0.2";

/// Decode a participant TCT's claims without verification. Builder- and
/// invariant-level peeks only; full verification (signature, typ, alg
/// pin) happens in [`crate::verify_session_bundle`] via
/// [`aitp_tct::verify_tct`].
pub(crate) fn peek_tct_claims(token: &str) -> Result<TctClaims, SessionBundleError> {
    let payload = aitp_crypto::jws::decode_payload_unverified(token)
        .map_err(|e| SessionBundleError::Canonicalization(format!("participant tct: {e}")))?;
    serde_json::from_slice(&payload)
        .map_err(|e| SessionBundleError::Canonicalization(format!("participant tct claims: {e}")))
}

/// Fluent builder for issuing a [`SessionTrustBundle`] as the
/// coordinator.
pub struct SessionBundleBuilder<'a> {
    coordinator_key: &'a AitpSigningKey,
    session_id: Option<Uuid>,
    participants: Vec<ParticipantEntry>,
    issued_at: Option<Timestamp>,
    extensions: Option<ExtensionsMap>,
}

impl<'a> SessionBundleBuilder<'a> {
    /// Begin a new bundle, signed by `coordinator_key`.
    pub fn new(coordinator_key: &'a AitpSigningKey) -> Self {
        Self {
            coordinator_key,
            session_id: None,
            participants: Vec::new(),
            issued_at: None,
            extensions: None,
        }
    }

    /// Set the session ID (UUIDv4). If unset, a fresh one is generated
    /// at `build()` time.
    pub fn session_id(mut self, id: Uuid) -> Self {
        self.session_id = Some(id);
        self
    }

    /// Override `issued_at`. Tests / fixtures only.
    pub fn issued_at(mut self, ts: Timestamp) -> Self {
        self.issued_at = Some(ts);
        self
    }

    /// Set the `extensions` namespace (RFC-AITP-0001 §7).
    ///
    /// Unset by default, which serializes as an absent `extensions` key —
    /// not `"extensions":{}`. Passing `ExtensionsMap::new()` here instead
    /// deliberately produces the present-but-empty form; the two are
    /// different signing inputs, so choose the one the caller means.
    pub fn extensions(mut self, extensions: ExtensionsMap) -> Self {
        self.extensions = Some(extensions);
        self
    }

    /// Add a participant. The TCT (compact JWS, carried verbatim) MUST
    /// be coordinator-issued (`iss == coordinator_key.aid()`) with
    /// `aud == aid`. These invariants are checked in `build()`.
    pub fn participant(mut self, aid: Aid, tct: String) -> Self {
        self.participants.push(ParticipantEntry { aid, tct });
        self
    }

    /// Construct, sign, and return the bundle.
    pub fn build(self) -> Result<SessionTrustBundle, SessionBundleError> {
        if self.participants.is_empty() {
            return Err(SessionBundleError::EmptyParticipants);
        }

        let coordinator = self.coordinator_key.aid().clone();
        let session_id = self.session_id.unwrap_or_else(Uuid::new_v4);
        let issued_at = self.issued_at.unwrap_or_else(Timestamp::now);

        // Validate every participant entry up front so build() returns
        // a structurally-correct bundle. The coordinator minted these
        // tokens itself; the peek is for invariant enforcement, not
        // trust.
        let mut min_exp: Option<Timestamp> = None;
        for entry in &self.participants {
            let claims = peek_tct_claims(&entry.tct)?;
            if claims.iss != coordinator {
                return Err(SessionBundleError::CoordinatorIssuerMismatch);
            }
            if claims.aud != entry.aid {
                return Err(SessionBundleError::AudienceMismatch);
            }
            min_exp = Some(match min_exp {
                Some(m) if m.0 <= claims.exp.0 => m,
                _ => claims.exp,
            });
        }

        // expires_at = min(participant TCT expiries) per RFC §6.
        let expires_at = min_exp.ok_or(SessionBundleError::EmptyParticipants)?;

        let canonical = bundle_signing_bytes(&BundleSigningBody {
            version: DEFAULT_BUNDLE_VERSION,
            session_id: &session_id,
            coordinator: &coordinator,
            issued_at: &issued_at,
            expires_at: &expires_at,
            participants: &self.participants,
            extensions: self.extensions.as_ref(),
        })?;
        let digest = Sha256::digest(&canonical);
        let signature = self.coordinator_key.sign(&digest);

        Ok(SessionTrustBundle {
            version: DEFAULT_BUNDLE_VERSION.to_string(),
            session_id,
            coordinator,
            issued_at,
            expires_at,
            participants: self.participants,
            extensions: self.extensions,
            signature: signature.into_string(),
        })
    }
}

/// The canonical signing input for a session trust bundle.
///
/// **The single definition of what gets signed.** The builder, the
/// verifier and the known-answer test all route through this, so signer,
/// verifier and test cannot drift apart.
///
/// The input is the JCS canonicalization of the bundle body **excluding
/// `signature`** (RFC-AITP-0001 §5.4.1, RFC-AITP-0010 §3). The
/// `{"session_bundle": …}` wrapper is the transport shape and is NOT
/// signed; RFC-AITP-0010 §5 step 6 requires verification against the inner
/// body and gives no allowance for accepting the wrapped form.
///
/// This deliberately states the rule and cites the RFCs rather than
/// pointing at a sibling artifact. The comment this replaced read "same
/// convention as the revocation snapshot", and that cross-reference is how
/// one misread vector became two divergent artifacts: correcting one and
/// leaving a pointer to it in the other keeps the propagation path open.
pub(crate) fn bundle_signing_bytes(
    body: &BundleSigningBody<'_>,
) -> Result<Vec<u8>, SessionBundleError> {
    jcs::canonicalize_serializable(body)
        .map_err(|e| SessionBundleError::Canonicalization(e.to_string()))
}

/// The signed body — every [`SessionTrustBundle`] field except
/// `signature`. Unlike the revocation snapshot, whose `signature` is a
/// sibling of the body, the bundle's `signature` is a member of it, so
/// this projection is what performs the exclusion.
#[derive(Serialize)]
pub(crate) struct BundleSigningBody<'a> {
    pub version: &'a str,
    pub session_id: &'a Uuid,
    pub coordinator: &'a Aid,
    pub issued_at: &'a Timestamp,
    pub expires_at: &'a Timestamp,
    pub participants: &'a [ParticipantEntry],
    /// Mirrors [`SessionTrustBundle::extensions`]'s presence-sensitivity:
    /// `None` omits the key, `Some(_)` (even an empty map) serializes it —
    /// never collapse one into the other here.
    #[serde(skip_serializing_if = "Option::is_none")]
    pub extensions: Option<&'a ExtensionsMap>,
}

#[cfg(test)]
mod signing_input_tests {
    use super::*;
    use crate::types::SessionTrustBundle;

    /// Drive the **production** signing-input helper against the spec's
    /// pinned `kat-session-bundle-001` bytes.
    ///
    /// This lives in-crate deliberately: `bundle_signing_bytes` is
    /// `pub(crate)`, so an integration test cannot reach it and would have
    /// to re-implement the canonicalization — which is exactly how a test
    /// ends up agreeing with itself instead of with the implementation.
    /// Verification measured the consequence: with the bundle KAT
    /// canonicalizing locally, a regression in `bundle_signing_bytes` was
    /// caught by only ONE test in the whole workspace. This restores the
    /// same redundancy the revocation path has.
    #[test]
    fn production_signing_bytes_match_the_pinned_vector() {
        let kat_path = std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR"))
            .parent()
            .and_then(|p| p.parent())
            .expect("repo root")
            .join("tests/schemas/known-answer/jcs-sha256.json");
        let kat: serde_json::Value =
            serde_json::from_slice(&std::fs::read(kat_path).expect("read kat")).unwrap();
        let v = kat["vectors"]
            .as_array()
            .unwrap()
            .iter()
            .find(|v| v["id"].as_str() == Some("kat-session-bundle-001"))
            .expect("kat-session-bundle-001 present");

        // The vector's `object` is the body without `signature`; add a
        // placeholder so it deserializes into the wire type, then project
        // it back out through the production body struct.
        let mut obj = v["object"].as_object().expect("object").clone();
        obj.insert("signature".into(), serde_json::Value::from(""));
        let bundle: SessionTrustBundle =
            serde_json::from_value(serde_json::Value::Object(obj)).expect("vector deserializes");

        let canonical = bundle_signing_bytes(&BundleSigningBody {
            version: &bundle.version,
            session_id: &bundle.session_id,
            coordinator: &bundle.coordinator,
            issued_at: &bundle.issued_at,
            expires_at: &bundle.expires_at,
            participants: &bundle.participants,
            extensions: bundle.extensions.as_ref(),
        })
        .expect("canonicalize");

        assert_eq!(
            canonical.len(),
            v["jcs_canonical_len_bytes"].as_u64().unwrap() as usize,
            "production signing bytes diverge in length from kat-session-bundle-001"
        );
        assert_eq!(
            hex::encode(&canonical),
            v["jcs_canonical_hex"].as_str().unwrap(),
            "production signing bytes diverge from kat-session-bundle-001 — the \
             implementation canonicalizes a different shape than the spec signs"
        );
    }
}