mtc_worker: Add API for the current landmark bundle

A "landmark bundle" consists of the minimum amount of information needed
to validate all MTCs issued before some checkpoint. It has the following
components:

1. The checkpoint, including co-signatures

2. The sequence of landmark subtrees determined by the landmark sequence
   up to the checkpoint

3. A proof of consistency of each subtree with the checkpoint.

Add an API to mtc_worker for fetching the current landmark bundle.

Author: cjpatton

crates/mtc_api/src/landmark.rs

@@ -165,6 +165,49 @@ impl LandmarkSequence {
             landmarks,
         })
     }
+
+    /// Iterate over the sequence of subtrees determined by the landmark sequence.
+    pub fn subtrees(&self) -> LandmarkSubtreesIterator<'_> {
+        LandmarkSubtreesIterator {
+            index: 1,
+            landmarks: &self.landmarks,
+            next_subtree: None,
+        }
+    }
+}
+
+/// An iterator over the subtrees determined by the landmark sequence.
+pub struct LandmarkSubtreesIterator<'a> {
+    index: usize,
+    landmarks: &'a VecDeque<u64>,
+    next_subtree: Option<Subtree>,
+}
+
+impl Iterator for LandmarkSubtreesIterator<'_> {
+    type Item = Subtree;
+
+    fn next(&mut self) -> Option<Subtree> {
+        if self.landmarks.len() < 2 {
+            return None;
+        }
+
+        if let Some(subtree) = self.next_subtree.take() {
+            self.next_subtree = None;
+            return Some(subtree);
+        }
+
+        if self.index == self.landmarks.len() {
+            return None;
+        }
+
+        let subtree;
+        (subtree, self.next_subtree) =
+            Subtree::split_interval(self.landmarks[self.index - 1], self.landmarks[self.index])
+                .unwrap();
+
+        self.index += 1;
+        Some(subtree)
+    }
 }
 
 #[cfg(test)]
@@ -233,4 +276,28 @@ mod tests {
         // Past last landmark.
         assert!(seq.subtree_for_index(200).is_none());
     }
+
+    #[test]
+    fn test_subtrees() {
+        let mut seq = LandmarkSequence::create(10);
+        assert!(seq.subtrees().next().is_none());
+
+        for i in 1..=5 {
+            seq.add(i * 10).unwrap();
+        }
+        let got = seq.subtrees().collect::<Vec<_>>();
+        let want = vec![
+            Subtree::new(0, 8).unwrap(),
+            Subtree::new(8, 10).unwrap(),
+            Subtree::new(8, 16).unwrap(),
+            Subtree::new(16, 20).unwrap(),
+            Subtree::new(20, 24).unwrap(),
+            Subtree::new(24, 30).unwrap(),
+            Subtree::new(30, 32).unwrap(),
+            Subtree::new(32, 40).unwrap(),
+            Subtree::new(40, 48).unwrap(),
+            Subtree::new(48, 50).unwrap(),
+        ];
+        assert_eq!(got, want);
+    }
 }

crates/mtc_worker/src/frontend_worker.rs

@@ -14,7 +14,9 @@ use der::{
 use generic_log_worker::{
     batcher_id_from_lookup_key, deserialize, get_cached_metadata, get_durable_object_stub,
     init_logging, load_cache_kv, load_public_bucket,
-    log_ops::{prove_subtree_inclusion, read_leaf, ProofError, CHECKPOINT_KEY},
+    log_ops::{
+        prove_subtree_consistency, prove_subtree_inclusion, read_leaf, ProofError, CHECKPOINT_KEY,
+    },
     put_cache_entry_metadata, serialize,
     util::now_millis,
     ObjectBackend, ObjectBucket, ENTRY_ENDPOINT, METRICS_ENDPOINT,
@@ -28,7 +30,9 @@ use serde::{Deserialize, Serialize};
 use serde_with::{base64::Base64, serde_as};
 use signed_note::{NoteVerifier, VerifierList};
 use std::time::Duration;
-use tlog_tiles::{open_checkpoint, LeafIndex, PendingLogEntry, PendingLogEntryBlob};
+use tlog_tiles::{
+    open_checkpoint, CheckpointText, LeafIndex, PendingLogEntry, PendingLogEntryBlob,
+};
 #[allow(clippy::wildcard_imports)]
 use worker::*;
 use x509_cert::{
@@ -71,6 +75,24 @@ pub struct GetCertificateResponse {
     pub landmark_id: usize,
 }
 
+/// GET response structure for the `/get-landmark-bundle` endpoint
+#[serde_as]
+#[derive(Serialize, Deserialize)]
+pub struct GetLandmarkBundleResponse {
+    #[serde_as(as = "Base64")]
+    pub checkpoint: Vec<u8>,
+    pub subtrees: Vec<SubtreeWithConsistencyProof>,
+}
+
+#[serde_as]
+#[derive(Serialize, Deserialize)]
+pub struct SubtreeWithConsistencyProof {
+    pub start: u64,
+    pub end: u64,
+    #[serde_as(as = "Base64")]
+    pub consistency_proof: Vec<u8>,
+}
+
 /// Start is the first code run when the Wasm module is loaded.
 #[event(start)]
 fn start() {
@@ -128,39 +150,13 @@ async fn main(req: Request, env: Env, _ctx: Context) -> Result<Response> {
                     let object_backend = ObjectBucket::new(load_public_bucket(&ctx.env, name)?);
                     // Fetch the current checkpoint to know which tiles to fetch
                     // (full or partials).
-                    let checkpoint_bytes = object_backend
-                        .fetch(CHECKPOINT_KEY)
-                        .await?
-                        .ok_or("no checkpoint in object storage".to_string())?;
-                    let origin = &load_origin(name);
-                    let verifiers = &VerifierList::new(
-                        load_checkpoint_signers(&ctx.env, name)
-                            .iter()
-                            .map(|s| s.verifier())
-                            .collect::<Vec<Box<dyn NoteVerifier>>>(),
-                    );
-                    let checkpoint = open_checkpoint(
-                        origin.as_str(),
-                        verifiers,
-                        now_millis(),
-                        &checkpoint_bytes,
-                    )
-                    .map_err(|e| e.to_string())?
-                    .0;
+                    let (checkpoint, _checkpoint_bytes) =
+                        get_current_checkpoint(&ctx.env, name, &object_backend).await?;
                     if leaf_index >= checkpoint.size() {
                         return Response::error("Leaf index is not in log", 422);
                     }
 
-                    let seq = if let Some(bytes) = object_backend.fetch(LANDMARK_KEY).await? {
-                        let max_landmarks = params
-                            .max_certificate_lifetime_secs
-                            .div_ceil(params.landmark_interval_secs)
-                            + 1;
-                        LandmarkSequence::from_bytes(&bytes, max_landmarks)
-                            .map_err(|e| e.to_string())?
-                    } else {
-                        return Err("failed to get landmark sequence".into());
-                    };
+                    let seq = get_landmark_sequence(name, &object_backend).await?;
                     if leaf_index < seq.first_index() {
                         return Response::error("Leaf index is before first active landmark", 422);
                     }
@@ -223,6 +219,9 @@ async fn main(req: Request, env: Env, _ctx: Context) -> Result<Response> {
 
                     Response::from_json(&GetCertificateResponse { data, landmark_id })
                 })
+                .get_async("/logs/:log/get-landmark-bundle", |_req, ctx| async move {
+                    get_landmark_bundle(&ctx.env, ctx.data).await
+                })
                 .get("/logs/:log/metadata", |_req, ctx| {
                     let name = ctx.data;
                     let params = &CONFIG.logs[name];
@@ -442,6 +441,93 @@ async fn add_entry(mut req: Request, env: &Env, name: &str) -> Result<Response>
     })
 }
 
+async fn get_landmark_bundle(env: &Env, name: &str) -> Result<Response> {
+    let object_backend = ObjectBucket::new(load_public_bucket(env, name)?);
+
+    // Fetch the current checkpoint.
+    let (checkpoint, checkpoint_bytes) = get_current_checkpoint(env, name, &object_backend).await?;
+
+    // Fetch the current landmark sequence.
+    let landmark_sequence = get_landmark_sequence(name, &object_backend).await?;
+
+    // Compute the sequence of landmark subtrees and, for each subtree, a proof of consistency with
+    // the checkpoint. Each signatureless MTC includes an inclusion proof in one of these subtrees.
+    let mut subtrees = Vec::new();
+    for landmark_subtree in landmark_sequence.subtrees() {
+        let consistency_proof = match prove_subtree_consistency(
+            *checkpoint.hash(),
+            checkpoint.size(),
+            landmark_subtree.lo(),
+            landmark_subtree.hi(),
+            &object_backend,
+        )
+        .await
+        {
+            Ok(p) => p,
+            Err(ProofError::Tlog(s)) => return Response::error(s.to_string(), 422),
+            Err(ProofError::Other(e)) => return Err(e.to_string().into()),
+        };
+
+        subtrees.push(SubtreeWithConsistencyProof {
+            start: landmark_subtree.lo(),
+            end: landmark_subtree.hi(),
+            consistency_proof: consistency_proof
+                .iter()
+                .flat_map(|h| h.0.to_vec())
+                .collect::<Vec<u8>>(),
+        });
+    }
+
+    Response::from_json(&GetLandmarkBundleResponse {
+        checkpoint: checkpoint_bytes,
+        subtrees,
+    })
+}
+
+async fn get_current_checkpoint(
+    env: &Env,
+    name: &str,
+    object_backend: &ObjectBucket,
+) -> Result<(CheckpointText, Vec<u8>)> {
+    let checkpoint_bytes = object_backend
+        .fetch(CHECKPOINT_KEY)
+        .await?
+        .ok_or("no checkpoint in object storage".to_string())?;
+
+    let origin = &load_origin(name);
+    let verifiers = &VerifierList::new(
+        load_checkpoint_signers(env, name)
+            .iter()
+            .map(|s| s.verifier())
+            .collect::<Vec<Box<dyn NoteVerifier>>>(),
+    );
+    let (checkpoint, _timestamp) =
+        open_checkpoint(origin.as_str(), verifiers, now_millis(), &checkpoint_bytes)
+            .map_err(|e| e.to_string())?;
+    Ok((checkpoint, checkpoint_bytes))
+}
+
+async fn get_landmark_sequence(
+    name: &str,
+    object_backend: &ObjectBucket,
+) -> Result<LandmarkSequence> {
+    let params = &CONFIG.logs[name];
+
+    let Some(landmark_sequence_bytes) = object_backend.fetch(LANDMARK_KEY).await? else {
+        return Err("failed to get landmark sequence".into());
+    };
+
+    let max_landmarks = params
+        .max_certificate_lifetime_secs
+        .div_ceil(params.landmark_interval_secs)
+        + 1;
+
+    let landmark_sequence = LandmarkSequence::from_bytes(&landmark_sequence_bytes, max_landmarks)
+        .map_err(|e| e.to_string())?;
+
+    Ok(landmark_sequence)
+}
+
 fn headers_from_http_metadata(meta: HttpMetadata) -> Headers {
     let h = Headers::new();
     if let Some(hdr) = meta.cache_control {