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Layer 1 · Merkle inclusion proofs

Canonical reference: digstore-core::merkle, codec in digstore-core::codec/primitives.rs. This is the always-on, fail-closed integrity gate 1.

Domain separation​

LEAF_TAG = b"dig-store:leaf:v1"     // merkle.rs:34  (raw-chunk build path only)
NODE_TAG = b"dig-store:node:v1"     // merkle.rs:39
node     = SHA-256( NODE_TAG || left(32) || right(32) )   // hash_pair, merkle.rs:113-119

The D5 per-resource leaf​

resource_leaf(ciphertext) = SHA-256(ciphertext)    // UNTAGGED — merkle.rs:140-142
The committed tree is per-resource

The committed generation tree has one leaf per resource: resource_leaf(concat_output(ordered chunk ciphertexts)). These D5 leaves are fed to MerkleTree::from_leaves already hashed and not re-tagged, so leaf↔node separation rests solely on NODE_TAG. (MerkleTree::build carries a separate raw-chunk LEAF_TAG path; the production read-path leaf is the per-resource one above.)

The single content→leaf binding is shared byte-for-byte by the producer (store.rs:209), the compiler (pipeline.rs:210-230), and the browser verifier (dig-client-wasm lib.rs:101-103).

Tree shape​

  • Leaves ordered ascending by raw 32-byte static_key.
  • CurrentRoot = MerkleTree::from_leaves(leaves).root().
  • An odd node is carried up UNCHANGED (no re-hash) — so a carried-up leaf skips a level.
  • Empty-tree root = SHA-256(&[]) (merkle.rs:161-167).
BINDING CONTRACT D8 — proof length ≤ ceil(log2 n)

Because an odd node is carried up unchanged, a proof path is ≤ ceil(log2 n) siblings. The ≤ (not =) is the binding contract D8 (merkle.rs:20-25).

Verify​

acc = leaf
for ProofStep { hash, is_left } in path:
    acc = is_left ? hash_pair(sibling, acc) : hash_pair(acc, sibling)
accept iff acc == root                          // MerkleProof::verify, merkle.rs:79-89

The guest leaf index is the rank of the served key among KeyTable entries by raw static_key order (resource_leaf_index, content.rs:434-456); build_real_proof rebuilds the tree from the embedded MerkleNodes and emits tree.prove(index) (content.rs:396-428).

Wire layout (Chia big-endian streamable codec)​

MerkleProof = leaf(32) || path_count:u32(BE) || ( sibling(32) || is_left:u8 ){path_count} || root(32)
  • is_left MUST be 0 or 1 (any other byte → InvalidTag) — merkle.rs:49-67.
  • On the wire: base64 of MerkleProof::to_bytes, carried as the X-Dig-Inclusion-Proof header / the inclusion_proof / merkle_proof_b64 field.

A limitation worth stating​

verify_module_root recomputes from_leaves(MerkleNodes).root() and requires it == the embedded CurrentRoot, plus embedded StoreId == expected (data_section.rs:253-314). This proves self-consistency for the requested store identity, NOT that the root is the latest authorized on-chain root — that is the job of anchored-root pinning (data_section.rs:238-243).