Implement NoDeletionProof

[RaimoMeltsas]

Add actual no deletion proof into GetNoDeletionProof API response

[mareksepp]

Scope update — Mar 23 2026 TGE sync

The scope of this issue has been significantly expanded based on the Mar 23 product sync. Three interrelated changes to the SMT tree format were agreed for TGE (breaking backward compat with old SDKs — accepted):

Change 1 — Leaf simplification

Reduce the stored leaf payload from the current large record (which includes the authenticator and other fields) to a single hash value. The authenticator is still validated by the aggregator at request time (anti-spam / anti-blocking) but does not need to be persisted in the leaf long-term. An append-only audit log should be kept separately by the aggregator. This significantly reduces memory footprint.

Change 2 — Non-inclusion proof simplification

The current proof format is a verbose op-code transcript that must be replayed twice. By fixing the tree topology, two op-codes can be removed, yielding a proof that is ~3-4× cheaper to verify. This translates directly to future ZK proof generation speed. Risto has a Python prototype (written Mar 23) that implements this — to be shared as reference.

Change 3 — Layer-number domain separation in hashing

Include the tree layer number as a domain separator in each internal hash computation (leaf = level 0, root = level 256). This eliminates theoretical bit-shifting attacks that require aggregator cooperation to execute. Necessary to support a future formal "no-trust-in-aggregator" claim.

Proof format: each step in the hash chain includes (a) layer number + (b) sibling hash. Layer number is included as a domain separator in the actual hashing, not just as a lookup aid — this is what makes it cryptographically binding and prevents bit-shift attacks. Format for each proof step: [layer_byte | sibling_hash].

Reference: Risto's Python prototype to be shared. Starting point for the diff between current spec and implementation: footnote in the Yellowpaper. Ahto to create a branch of aggregator-go and prototype the level-based hashing to assess effort.

Assignee: ahtotruu (branch + prototype), ristik (Python reference + review)