feat(confidential): escrow commitment blindings to the sender-auditor in sponge lane 2 - #853
feat(confidential): escrow commitment blindings to the sender-auditor in sponge lane 2#853brozorec wants to merge 12 commits into
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Widens the sender-auditor sponge from two lanes to three and adds an auditor-side escrow of the delegation viewing key. Lane 2 carries the new spendable blinding on the checkpoint operations (W_a5, T_a9, S_a6) and dvk_i on spender transfers (O_a9); S14 escrows dvk_i to the owner's auditor at set_spender under a new domain tag. The auditor can now recover the full Pedersen opening of C_spend and C_a rather than the value alone.
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WalkthroughConfidential token circuits now derive three sender-auditor sponge outputs. They escrow spendable blindings and, for delegated operations, delegation viewing keys. Payloads, proofs, events, tests, documentation, baselines, and verification keys reflect the added fields. ChangesAuditor escrow expansion
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to The PR adds auditor-visible opening data and changes payload, public-input, and event contracts, but current specifications and SDK guidance still describe the old two-lane behavior and make conflicting claims about which openings are recoverable. This could lead downstream clients or auditors to derive incorrect data, so the documentation contract should be corrected or explicitly accepted before merge. Sequence Diagram(s)sequenceDiagram
participant Wallet
participant ConfidentialCircuit
participant ProofVerifier
participant ConfidentialEvent
Wallet->>ConfidentialCircuit: submit operation proof inputs and auditor ciphertexts
ConfidentialCircuit->>ConfidentialCircuit: derive three sender-auditor masks
ConfidentialCircuit->>ProofVerifier: return proof with expanded public inputs
ProofVerifier->>ConfidentialEvent: emit verified auditor ciphertext fields
Poem
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 52.94% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 4 files. (25 skipped: 25 unsupported.) Full details: Description checkExplanation The description clearly explains the three-lane sponge change, escrow behavior, circuit impacts, wire-format changes, security tradeoffs, tests, and validation results. It does not include the issue reference or the template checklist, but these omissions are non-critical because the description is otherwise complete and directly related to the changes. ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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In `@packages/tokens/src/confidential/CLAUDE.md`:
- Around line 62-68: The confidential documentation duplicates protocol
semantics that should be cited instead. In
packages/tokens/src/confidential/CLAUDE.md lines 62-68, replace the domain-tag
and lane-assignment tables with citations to DESIGN_cont.md §13 and DESIGN.md
§2.5; in packages/tokens/src/confidential/circuits/CLAUDE.md lines 39-41,
replace the copied sponge construction and lane rules with a citation to
DESIGN.md §2.5. No other protocol details require changes.
In `@packages/tokens/src/confidential/docs/DESIGN_cont.md`:
- Line 18: The sender-auditor channel must be documented consistently as
three-lane: in DESIGN_cont.md at lines 29, 37, and 94, add sender lane 2 and use
SpongeSqueeze_3 wherever it is read; in SDK.md at lines 464-473, update the
auditor-client procedure to derive sender lane 2 while preserving the recipient
channel’s two-lane behavior.
In `@packages/tokens/src/confidential/docs/DESIGN.md`:
- Around line 146-154: Move the newly added mathematical equations and
constraint rows in §2.5 and §§7.5–7.8 from DESIGN.md into DESIGN_cont.md,
following the placement guidance in CLAUDE.md. Replace the removed material in
DESIGN.md with concise citations pointing to the corresponding DESIGN_cont.md
sections, preserving the existing section structure and references.
- Line 293: Update the viewing-key description’s contract-binding parenthetical
to include V2 alongside R2, W2, T2, and S2, matching the V2 definition in
RevokeSpender.
In `@packages/tokens/src/confidential/docs/OVERVIEW.md`:
- Around line 94-95: Update the visibility table associated with the withdrawal
flow to include the post-withdrawal blinding factor available to the sender
auditor and the sender-side blinding exposed through transfer checkpoints, or
explicitly label the table as non-exhaustive. Keep the existing post-withdrawal
balance entry and align the table with the auditor contract described in the
Wallet and Contract summary.
In `@packages/tokens/src/confidential/docs/SELECTIVE_DISCLOSURE.md`:
- Line 100: Qualify the auditor trust-scope statement around “full Pedersen
openings” to limit C_spend openings to event-scoped checkpoints, consistent with
the post-merge limitation; alternatively document the mechanism that provides
post-merge blinding. Update the D-auditor disclosure wording so it does not
imply auditors can open every post-merge C_spend.
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packages/tokens/src/confidential/CLAUDE.mdpackages/tokens/src/confidential/circuits/CLAUDE.mdpackages/tokens/src/confidential/circuits/constraints.baselinepackages/tokens/src/confidential/circuits/lib/src/lib.nrpackages/tokens/src/confidential/circuits/lib/src/tests.nrpackages/tokens/src/confidential/circuits/lib/testdata/encrypt_esc_dvk_auditor.jsonpackages/tokens/src/confidential/circuits/lib/testdata/sponge_squeeze_3.jsonpackages/tokens/src/confidential/circuits/set_spender/src/main.nrpackages/tokens/src/confidential/circuits/set_spender/src/tests.nrpackages/tokens/src/confidential/circuits/spender_transfer/src/main.nrpackages/tokens/src/confidential/circuits/spender_transfer/src/tests.nrpackages/tokens/src/confidential/circuits/transfer/src/main.nrpackages/tokens/src/confidential/circuits/transfer/src/tests.nrpackages/tokens/src/confidential/circuits/vks/set_spender.vk.jsonpackages/tokens/src/confidential/circuits/vks/spender_transfer.vk.jsonpackages/tokens/src/confidential/circuits/vks/transfer.vk.jsonpackages/tokens/src/confidential/circuits/vks/withdraw.vk.jsonpackages/tokens/src/confidential/circuits/withdraw/src/main.nrpackages/tokens/src/confidential/circuits/withdraw/src/tests.nrpackages/tokens/src/confidential/compliance/test.rspackages/tokens/src/confidential/docs/DESIGN.mdpackages/tokens/src/confidential/docs/DESIGN_cont.mdpackages/tokens/src/confidential/docs/INDEXER.mdpackages/tokens/src/confidential/docs/OVERVIEW.mdpackages/tokens/src/confidential/docs/SDK.mdpackages/tokens/src/confidential/docs/SELECTIVE_DISCLOSURE.mdpackages/tokens/src/confidential/mod.rspackages/tokens/src/confidential/storage.rspackages/tokens/src/confidential/test.rs
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…hout DESIGN_cont §8.1/§8.2/§8.4/§8.5 still read the sender channel two-wide and still claimed no auditor can open C_spend; SDK §11 and OVERVIEW's visibility table omitted lane 2 entirely. Scope the spend-side opening to the checkpoint events that escrow it, restore V2 in the vk contract-binding lists, and move the lane-2 semantics out of DESIGN §2.5 into §8.1.
`SetSpender` gains `sigma_a` and `SpenderTransfer` gains `sigma_a_new`, both already proof-bound public inputs. Without them the owner's auditor cannot derive `r_a` and open `C_a`, contradicting DESIGN_cont §8.5. Event assertions in the happy-path tests now compare the typed `#[contractevent]` struct rather than counting events.
…d helper Adds the missing negative tests for W_a5, T_a9, S_a6, S14 and O_a9 -- each constraint could be deleted without failing a test. `encrypt_auditor_sender_balance` has no circuit caller since the sender channel went three-lane; `sponge_squeeze_2` is now the prefix of `sponge_squeeze_3` so their agreement is structural rather than test-pinned. Also corrects the RevokeSpender and `op_i` rationales, which did not hold.
…7 home The lane-2 escrow does not lapse at a merge: one auditor_id serves both of an account's channels, so the same key holds every inbound blinding and carries the opening forward by addition. §8.1, §9.4, SDK §11, OVERVIEW and SELECTIVE_DISCLOSURE said otherwise. Also fills in the event and payload tables with the fields this branch added, specifies the auditor-side dvk escrow in §8.5 (DESIGN.md is over its LaTeX budget), and makes the COMPLIANCE §5.3 clawback sketch consume and re-emit the escrow instead of leaving r_s unresolved.
S14 and O_a9 escrowed the delegation viewing key, which is deterministic and permanent per (owner, spender) and survives revoke-then-re-delegate, so one leaked ciphertext opened every allowance state for that pair, past and future. Both now escrow the blinding of the allowance commitment the operation writes -- r_a at set_spender, r_a' at spender_transfer -- which is what the auditor actually needs to open C_a. Tag 17 becomes ESCROWED_ALLOWANCE_BLINDING_AUDITOR (value unchanged) and lane 2 is uniformly a commitment blinding, never a key. Zero ACIR delta: both blindings were already constrained witnesses in scope. Payload and event fields are renamed, so this is an SDK-visible ABI change. DESIGN_cont §8.5's forward-only and rotation-remediation claims were false and are replaced with an event-scoped claim: a rotated-in key holds no opening until the delegation's next state change, and a retired key that already holds one can still carry it through a public homomorphic fold.
brozorec
changed the title
Seven spots still described S14 and SpenderTransfer lane 2 as escrowing dvk_i. The SELECTIVE_DISCLOSURE §8 note was the substantive one: it ruled out a lane-2 disclosure variant on the grounds that the lane masks a key, which no longer holds now that it carries r_a'.
The leak scope it describes is already stated by the dvk_i comparison paragraph above it, which ends "a leaked r_a ciphertext costs one state".
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Only 2 comments that I couldn't find the answers on my own:
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σ_a on SetSpender and σ_a' on SpenderTransfer are new event fields, and the description never mentions them. Neither is needed by the decryption paths §8.5 specifies (S14's pad keys off op_i, not σ_a). Presumably they're there to back §8.5's "reconstructed from the event alone with no storage read". Ask why, and whether the auditor actually needs them or whether it's for the spender/indexer.
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Pad reuse now costs more than it used to. r_e = Poseidon(δ_eph, vk_op, σ_a) is derived from the consumed salt, and spender_transfer/src/main.nr:276 only asserts sigma_a_new != sigma_a, so an A→B→A alternation reproduces a pad exactly. Previously a repeated pad leaked a value; now lane 2 makes it leak the other half of an opening. Nothing enforces global salt uniqueness and the PR characterizes this in tests (alternating_sigma_a_reproduces_an_earlier_allowance_blinding, repeated_r_e_and_sigma_a_repeat_every_channel_pad) rather than preventing it. That's a defensible call, but it's the sharpest security question here and I'd want the docs to say the blast radius grew.
A reverted `confidential_transfer_from` leaves the delegation entry untouched, so the stored `sigma_a` recurred on the retry along with the ephemeral scalar and every channel pad derived from it, O_a9's lane[2] included; a retry that changed the amount published the difference in the clear. O7, O9, O_a2 and O_a6 now absorb the prover-chosen `sigma_a'`, which `SpenderTransfer` emits in place of the stored salt, and `SetSpender` drops its salt since no auditor path reads either. Documents the rotation assert as O14 and regenerates the spender-transfer VK and fixtures.
Bare cardinals read as ordinals -- "lane 2" invites "second lane" when it means the third slot, index 2. Every specific-lane reference now uses the zero-based index notation, defined normatively in DESIGN.md 2.5, and the ambiguous [0..1] range in the prefix-property formulas is replaced by an explicit per-index equality.
2 participants
Stack 1/3. Base:
main. Followed by proofless revoke → clawback.What
Widens the sender-auditor Poseidon2 sponge from two lanes to three, and adds an auditor-side escrow of the allowance blinding.
sponge_squeeze_3incircuits/lib/src/lib.nrr_a'on spender transfers (O_a9)r_ato the owner's auditor atset_spender, under new domain tag 17 (ESCROWED_ALLOWANCE_BLINDING_AUDITOR) — a single-output pad, becauseset_spender's lane 2 is already taken by S_a6Net effect: the auditor recovers the full Pedersen opening of
C_spendandC_a, not just the value.Scope of the capability
The escrow is of a per-state blinding, not of
dvk_i. That matters becausedvk_i = Poseidon(δ_dvk, vk, op_i)is permanent per(owner, spender)— no salt or generation counter enters it, so revoke-then-re-delegate regenerates it exactly — andσ_ais public. Escrowing it would make one leaked ciphertext open every allowance state for that pair, past and future, with neither rotation nor revocation remediating.So decryption is event-scoped: a key can decrypt exactly the ciphertexts produced while it was active. A rotated-in key holds no opening for a live delegation until that delegation's next state change. It does not follow that a retired key loses track of post-rotation state — proofless folds recompute
C_spend' = C_spend + C_apublicly with no new encrypted checkpoint, so a holder of a known opening carries it forward by the same addition the contract performs.DESIGN_cont.md§8.5 states this and claims nothing stronger.Tradeoff: the auditor must have observed the event; there is no cold state-based recovery. This extends an existing dependency rather than creating one (DESIGN §5.2 already makes a durable event archive normative, and every other auditor capability is event-scoped). New
INDEXER.md§7.1 covers the consequences — a fails-closed check of each reconstructed opening against the storedallowance_commitment, and delegation-entry TTL as the bound on how long that check stays possible.Why it's safe to widen
sponge_squeeze_3(d, s, σ)[0..2] == sponge_squeeze_2(d, s, σ)by construction — the absorb fits one rate-3 block, so both read the same permutation. No existing mask value changes, and the widening costs no extra constraints. Pinned bysponge_squeeze_3_agrees_with_squeeze_2_on_shared_lanes.Tag 17 is separate from
ESCROWED_DELEGATION_VIEWING_KEY(10) even though reuse would be secure: sharing it would drop one leg of DESIGN §5.3's "why reusingr_eis safe" argument. S14 is the thinnest construction in the system — it reuses the S_a2 shared scalar and absorbsop_i(a per-delegation constant, not a nonce), so it stands on tag distinctness plus the freshness ofs_{a,s}alone. §5.3 now says so plainly.Review notes
Read
sponge_squeeze_3once, then check four near-identical applications. Constraint counts againstmain:Both escrowed blindings were already constrained witnesses in scope above their escrow sites (S6/S7, O10/O11), so S14 costs one Poseidon and O_a9 one field addition.
One thing that reads as a mismatch and is not: O_a9's lane-2 pad keys off the consumed
σ_a(the whole sponge does, per O_a6) while the escrowed value derives from the writtenσ_a'. Sound — the pad needs only to be fresh and unpredictable to everyone but the auditor — andDESIGN_cont§8.4 states it explicitly.Payloads gain
r_tilde_aud_s(all four operations) andr_a_tilde_aud_s(set_spender); PI order and event shapes follow. Four VKs regenerate. This is a wire-format change — mixed old/new proofs and verification keys fail cleanly; treat as an SDK migration and call it out in release notes.New tests worth a look:
rejects_escrow_of_blinding_for_the_wrong_sigma_aandrejects_escrow_of_the_consumed_allowance_blindingpin that each circuit escrows the blinding of the state it writes; five characterization tests document what happens underσ_a/r_ereuse (including the A→B→A alternation O10 permits) rather than asserting it is prevented, since nothing enforces global uniqueness; andauditor_key_rotation_rescopes_the_escrowed_allowance_openingwalks set → rotate → spender transfer → revoke, asserting which key version each operation's ciphertexts were produced for.Checks
nargo test169 passed ·cargo test -p stellar-tokens714 passed ·cargo +nightly fmt --checkclean ·cargo clippy --release --locked --all-targets -D warningsclean ·constraints.baselineregenerated · VKs regenerated