Fix escrow owner memo nonce - #825
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WalkthroughEscrow-spend randomness now includes a persistent owner/spender epoch. The contract increments the epoch before deriving spender ciphertext, owner ciphertext, and owner-memo randomness. Tests cover repeated ChangesEscrow spend randomness
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟠 High · up to The escrow spend derivation may reuse spender randomness when different owners repeat a seed, allowing ciphertext comparison to reveal relationships between escrow values. This is a concrete confidentiality risk, so the PR is not merge-ready until the derivation is owner/spender-scoped and covered by a repeated-seed cross-owner test. Suggested reviewers: Poem
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Inline comments:
In `@contracts/src/token/ConfidentialFungibleToken.compact`:
- Around line 1135-1141: Update the rSpender derivation in the spend flow around
_expandSpendRandomness so its input is unique per owner/spender context: either
bind fromAddress (or the owner/spender pair) into the domain/derivation inputs,
or use an epoch counter scoped to the spender. Preserve the existing owner and
memo derivations, and add a test covering repeated seeds across different owners
to verify rSpender values do not collide.
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contracts/src/token/ConfidentialFungibleToken.compactcontracts/src/token/test/ConfidentialFungibleToken.test.ts
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| * `value`. The amount is hidden. | ||
| * | ||
| * @circuitInfo k=16, rows=63207 | ||
| * @circuitInfo k=17, rows=70164 |
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transferFrom crosses into k=17
🔴 blocking: the five-field persistentHash(seed, tag, owner, spender, epoch) costs one more SHA-256 block per scalar than the two-field expandRandomness it replaces, so transferFrom goes 63207 → 70164 rows and over the k=16 boundary. k=17 doubles its proving table, and the header two hundred lines up already warns the full CFT surface exceeded the local ledger-8 block byte budget.
This PR introduces the derivation, so it is not pre-existing debt. Hashing (owner, spender, epoch) once into a 32-byte nonce and expanding that three times should hold k=16; worth measuring before this lands on the release branch rather than tracking it after.
added by claude (dev3-midnight-basic-review)
| * reveal the amount difference. `_spendEscrow` is the exception: every value it | ||
| * derives folds in the spend epoch AND both counterparties, so its randomness | ||
| * is unique per spend regardless of the seed. The credit path folds the | ||
| * memo-list length in as a nonce (defense in depth), but that counter RESETS on |
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Credit-nonce reset wording collides with #819 / #821
🔵 followup: this paragraph still says the credit nonce RESETS on clearMemos, but #819 and #821 replace that with _creditEpochs. Not fixable here, since the correct wording depends on which of the three merges first. Whichever lands last reconciles the sentence, in the header and in the wit_RandomnessSeed doc.
added by claude (dev3-midnight-basic-review)
| // | ||
| // Kept out of `EscrowEntry`: `approve` replaces that entry wholesale, so a | ||
| // counter living there would restart on every re-approve. | ||
| export ledger _escrowSpendEpochs: Map<Bytes<32>, Map<Bytes<32>, Counter>>; |
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New public state deserves a one-line justification
⚪ nitpick: _escrowSpendEpochs is new persisted public state, a per-pair spend count. It leaks nothing beyond what tx history already shows, but saying so in the ledger comment saves the next reader asking whether it is a disclosure.
added by claude (dev3-midnight-basic-review)
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Add
_escrowSpendEpochs, a per-(owner, spender) counter folded into all three derivations (rSpender,rOwnerandeOwnerMemo), so each spend is uniquely randomized regardless of the seed. It's kept out ofEscrowEntrybecause approve replaces that entry wholesale and keyed to the pair_spendEscrowalready reads so no new state is pinnedResolves #796
Summary by CodeRabbit
Bug Fixes
Tests