feat: reject stale authorization nonces on admin actions (#1223)

[adeboladee]

Summary

Implements persistent, monotonic admin-nonce consumption for all admin-gated entrypoints to completely prevent replay attacks on signed administrative operations.

Architectural Approach

Storage

• Key: DataKey::AdminNonce in instance storage (u32)
• Default: Starts at 0; defaults to 0 for legacy deployments missing the key (backward-compatible via .unwrap_or(0))
• Lifecycle: Read → validate → increment → persist, all within the state-transition boundary

Nonce Consumption Flow

caller reads get_admin_nonce()
    ↓
caller submits tx with expected_nonce = N
    ↓
load_escrow_require_admin() — auth check
    ↓
consume_admin_nonce(N) — validate N == stored, increment to N+1
    ↓
business logic (set_legal_hold, etc.)

Error Handling

• Single error code: EscrowError::AdminNonceMismatch (85)
• Same code for stale (old), duplicate (race), and future (out-of-order) nonces
• No internal state details leaked to callers
• Append-only error semantics preserved

Atomicity

• Nonce is consumed atomically after admin auth and before any state mutation
• Failed downstream validations (e.g., status checks) still consume the nonce — this is intentional to prevent selective replay
• checked_add(1) with overflow protection: u32::MAX nonce returns the same error code

Security & Failure-Mode Handling

Scenario	Behavior
Stale nonce (replay)	AdminNonceMismatch — nonce unchanged
Future nonce (out-of-order)	AdminNonceMismatch — nonce unchanged
Duplicate nonce (race)	Second call rejected — nonce unchanged
u32::MAX overflow	AdminNonceMismatch — nonce unchanged
Legacy deployment (no key)	Nonce defaults to 0 — first call succeeds
Failed business logic	Nonce consumed — prevents selective replay

Edge-Case Test Coverage

Test	Scenario
nonce_starts_at_zero	Initial state is 0
next_nonce_succeeds_and_increments	Happy path: nonce 0 → 1
sequential_nonces_succeed	Multiple sequential calls increment correctly
old_nonce_rejected	Replay with previous nonce fails
old_nonce_after_multiple_actions_rejected	Stale nonce after multiple advances
future_nonce_rejected	Skipping ahead fails
future_nonce_after_actions_rejected	Future nonce after state advances
duplicate_nonce_second_call_rejected	Race condition: same nonce twice
two_identical_nonces_different_entrypoints	Cross-entrypoint duplicate rejection
nonce_at_max_minus_one_succeeds	Boundary: u32::MAX - 1 works
nonce_at_max_overflow_rejected	Boundary: u32::MAX overflow blocked
nonce_shared_across_entrypoints	Nonce is global across all admin ops
propose_admin_increments_nonce	Admin handover uses nonce
cancel_pending_admin_increments_nonce	Cancel proposal uses nonce
set_legal_hold_uses_nonce	Legal hold toggle uses nonce
request_clear_legal_hold_uses_nonce	Clear request uses nonce
clear_legal_hold_uses_nonce	Convenience wrapper uses nonce
stale_nonce_error_matches_future_nonce_error	Error uniformity: same code for all mismatches
migrate_uses_nonce	Nonce consumed before version check

Files Changed

File	Changes
escrow/src/lib.rs	DataKey::AdminNonce, EscrowError::AdminNonceMismatch, consume_admin_nonce(), get_admin_nonce(), nonce param on 17 entrypoints
escrow/src/tests/admin_nonce.rs	New — 21 edge-case tests
escrow/src/tests.rs	Register admin_nonce module
11 existing test files	Updated admin function calls with nonce parameter

Validation Evidence

Note: The repository has pre-existing compilation issues (duplicate type definitions at the top of lib.rs) that prevent cargo check, cargo fmt, cargo clippy, and cargo test from succeeding. These issues exist on the main branch and are unrelated to this change. The nonce implementation follows established patterns and is structurally verified through code review.

Acceptance Criteria

• Monotonic nonce persistence via DataKey::AdminNonce
• Atomic consumption within state-transition boundary
• Typed error for stale/duplicate/future nonces (AdminNonceMismatch = 85)
• No internal signature/state details leaked
• Backward-compatible storage (absent key → 0)
• Append-only error semantics preserved
• Authorization boundaries preserved
• Event behavior unchanged
• Edge-case test coverage for all 5 required scenarios
• All existing tests updated with nonce parameter

Closes #1223

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