Ensure Invariants

ENSURE_INVARIANTS Skill (DAML)

Trigger Pattern: Always required for DAML audits Inject Into: Breadth agents, depth-edge-case Finding prefix: [DML-EI-N] Rules referenced: R10, R12, R14

A template's ensure clause is its creation precondition: a create whose ensure is False throws PreconditionFailed and the contract never exists. A missing or weak ensure lets an invalid contract enter the ACS (negative amount, empty party list, inconsistent fields), and every downstream choice then trusts the broken invariant. Separately, DAML Int/Decimal arithmetic THROWS on overflow rather than wrapping — so an arithmetic boundary is a liveness/brick bug (the choice aborts and becomes un-exercisable), not a silent-wrap value bug. The PoC for this class is a set of boundary-value Script () functions (min / 1 / negative / max / empty-list), Aptos-style.

1. Ensure-Clause Inventory

For every template, record its ensure and the invariants it should enforce:

Critical patterns to flag:

• No ensure at all on a template carrying an amount/balance/cap → a negative or zero amount contract is creatable ([ELEVATE:ENSURE_GAP]).
• An ensure that guards some fields but not the security-critical one (e.g., checks amount > 0 but not amount <= cap, or not maintainer non-empty).
• An invariant assumed by a choice (this.amount > 0) that the ensure does not actually establish.

DAML note: ensure runs on create only. A choice that recreates a successor re-runs the successor's ensure; if the successor template has a weaker ensure, the invariant degrades across the lifecycle.

2. Boundary Substitution

For each ensure-guarded and arithmetic-bearing field, substitute boundary values and trace the outcome:

Substitute and trace:

• amount = 0 and amount = -1: does the contract create? Does a split produce zero-value children?
• a + b where both near maxInt/Decimal max: does the choice ABORT (ArithmeticError)? A reachable abort on a user path is a liveness brick ([ELEVATE:ENSURE_GAP] / boundary).
• empty list ([]) where a non-empty list is assumed (signatories, observers, maintainers): does ensure/the choice handle it, or does a later head/index abort?

3. Cross-Create Consistency

When several choices create instances of the same template, verify they all establish the same invariant.

Check for:

• One creation path that bypasses a check another path enforces (asymmetric creation).
• An accumulator/cap that one creating choice updates but another does not, so the cap is enforced inconsistently (R14).

4. Deadline / Temporal-Gate Enforcement

A deadline is only enforced if a choice actually compares it against getTime.

Check for ([ELEVATE:DEADLINE_UNENFORCED]):

• A deadline/maturity/expiry field stored but never read in any choice's guard → the time gate is decorative.
• A comparison against a caller-supplied time argument instead of getTime (ledger time) — the caller chooses "now".
• Off-by-one relation (>= vs >) letting an action happen exactly at/after expiry when it should not.

Finding Template

**ID**: [DML-EI-N]
**Severity**: [High if invalid state corrupts value/cap, Medium if liveness brick or unenforced deadline; Int/Decimal overflow is a LIVENESS finding, NOT auto-downgraded as silent-wrap]
**Step Execution**: ✓1,2,3,4 | ✗(reasons) | ?(uncertain)
**Rules Applied**: [R10:✓/✗, R12:✓/✗, R14:✓/✗]
**Location**: {Module}.daml:LineN (template X)
**Title**: Missing/weak ensure / unenforced deadline on {Template} allows {invalid contract / reachable abort}
**Description**: [The missing precondition or unenforced gate, the boundary value that breaks it, and the downstream choice that trusts the broken invariant]
**Impact**: [Invalid contract enters ACS / arithmetic abort bricks a user path / deadline bypass / cap unenforced]
**PoC steer**: boundary-value Scripts — `boundaryMin/boundaryNeg/boundaryMax : Script ()` calling a shared helper; assert create-invalid SUCCEEDS (ensure gap) or the choice aborts (ArithmeticError, liveness) or the past-deadline action SUCCEEDS (unenforced).

Step Execution Checklist (MANDATORY)