mvx_constant_time_skill

Overview

This skill helps you verify that cryptographic secrets are handled in constant time to prevent timing attacks in MultiversX smart contracts. It focuses on patterns that introduce variable-time behavior and on practical mitigations you can apply during development and review. The goal is to reduce secret-dependent timing leakage by preferring VM-managed crypto and safe comparison primitives.

How this skill works

The skill inspects code patterns that perform secret-dependent branching, early returns, and short-circuit boolean logic when operating on secrets. It recommends using VM-provided crypto primitives and managed types that implement constant-time comparisons, or well-reviewed crates like subtle for manual slice comparisons. Verification guidance explains why measuring constant time on-chain is unreliable and why relying on the platform crypto is safest.

When to use it

• When implementing custom elliptic curve math, signature verification, or ZK verification inside WASM.
• When comparing secrets such as password hashes, MACs, or authentication tokens.
• During code reviews to catch early-exit or short-circuit patterns on secret data.
• Before shipping contracts that handle private keys, seeds, or user secrets.

Best practices

• Avoid early return on byte-by-byte comparisons; do full-scan comparisons that do not short-circuit.
• Do not use && or || short-circuit logic on secret-dependent conditions.
• Prefer VM-managed crypto functions (self.crypto().verify_signature(...)) instead of custom WASM implementations.
• Use ManagedBuffer comparisons provided by the API when available; they are often constant time in the VM.
• If you must compare slices manually, use a reviewed constant-time library such as subtle::ConstantTimeEq.

Example use cases

• Reviewing a contract that manually implements signature verification and replacing it with the VM crypto call.
• Replacing an early-exit byte comparison used for stored hash checks with a constant-time comparison primitive.
• Auditing authentication flows to ensure token equality checks do not reveal mismatch index.
• Designing a new contract that handles secrets and choosing ManagedBuffer or subtle-based comparisons up front.
• Teaching developers safe comparison patterns during security onboarding for smart contract teams.