constant-time-testing

Overview of constant-time-testing skill

constant-time-testing is a practical skill for auditing cryptographic code for timing side channels. Use the constant-time-testing skill when you need to check whether secret-dependent branches, memory access patterns, or microarchitectural behavior could leak keys, nonces, or other sensitive values.

Who should use constant-time-testing

This skill is best for security auditors, crypto engineers, and reviewers of implementation-level cryptography. It is especially useful when you already have code, a test harness, or a suspected hot path and need a focused constant-time-testing guide rather than a generic secure-coding checklist.

What problem it solves

The real job is not “understand timing attacks in theory,” but “find whether this code behaves differently when secrets change.” constant-time-testing helps you turn that question into a repeatable review workflow: identify sensitive inputs, inspect the code paths they influence, and design tests that can expose leakage.

What makes it useful

The main value is specificity. A good constant-time-testing skill should push you toward:

• the exact code regions that matter most,
• the kinds of comparisons and lookups that often leak,
• and the evidence you need before deciding whether a finding is real.

How to Use constant-time-testing skill

Install and open the source files

For constant-time-testing install, add the skill from trailofbits/skills and then read SKILL.md first. If you are using this in an agent workflow, also inspect adjacent repo files that define behavior or conventions before you write your prompt.

Start with the right input shape

The skill works best when you provide a concrete target, not a vague request. Good inputs include:

• the repository or file path to review,
• the secret values or API calls that must remain hidden,
• the threat model, such as local attacker, remote timing observer, or benchmark noise,
• and the language or platform, since C, Rust, assembly, and high-level code leak differently.

A strong prompt looks like: “Use constant-time-testing on src/crypto.rs to check whether verify_tag() branches on secret bytes. Assume a remote attacker and report likely leakage points, test ideas, and false positives.”

Suggested review workflow

A practical constant-time-testing usage pattern is:

1. Identify secrets and all code paths they influence.
2. Look for branches, early returns, table lookups, and variable-time primitives.
3. Test whether behavior changes with secret-dependent inputs.
4. Separate true leakage from compiler, runtime, or measurement noise.
5. Report findings with the exact inputs, code locations, and confidence level.

Read first for better results

Prioritize SKILL.md, then any repository examples of constant-time checks, measurement scripts, or policy notes. If the repo includes multiple implementation targets, read the one closest to your production stack first so the constant-time-testing guide stays aligned with your environment.

constant-time-testing skill FAQ

Is constant-time-testing only for crypto libraries?

No. It is for any code where secret-dependent timing could matter, including authentication checks, key comparison, parsing of secret-bearing formats, and protocol logic. The constant-time-testing for Security Audit use case is broader than pure cryptography.

Do I need a benchmark setup before using it?

Not always. You can start with static review and targeted tests, then add timing measurements if the code path looks suspicious. For many audits, the first pass is about narrowing where leakage could exist.

How is this different from a normal prompt?

A normal prompt often asks for a generic explanation of timing attacks. constant-time-testing is more actionable: it is meant to drive a review of a specific codebase, a specific secret, and a specific threat model, which usually produces better audit output.

When should I not use it?

Do not rely on it if you need full dynamic analysis infrastructure, formal verification, or hardware-specific certification. It is most useful as a focused review and testing guide, not a replacement for deeper assurance methods.

How to Improve constant-time-testing skill

Give the skill the secrets and invariants

The biggest quality boost comes from naming what must stay secret and what must not change. Tell the model which values are sensitive, which operations are allowed to vary, and which code paths are security-critical. That makes the constant-time-testing skill much more precise.

Share code locations and suspected patterns

If you already suspect a function, branch, or lookup table, point to it directly. Example: “Inspect verify(), ct_eq(), and the S-box lookup in src/aes.c.” This reduces guesswork and helps the output focus on the highest-risk leakage paths.

Ask for evidence, not just judgment

Good results usually include a concrete test plan: input pairs to compare, where to instrument, what counts as leakage, and how to interpret noisy measurements. Ask for those artifacts explicitly so the answer goes beyond “looks constant-time” or “not constant-time.”

Iterate after the first pass

Use the first result to refine scope. If it flags a branch, ask for a narrower constant-time-testing usage check on that function alone; if it looks clean, ask for adjacent helpers, compiler effects, or platform-specific behavior that could still leak.