Hw Verify

Ask Claude "is this Verilog constant-time?" and get a formal answer with the leaking signals named — not a guess.

License MCP Python CI

▶ Try it in your browser — paste Verilog, get a formal constant-time verdict with the leaking signals named. No install, nothing uploaded.

Why this exists

An LLM asked to write constant-time RTL will produce something plausible. Plausible is exactly the failure mode: early-exit comparisons and data-dependent loop bounds look fine. The model has no way to check, and neither does the person reading the diff.

This server gives the agent a checker it cannot argue with. The loop is:

1. the agent writes RTL;
2. the server refuses it and names the secrets that reach the completion signal;
3. the agent applies the suggested repair;
4. the server confirms — or refuses again.

The agent cannot declare success. Every refusal carries a next_step telling it what to change, and every tool description says a self-asserted verdict does not count. That constraint is the product: it makes the checker's refusal semantics the grammar the agent learns to think in.

Install

Not yet on PyPI. Install from a checkout:

git clone https://github.com/nickharris808/hw-verify-mcp.git && cd hw-verify-mcp
pip install .

This pulls in ctbench, ct-mask, and patchproof, which do the actual analysis.

30-second quickstart

Add to your Claude Desktop config (claude_desktop_config.json):

{
  "mcpServers": {
    "hw-verify": {
      "command": "hw-verify-mcp"
    }
  }
}

Then ask: "Write me a constant-time 8-bit tag comparator, and check it with hw-verify."

Verify the server is wired up before you trust the agent's answers:

$ hw-verify-mcp --version 2>/dev/null; python -c "
from hwverify.server import TOOLS
from hwverify.tools import AVAILABILITY
print(len(TOOLS), 'tools'); print(AVAILABILITY)"
12 tools
{'ctbench': True, 'ct-mask': True, 'patchproof': True}

If any backend reports False, the tools that need it return a clear error rather than a wrong answer.

Worked example — the loop, verbatim

The agent writes the obvious early-exit comparator and calls check_constant_time:

{
  "verdict": "LEAKY",
  "observation": "done",
  "reaching_secrets": ["x", "y"],
  "cone_size": 9,
  "model": "Syntactic fan-in cone of the observation signal, including every enclosing if/case guard. Over-approximate within the supported subset, so CONSTANT_TIME is conservative there; anything outside the subset returns UNKNOWN rather than a verdict.",
  "next_step": "The completion signal depends on x, y. Make the completion condition a function of a data-oblivious counter rather than of operand values: run the loop a fixed number of cycles and drop any early-exit branch. Then call this tool again — a verdict you assert yourself does not count."
}

The agent replaces the early exit with a fixed counter and re-submits:

{
  "verdict": "CONSTANT_TIME",
  "reaching_secrets": [],
  "next_step": "No secret reaches the completion signal. Note this covers completion timing only, not power, EM, or cache channels."
}

That exact sequence is a test (test_agent_loop_refuse_fix_confirm), so the loop is verified rather than illustrated.

The tools

Tool	What it does
check_constant_time	CONSTANT_TIME or LEAKY for a Verilog module, with the reaching secrets named
find_leak	just the localisation: which secrets reach the completion signal, and the cone size
list_benchmark_fixtures	the ctbench matched-pair corpus, with expected verdicts
get_benchmark_fixture	the Verilog source of one fixture, so the agent can reason about it
score_benchmark_submission	grade a set of verdicts; unsound is reported separately from imprecise
run_reference_checker	run the bundled baseline over the whole corpus
check_masking	first-order masking verification of a gadget, by name or as a JSON netlist
list_masking_gadgets	the masking corpus, and the netlist format for your own
check_patch_complete	does a bounds-check repair eliminate every violating input?
list_defect_classes	the modelled defect classes, and what a COMPLETE verdict excludes
replay_certificate	re-check an elimination certificate using integer arithmetic, no solver
prove_confidential	not available — see below

Secrets are never inferred

check_constant_time refuses to guess which inputs are sensitive:

{ "error": "no secrets declared. Secrets are a specification choice and are never inferred: pass the input names that carry sensitive values." }

Guessing here would be worse than useless — it would produce confident verdicts about the wrong property.

Errors are data, not faults

A refusal is a normal outcome. Unknown tools, bad arguments, and malformed netlists all come back as {"error": ...} rather than as transport failures, because an agent recovers from a JSON error and cannot recover from a broken connection.

Honest scope

Everything the server inherits from its three backends, it also inherits the limits of:

• Constant-time verdicts cover completion timing against declared secrets — not power, EM, cache, or microarchitectural channels. The checker is a syntactic over-approximation within the supported subset, so CONSTANT_TIME is conservative there and LEAKY may be pessimistic. A design outside the subset (submodule instantiation, for, generate, function, macro) returns UNKNOWN with a next_step telling the agent it has not been shown constant-time; find_leak returns leaks: null rather than false, so an agent cannot read it as clean.
• Masking is glitch-free, first-order (d=1), 2-share probing. The report separates mean-invariance from whole-distribution invariance and says which was established.
• Patch completeness is reachability in modelled bit semantics — not an RCE claim — and list_defect_classes returns the shapes deliberately outside the model.

prove_confidential

The tool is in the list, and calling it tells you why:

Every tool in this server analyses a design you supply in full. Proving a property to a third party who never receives the design is a different problem: it needs the result bound to a commitment of a design that stays hidden. That capability is commercial and is not part of this package.

It is listed rather than omitted deliberately. An agent that discovers the boundary is more useful than one that silently never learns it exists.

Development

pip install -e . && pytest tests -q && ruff check .

24 tests: the tool functions directly, one real MCP session over the in-memory transport, and one that drives the installed hw-verify-mcp binary over stdio JSON-RPC (skipped if the package is not on PATH). A further test asserts mcp-manifest.json lists exactly the tools the server exposes, so the manifest cannot drift.

Note for anyone writing their own client: keep stdin open. Closing it immediately after writing makes the server shut down before later replies are flushed — that is correct stdio behaviour, and it will look like a hang or a dropped response if you batch-write.

Documentation

• SCOPE.md — what the three checkers prove, and why UNKNOWN reaches the agent as leaks: null rather than false.

Part of the hw-verify toolkit

Open tools for proving security properties of hardware and bounds checks. They share one boundary: everything open analyses a design you disclose in full.

Project	What it does
▶ Live demo	Constant-time checker in your browser — the real analyzer via Pyodide
Docs & overview	What the toolkit proves, and what it refuses to answer
hw-verify	One install, one command, all three checkers
ctbench	Matched-pair constant-time RTL benchmark + leaderboard
patchproof	Prove a bounds-check fix eliminates every violating input
patchproof-verify	Re-check its certificates in Rust, with no shared code
ct-mask	First-order masking verification by two certificates
hw-verify-mcp (you are here)	MCP server — the checkers, callable by AI agents
ct-audit-action	GitHub Action — fail a PR on a leaky completion signal
verdicts · witness paths	Two datasets: what each design is, and why

The commercial boundary. Proving a property to a third party who never receives the design — a verdict bound to a commitment of a design that stays hidden — is a different problem and a commercial one. It is not in any of these packages.

Citation

If you use this in academic work, please cite it — CITATION.cff has the metadata, and GitHub renders a "Cite this repository" button from it.

Contributing

A tool an agent misuses, or a refusal whose next_step did not help, is the most valuable report. See CONTRIBUTING.md.

License

Apache-2.0. See LICENSE. Contributing: CONTRIBUTING.md.