--- name: protect-mcp-setup description: Configure Cedar policy enforcement and Ed25519 signed receipts for Claude Code tool calls. Use when setting up projects that need cryptographic audit trails, policy-gated tool execution, or compliance-ready evidence of agent actions. --- # protect-mcp — Policy Enforcement + Signed Receipts Cryptographic governance for every Claude Code tool call. Each invocation is evaluated against a Cedar policy and produces an Ed25519-signed receipt that anyone can verify offline. ## Overview Claude Code runs powerful tools: `Bash`, `Edit`, `Write`, `WebFetch`. By default there is no audit trail, no policy enforcement, and no way to prove what was decided after the fact. `protect-mcp` closes all three gaps: - **Cedar policies** (AWS's open authorization engine) evaluate every tool call before execution. Cedar deny is authoritative. - **Ed25519 receipts** record the name of each tool that ran, signed with your key. - **Offline verification** via `npx @veritasacta/verify`. No server, no account, no trust in the operator. ## Problem AI agents make decisions that affect money, safety, and rights. The Claude Code session log records what happened, but the log is: - Mutable — anyone with access can edit it - Unsigned — there is no way to prove integrity - Operator-bound — verification requires trusting whoever holds the log For compliance contexts (finance, healthcare, regulated research), this is not sufficient. You need tamper-evident evidence that can be verified by third parties without trusting you. ## Solution Add `protect-mcp` to your Claude Code project: ```bash # 1. Install the plugin (adds hooks + skill to your project) claude plugin install wshobson/agents/protect-mcp # 2. Create ./protect.cedar (see below). The plugin installs the hooks. # 3. Create the signing key once (protect-mcp 0.7.4 sign does not create it). # An existing key is never replaced. See references/receipt-format.md to rotate. if [ ! -e ./protect-mcp.key ]; then d=$(mktemp -d) && npx protect-mcp@0.7.4 init --dir "$d" && mv "$d/keys/gateway.json" ./protect-mcp.key fi echo "/protect-mcp.key" >> .gitignore # 4. Use Claude Code normally. Every tool call is now policy-evaluated # and produces a signed receipt in ./receipts/ ``` ## Hook Configuration Installing the plugin adds both hooks from `hooks/hooks.json`. Each hook runs a script bundled with the plugin: ```json { "hooks": { "PreToolUse": [ { "matcher": ".*", "hooks": [ { "type": "command", "command": "\"${CLAUDE_PLUGIN_ROOT}\"/hooks/evaluate.sh" } ] } ], "PostToolUse": [ { "matcher": ".*", "hooks": [ { "type": "command", "command": "\"${CLAUDE_PLUGIN_ROOT}\"/hooks/sign.sh" } ] } ] } } ``` Claude Code passes the hook event to the command as JSON on stdin and does not set `TOOL_NAME` or `TOOL_INPUT` variables. `evaluate.sh` reads `tool_name` and `tool_input` from that payload and passes them to `protect-mcp` as flags; `sign.sh` reads `tool_name` only, because the 0.7.4 signer records nothing else. Set `PROTECT_MCP_POLICY`, `PROTECT_MCP_RECEIPTS`, and `PROTECT_MCP_KEY` to change the default paths. When the policy file is missing, the PreToolUse hook prints a warning to stderr and allows the call. ### What each hook does **PreToolUse** — Runs BEFORE the tool executes. Evaluates the tool call against your Cedar policy file. If Cedar returns `deny`, the hook exits with code 2 and Claude Code blocks the tool call entirely. **PostToolUse** runs AFTER the tool completes. It signs a receipt that names the tool and appends it to `./receipts/receipts.jsonl`. protect-mcp 0.7.4 does not record the tool input or output. ## Cedar Policy File Create `./protect.cedar` at the project root: ```cedar // Read-only tools: one rule can name several tools in `when`. Add WebFetch // with your own URL rule. permit (principal, action == Action::"MCP::Tool::call", resource) when { resource == Tool::"Read" || resource == Tool::"Glob" || resource == Tool::"Grep" }; // Safe commands only; git limited to read subcommands permit (principal, action == Action::"MCP::Tool::call", resource == Tool::"Bash") when { context has input && context.input has command && (context.input.command like "git status*" || context.input.command like "git diff*" || context.input.command like "git log*" || context.input.command like "git show*" || context.input.command like "npm*" || context.input.command like "ls*" || context.input.command like "cat*" || context.input.command like "echo*" || context.input.command like "pwd*" || context.input.command like "test*") }; // No chaining (`&` also denies `2>&1`), `$` expansion, redirection (`>` or // `<`, which covers `<(`), file output (`git diff --output`), or rm -rf forbid (principal, action == Action::"MCP::Tool::call", resource == Tool::"Bash") when { context has input && context.input has command && (context.input.command like "*;*" || context.input.command like "*&*" || context.input.command like "*|*" || context.input.command like "*$*" || context.input.command like "*`*" || context.input.command like "*>*" || context.input.command like "*<*" || context.input.command like "*\n*" || context.input.command like "*--output*" || context.input.command like "*rm -rf*") }; // Writes only inside the project (paths are absolute), never via `..` permit (principal, action == Action::"MCP::Tool::call", resource) when { (resource == Tool::"Write" || resource == Tool::"Edit") && context has input && context.input has file_path && context.input.file_path like "/path/to/project/*" }; forbid (principal, action == Action::"MCP::Tool::call", resource) when { (resource == Tool::"Write" || resource == Tool::"Edit") && context has input && context.input has file_path && (context.input.file_path like "*/../*" || context.input.file_path like "*/..") }; ``` String matching is best-effort: `like` checks the raw string, not a resolved path, and an `npm*` permit runs arbitrary code, so it is only as safe as the project's scripts. ## Verification Verify every receipt against the public key in `./protect-mcp.key`: ```bash PUB=$(node -p 'JSON.parse(require("fs").readFileSync("./protect-mcp.key")).publicKey') npx @veritasacta/verify@0.9.2 --replay-chain ./receipts/receipts.jsonl --key "$PUB" # Exit 0 = every receipt verified # Exit 1 = a receipt failed (tampered, wrong key, or malformed line) # Exit 2 = the file could not be read ``` The plugin's slash commands do the same inside Claude Code. `/verify-receipt` takes one receipt in its own file, e.g., from `tail -n 1 ./receipts/receipts.jsonl > receipt.json`. ``` /verify-receipt receipt.json /audit-chain --last 20 ``` ## Receipt Format Each receipt is one line of `./receipts/receipts.jsonl`. See [`references/receipt-format.md`](references/receipt-format.md) for a sample. - **Ed25519** signatures (RFC 8032) over all fields but `signature` - **JCS canonicalization** (RFC 8785) before signing - **No public key** in the receipt, so pass it with `--key` - **No link to the previous receipt**, so a deleted line goes undetected ## Why This Matters | Before | After | |--------|-------| | "Trust me, the agent only read files" | Cryptographically provable: every Read logged and signed | | "The log shows it happened" | The receipt proves it happened, and no one can edit it | | "You'd have to audit our system" | Anyone can verify every receipt offline | | "Logs might be different by now" | Ed25519 signatures lock the record at signing time | ## Standards - **Ed25519** — RFC 8032 (digital signatures) - **JCS** — RFC 8785 (deterministic JSON canonicalization) - **Cedar** — AWS's open authorization policy language - **IETF draft** — [draft-farley-acta-signed-receipts](https://datatracker.ietf.org/doc/draft-farley-acta-signed-receipts/) ## Related - **npm**: [protect-mcp](https://www.npmjs.com/package/protect-mcp) - **Verify CLI**: [@veritasacta/verify](https://www.npmjs.com/package/@veritasacta/verify) - **Source**: [github.com/ScopeBlind/scopeblind-gateway](https://github.com/ScopeBlind/scopeblind-gateway) - **Protocol**: [veritasacta.com](https://veritasacta.com) - **Integrations**: Microsoft Agent Governance Toolkit (PR #667), AWS cedar-policy/cedar-for-agents (PR #64)