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Step-by-step cookbook for setting up cryptographically signed audit trails on Claude Code tool calls. Use when explaining, evaluating, or demonstrating the pattern before committing to the protect-mcp runtime hooks. Covers Cedar policy, Ed25519 receipts, offline verification,
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Step-by-step cookbook for setting up cryptographically signed audit trails on Claude Code tool calls. Use when explaining, evaluating, or demonstrating the pattern before committing to the protect-mcp runtime hooks. Covers Cedar policy, Ed25519 receipts, offline verification, 📦 Ships with wshobson-agents
</> SKILL.md
signed-audit-trails-recipe.SKILL.md
---
name : signed-audit-trails-recipe
description : Step-by-step cookbook for setting up cryptographically signed audit trails on Claude Code tool calls. Use when explaining, evaluating, or demonstrating the pattern before committing to the protect-mcp runtime hooks. Covers Cedar policy, Ed25519 receipts, offline verification, tamper detection, CI/CD integration, and SLSA composition.
---
# Signed Audit Trails for Claude Code Tool Calls
Cookbook-style walkthrough for cryptographically signed receipts on every
Claude Code tool call. This is the teaching skill. For the runtime
implementation, install the [`protect-mcp`](../../protect-mcp/) plugin.
## What this gives you
Every tool call (`Bash`, `Edit`, `Write`, `WebFetch`) is:
1. **Evaluated against a Cedar policy** before execution. If the policy denies
the call, the tool does not run.
2. **Signed as an Ed25519 receipt** after execution. Receipts are
JCS-canonical, hash-chained, and verifiable offline by anyone with the
public key.
An auditor, regulator, or counterparty can verify the full chain later with a
single CLI command (`npx @veritasacta/verify receipts/*.json`). No network
call, no vendor lookup, no trust in the operator.
## When to use the pattern
- **Regulated environments** (finance, healthcare, critical infrastructure)
Flowy AI Flows that just work. Hand-picked Claude Code plugins that fire the right skill as you prompt.
where you need tamper-evident evidence of agent behavior
- **CI/CD pipelines** where you want to prove that a policy gate held for
every automated build step
- **Multi-party collaboration** where a counterparty wants to verify your
agent's behavior without trusting your operator
- **Compliance contexts** (EU AI Act Article 12, SLSA provenance for
agent-built software) where standard logging is not sufficient
## Step 1: Install the hook configuration
Create `.claude/settings.json` in your project root:
```json
{
"hooks": {
"PreToolUse": [
{
"matcher": ".*",
"hook": {
"type": "command",
"command": "npx protect-mcp@latest evaluate --policy ./protect.cedar --tool \"$TOOL_NAME\" --input \"$TOOL_INPUT\" --fail-on-missing-policy false"
}
}
],
"PostToolUse": [
{
"matcher": ".*",
"hook": {
"type": "command",
"command": "npx protect-mcp@latest sign --tool \"$TOOL_NAME\" --input \"$TOOL_INPUT\" --output \"$TOOL_OUTPUT\" --receipts ./receipts/ --key ./protect-mcp.key"
}
}
]
}
}
```
The first run of `protect-mcp sign` generates `./protect-mcp.key` (Ed25519
private key) if one does not exist. Commit the **public** key fingerprint
(visible in any receipt's `public_key` field); do not commit the private
key.
Add the private key and receipt directory to `.gitignore`:
```bash
echo "./protect-mcp.key" >> .gitignore
echo "./receipts/" >> .gitignore
```
## Step 2: Write a Cedar policy
Create `./protect.cedar`:
```cedar
// Allow all read-oriented tools by default.
permit (
principal,
action in [Action::"Read", Action::"Glob", Action::"Grep", Action::"WebSearch"],
resource
);
// Allow Bash commands from a safe list only.
permit (
principal,
action == Action::"Bash",
resource
) when {
context.command_pattern in [
"git", "npm", "pnpm", "yarn", "ls", "cat", "pwd",
"echo", "test", "node", "python", "make"
]
};
// Explicit deny on destructive commands. Cedar deny is authoritative.
forbid (
principal,
action == Action::"Bash",
resource
) when {
context.command_pattern in ["rm -rf", "dd", "mkfs", "shred"]
};
// Restrict writes to the project directory.
permit (
principal,
action in [Action::"Write", Action::"Edit"],
resource
) when {
context.path_starts_with == "./"
};
```
Four rules:
- Read-oriented tools always allowed
- `Bash` allowed for safe command patterns (`git`, `npm`, etc.)
- `Bash rm -rf` and similar destructive commands explicitly denied
- Writes allowed only within the project (`./` prefix)
Cedar `forbid` rules take precedence over `permit` rules, so destructive
commands cannot be bypassed by a later permissive rule.
## Step 3: Use Claude Code normally
Start Claude Code. Every tool call goes through both hooks:
```
You: Please read the README and summarize it.
Claude: I will read README.md.
[PreToolUse: Read ./README.md -> allow]
[Tool: Read executes]
[PostToolUse: receipt rcpt-a8f3c9d2 signed to ./receipts/]
... summary of README ...
```
A session of 20 tool calls produces 20 receipts, each hash-chained to its
predecessor.
## Step 4: Inspect a receipt
```bash
cat ./receipts/$(ls -t ./receipts/ | head -1)
```
```json
{
"receipt_id": "rcpt-a8f3c9d2",
"receipt_version": "1.0",
"issuer_id": "claude-code-protect-mcp",
"event_time": "2026-04-17T12:34:56.123Z",
"tool_name": "Read",
"input_hash": "sha256:a3f8c9d2e1b7465f...",
"decision": "allow",
"policy_id": "protect.cedar",
"policy_digest": "sha256:b7e2f4a6c8d0e1f3...",
"parent_receipt_id": "rcpt-3d1ab7c2",
"public_key": "4437ca56815c0516...",
"signature": "4cde814b7889e987..."
}
```
Every field except `signature` and `public_key` is covered by the Ed25519
signature. Modifying any field after signing invalidates the signature.
## Step 5: Verify the receipt chain
```bash
npx @veritasacta/verify ./receipts/*.json
```
Exit codes:
| Code | Meaning |
|------|---------|
| `0` | All receipts verified; chain intact |
| `1` | A receipt failed signature verification (tampered, or wrong key) |
| `2` | A receipt was malformed |
## Step 6: Demonstrate tamper detection
Modify any receipt's `decision` field from `allow` to `deny`:
```bash
python3 -c "
import json, os
path = './receipts/' + sorted(os.listdir('./receipts'))[-1]
r = json.loads(open(path).read())
r['decision'] = 'deny'
open(path, 'w').write(json.dumps(r))
"
npx @veritasacta/verify ./receipts/*.json
```
The verifier exits with code `1` and reports which receipt failed. The
Ed25519 signature no longer matches the JCS-canonical bytes of the
tampered payload.
Restore the field and verification passes again.
## How the cryptography works
Three invariants make receipts verifiable offline across any conformant
implementation:
1. **JCS canonicalization (RFC 8785)** before signing. Keys sorted,
whitespace minimized, strings NFC-normalized. Two independent
implementations produce byte-identical signing payloads for the same
receipt content.
2. **Ed25519 signatures (RFC 8032)** over the canonical bytes.
Deterministic, fixed-size, no nonce dependency.
3. **Hash chain linkage.** Each receipt's `parent_receipt_hash` is the
SHA-256 of the predecessor's canonical form. Insertions, deletions, and
reorderings break later receipts.
For the formal wire format see
[draft-farley-acta-signed-receipts](https://datatracker.ietf.org/doc/draft-farley-acta-signed-receipts/).
## Cross-implementation interop
The receipt format has four independent implementations today:
| Implementation | Language | Use case |
|----------------|----------|----------|
| [protect-mcp](https://www.npmjs.com/package/protect-mcp) | TypeScript | Claude Code, Cursor, MCP hosts |
| [protect-mcp-adk](https://pypi.org/project/protect-mcp-adk/) | Python | Google Agent Development Kit |
| [sb-runtime](https://github.com/ScopeBlind/sb-runtime) | Rust | OS-level sandbox (Landlock + seccomp) |
| APS governance hook | Python | CrewAI, LangChain |
A receipt produced by any of them verifies against
[`@veritasacta/verify`](https://www.npmjs.com/package/@veritasacta/verify).
The auditor does not need to trust the operator's tooling choice: the format
is the contract.
## CI/CD integration
Gate merges on receipt chain verification so no build lands with a broken
evidence chain:
```yaml
# .github/workflows/verify-receipts.yml
name : Verify Decision Receipts
on : [push, pull_request]
jobs:
verify:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: actions/setup-node@v4
with: { node-version: '20' }
- name: Run governed agent
run: python scripts/run_agent.py > receipts.jsonl
- name: Verify receipt chain
run: npx @veritasacta/verify receipts.jsonl
```
Archive the receipts as an artifact so the chain survives beyond the job run:
```yaml
- name: Upload receipts
if: always()
uses: actions/upload-artifact@v4
with:
name: decision-receipts
path: receipts/
```
## Composition with SLSA provenance for agent-built software
When Claude Code builds and releases software (running `npm install`,
`npm build`, `npm publish` as tool calls), the receipt chain is the
per-step build log. SLSA Provenance v1 has an extension point for this: the
`byproducts` field can reference the receipt chain alongside the build
attestation.
The [agent-commit build type](https://refs.arewm.com/agent-commit/v0.2)
documents the pattern using the ResourceDescriptor shape:
```json
{
"name": "decision-receipts",
"digest": { "sha256": "..." },
"uri": "oci://registry/org/build-xyz/receipts:sha256-...",
"annotations": {
"predicateType": "https://veritasacta.com/attestation/decision-receipt/v0.1",
"signerRole": "supervisor-hook"
}
}
```
The SLSA provenance is signed by the builder identity; the receipt
attestation is signed by the supervisor-hook identity. Two trust domains,
cross-referenced at the byproduct layer. See
[slsa-framework/slsa#1594](https://github.com/slsa-framework/slsa/issues/1594)
for the composition discussion.
## Common pitfalls
**Private key in version control.** The generated `./protect-mcp.key` must
not be committed. The examples above add it to `.gitignore`. If a key is
accidentally committed, rotate immediately (delete the key file and let the
hook regenerate on next run).
**Hook command quoting.** The hooks receive `$TOOL_NAME` and `$TOOL_INPUT`
as environment variables. Keep the quoting `"$TOOL_INPUT"` so inputs with
spaces or special characters pass through intact.
**Receipts directory in CI.** If Claude Code runs in CI, upload receipts as
an artifact at the end of the job or the chain is lost at job end.
**Policy is missing.** The example `PreToolUse` hook uses
`--fail-on-missing-policy false` so an absent `./protect.cedar` does not
break Claude Code out of the box. Remove this flag in production so a
missing policy is treated as a hard failure.
## Related in this marketplace
- [`protect-mcp`](../../protect-mcp/) — the runtime hook implementation
(use this plugin in production)
- [`review-agent-governance`](../../review-agent-governance/) — require
human approval before review-surface actions; composes with protect-mcp
## References
- [`draft-farley-acta-signed-receipts`](https://datatracker.ietf.org/doc/draft-farley-acta-signed-receipts/) — IETF draft, receipt wire format
- [RFC 8032](https://datatracker.ietf.org/doc/html/rfc8032) — Ed25519
- [RFC 8785](https://datatracker.ietf.org/doc/html/rfc8785) — JCS
- [Cedar policy language](https://docs.cedarpolicy.com/)
- [protect-mcp on npm](https://www.npmjs.com/package/protect-mcp)
- [@veritasacta/verify on npm](https://www.npmjs.com/package/@veritasacta/verify)
- [in-toto/attestation#549](https://github.com/in-toto/attestation/pull/549) — Decision Receipt predicate proposal
- [agent-commit build type](https://refs.arewm.com/agent-commit/v0.2) — SLSA provenance for agent-produced commits
- [Microsoft Agent Governance Toolkit](https://github.com/microsoft/agent-governance-toolkit) (`examples/protect-mcp-governed/`)
- [AWS Cedar for Agents](https://github.com/cedar-policy/cedar-for-agents)
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