# ATCR Signature Verification Integration Strategy

## Overview

This document provides a comprehensive overview of how to integrate ATProto signature verification into various tools and workflows. ATCR uses a layered approach that provides maximum compatibility while maintaining ATProto's decentralized philosophy.

## Architecture Layers

```
┌─────────────────────────────────────────────────────────┐
│ Layer 4: Applications & Workflows                      │
│  - CI/CD pipelines                                      │
│  - Kubernetes admission control                         │
│  - Runtime verification                                 │
│  - Security scanning                                    │
└──────────────────────┬──────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────┐
│ Layer 3: Integration Methods                           │
│  - Plugins (Ratify, Gatekeeper, Containerd)            │
│  - CLI tools (atcr-verify)                              │
│  - External services (webhooks, APIs)                   │
│  - (Optional) X.509 certificates (hold-as-CA)           │
└──────────────────────┬──────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────┐
│ Layer 2: Signature Discovery                           │
│  - OCI Referrers API (GET /v2/.../referrers/...)        │
│  - ORAS artifact format                                 │
│  - artifactType: application/vnd.atproto.signature...   │
└──────────────────────┬──────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────┐
│ Layer 1: ATProto Signatures (Foundation)               │
│  - Manifests signed by PDS (K-256)                      │
│  - Signatures in ATProto repository commits             │
│  - Public keys in DID documents                         │
│  - DID-based identity                                   │
└─────────────────────────────────────────────────────────┘
```

## Integration Approaches

### Approach 1: Plugin-Based (RECOMMENDED) ⭐

**Best for:** Kubernetes, standard tooling, production deployments

Integrate through plugin systems of existing tools:

#### Ratify Verifier Plugin
- **Use case:** Kubernetes admission control via Gatekeeper
- **Effort:** 2-3 weeks to build
- **Maturity:** CNCF Sandbox project, growing adoption
- **Benefits:**
  - ✅ Standard plugin interface
  - ✅ Works with existing Ratify deployments
  - ✅ Policy-based enforcement
  - ✅ Multi-verifier support (can combine with Notation, Cosign)

**Implementation:**
```go
// Ratify plugin interface
type ReferenceVerifier interface {
    VerifyReference(
        ctx context.Context,
        subjectRef common.Reference,
        referenceDesc ocispecs.ReferenceDescriptor,
        store referrerStore.ReferrerStore,
    ) (VerifierResult, error)
}
```

**Deployment:**
```yaml
apiVersion: config.ratify.deislabs.io/v1beta1
kind: Verifier
metadata:
  name: atcr-verifier
spec:
  name: atproto
  artifactType: application/vnd.atproto.signature.v1+json
  parameters:
    trustedDIDs:
      - did:plc:alice123
```

See [Ratify Integration Guide](./SIGNATURE_INTEGRATION.md#ratify-plugin)

---

#### OPA Gatekeeper External Provider
- **Use case:** Kubernetes admission control with OPA policies
- **Effort:** 2-3 weeks to build
- **Maturity:** Very stable, widely adopted
- **Benefits:**
  - ✅ Rego-based policies (flexible)
  - ✅ External data provider API (standard)
  - ✅ Can reuse existing Gatekeeper deployments

**Implementation:**
```go
// External data provider
type Provider struct {
    verifier *atproto.Verifier
}

func (p *Provider) Provide(ctx context.Context, req ProviderRequest) (*ProviderResponse, error) {
    image := req.Keys["image"]
    result, err := p.verifier.Verify(ctx, image)
    return &ProviderResponse{
        Data: map[string]bool{"verified": result.Verified},
    }, nil
}
```

**Policy:**
```rego
package verify

violation[{"msg": msg}] {
  container := input.review.object.spec.containers[_]
  startswith(container.image, "atcr.io/")

  response := external_data({
    "provider": "atcr-verifier",
    "keys": ["image"],
    "values": [container.image]
  })

  response.verified != true
  msg := sprintf("Image %v has no valid ATProto signature", [container.image])
}
```

See [Gatekeeper Integration Guide](./SIGNATURE_INTEGRATION.md#opa-gatekeeper-external-provider)

---

#### Containerd 2.0 Image Verifier Plugin
- **Use case:** Runtime verification at image pull time
- **Effort:** 1-2 weeks to build
- **Maturity:** New in Containerd 2.0 (Nov 2024)
- **Benefits:**
  - ✅ Runtime enforcement (pull-time verification)
  - ✅ Works for Docker, nerdctl, ctr
  - ✅ Transparent to users
  - ✅ No Kubernetes required

**Limitation:** CRI plugin integration still maturing

**Implementation:**
```bash
#!/bin/bash
# /usr/local/bin/containerd-verifiers/atcr-verifier
# Binary called by containerd on image pull

# Containerd passes image info via stdin
read -r INPUT

IMAGE=$(echo "$INPUT" | jq -r '.reference')
DIGEST=$(echo "$INPUT" | jq -r '.descriptor.digest')

# Verify signature
if atcr-verify "$IMAGE@$DIGEST" --quiet; then
  exit 0  # Verified
else
  exit 1  # Failed
fi
```

**Configuration:**
```toml
# /etc/containerd/config.toml
[plugins."io.containerd.image-verifier.v1.bindir"]
  bin_dir = "/usr/local/bin/containerd-verifiers"
  max_verifiers = 5
  per_verifier_timeout = "10s"
```

See [Containerd Integration Guide](./SIGNATURE_INTEGRATION.md#containerd-20)

---

### Approach 2: CLI Tool (RECOMMENDED) ⭐

**Best for:** CI/CD, scripts, general-purpose verification

Use `atcr-verify` CLI tool directly in workflows:

#### Command-Line Verification
```bash
# Basic verification
atcr-verify atcr.io/alice/myapp:latest

# With trust policy
atcr-verify atcr.io/alice/myapp:latest --policy trust-policy.yaml

# JSON output for scripting
atcr-verify atcr.io/alice/myapp:latest --output json

# Quiet mode for exit codes
atcr-verify atcr.io/alice/myapp:latest --quiet && echo "Verified"
```

#### CI/CD Integration

**GitHub Actions:**
```yaml
- name: Verify image
  run: atcr-verify ${{ env.IMAGE }} --policy .github/trust-policy.yaml
```

**GitLab CI:**
```yaml
verify:
  image: atcr.io/atcr/verify:latest
  script:
    - atcr-verify ${IMAGE} --policy trust-policy.yaml
```

**Universal Container:**
```bash
docker run --rm atcr.io/atcr/verify:latest verify IMAGE
```

**Benefits:**
- ✅ Works everywhere (not just Kubernetes)
- ✅ Simple integration (single binary)
- ✅ No plugin installation required
- ✅ Offline mode support

See [atcr-verify CLI Documentation](./ATCR_VERIFY_CLI.md)

---

### Approach 3: External Services

**Best for:** Custom admission controllers, API-based verification

Build verification as a service that tools can call:

#### Webhook Service
```go
// HTTP endpoint for verification
func (h *Handler) VerifyImage(w http.ResponseWriter, r *http.Request) {
    image := r.URL.Query().Get("image")

    result, err := h.verifier.Verify(r.Context(), image)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    json.NewEncoder(w).Encode(map[string]interface{}{
        "verified": result.Verified,
        "did": result.Signature.DID,
        "signedAt": result.Signature.SignedAt,
    })
}
```

#### Usage from Kyverno
```yaml
verifyImages:
- imageReferences:
  - "atcr.io/*/*"
  attestors:
  - entries:
    - api:
        url: http://atcr-verify.kube-system/verify?image={{ image }}
```

**Benefits:**
- ✅ Flexible integration
- ✅ Centralized verification logic
- ✅ Caching and rate limiting
- ✅ Can add additional checks (vulnerability scanning, etc.)

---

### Approach 4: Hold-as-CA (OPTIONAL, ENTERPRISE ONLY)

**Best for:** Enterprise X.509 PKI compliance requirements

⚠️ **WARNING:** This approach introduces centralization trade-offs. Only use if you have specific X.509 compliance requirements.

Hold services act as Certificate Authorities that issue X.509 certificates for users, enabling standard Notation verification.

**When to use:**
- Enterprise requires standard X.509 PKI
- Cannot deploy custom plugins
- Accept centralization trade-off for tool compatibility

**When NOT to use:**
- Default deployments (use plugins instead)
- Maximum decentralization required
- Don't need X.509 compliance

See [Hold-as-CA Architecture](./HOLD_AS_CA.md) for complete details and security implications.

---

## Tool Compatibility Matrix

| Tool | Discover | Verify | Integration Method | Priority | Effort |
|------|----------|--------|-------------------|----------|--------|
| **Kubernetes** | | | | | |
| OPA Gatekeeper | ✅ | ✅ | External provider | **HIGH** | 2-3 weeks |
| Ratify | ✅ | ✅ | Verifier plugin | **HIGH** | 2-3 weeks |
| Kyverno | ✅ | ⚠️ | External service | MEDIUM | 2 weeks |
| Portieris | ❌ | ❌ | N/A (deprecated) | NONE | - |
| **Runtime** | | | | | |
| Containerd 2.0 | ✅ | ✅ | Bindir plugin | **MED-HIGH** | 1-2 weeks |
| CRI-O | ⚠️ | ⚠️ | Upstream contribution | MEDIUM | 3-4 weeks |
| Podman | ⚠️ | ⚠️ | Upstream contribution | MEDIUM | 3-4 weeks |
| **CI/CD** | | | | | |
| GitHub Actions | ✅ | ✅ | Custom action | **HIGH** | 1 week |
| GitLab CI | ✅ | ✅ | Container image | **HIGH** | 1 week |
| Jenkins/CircleCI | ✅ | ✅ | Container image | HIGH | 1 week |
| **Scanners** | | | | | |
| Trivy | ✅ | ❌ | N/A (not verifier) | NONE | - |
| Snyk | ❌ | ❌ | N/A (not verifier) | NONE | - |
| Anchore | ❌ | ❌ | N/A (not verifier) | NONE | - |
| **Registries** | | | | | |
| Harbor | ✅ | ⚠️ | UI integration | LOW | - |
| **OCI Tools** | | | | | |
| ORAS CLI | ✅ | ❌ | Already works | Document | - |
| Notation | ⚠️ | ⚠️ | Hold-as-CA | OPTIONAL | 3-4 weeks |
| Cosign | ❌ | ❌ | Not compatible | NONE | - |
| Crane | ✅ | ❌ | Already works | Document | - |
| Skopeo | ⚠️ | ⚠️ | Upstream contribution | LOW | 3-4 weeks |

**Legend:**
- ✅ Works / Feasible
- ⚠️ Partial / Requires changes
- ❌ Not applicable / Not feasible

---

## Implementation Roadmap

### Phase 1: Foundation (4-5 weeks) ⭐

**Goal:** Core verification capability

1. **atcr-verify CLI tool** (Week 1-2)
   - ATProto signature verification
   - Trust policy support
   - Multiple output formats
   - Offline mode

2. **OCI Referrers API** (Week 2-3)
   - AppView endpoint implementation
   - ORAS artifact serving
   - Integration with existing SBOM pattern

3. **CI/CD Container Image** (Week 3)
   - Universal verification image
   - Documentation for GitHub Actions, GitLab CI
   - Example workflows

4. **Documentation** (Week 4-5)
   - Integration guides
   - Trust policy examples
   - Troubleshooting guides

**Deliverables:**
- `atcr-verify` binary (Linux, macOS, Windows)
- `atcr.io/atcr/verify:latest` container image
- OCI Referrers API implementation
- Complete documentation

---

### Phase 2: Kubernetes Integration (3-4 weeks)

**Goal:** Production-ready Kubernetes admission control

5. **OPA Gatekeeper Provider** (Week 1-2)
   - External data provider service
   - Helm chart for deployment
   - Example policies

6. **Ratify Plugin** (Week 2-3)
   - Verifier plugin implementation
   - Testing with Ratify
   - Documentation

7. **Kubernetes Examples** (Week 4)
   - Deployment manifests
   - Policy examples
   - Integration testing

**Deliverables:**
- `atcr-gatekeeper-provider` service
- Ratify plugin binary
- Kubernetes deployment examples
- Production deployment guide

---

### Phase 3: Runtime Verification (2-3 weeks)

**Goal:** Pull-time verification

8. **Containerd Plugin** (Week 1-2)
   - Bindir verifier implementation
   - Configuration documentation
   - Testing with Docker, nerdctl

9. **CRI-O/Podman Integration** (Week 3, optional)
   - Upstream contribution (if accepted)
   - Policy.json extension
   - Documentation

**Deliverables:**
- Containerd verifier binary
- Configuration guides
- Runtime verification examples

---

### Phase 4: Optional Features (2-3 weeks)

**Goal:** Enterprise features (if demanded)

10. **Hold-as-CA** (Week 1-2, optional)
    - Certificate generation
    - Notation signature creation
    - Trust store distribution
    - **Only if enterprise customers request**

11. **Advanced Features** (Week 3, as needed)
    - Signature transparency log
    - Multi-signature support
    - Hardware token integration

**Deliverables:**
- Hold co-signing implementation (if needed)
- Advanced feature documentation

---

## Decision Matrix

### Which Integration Approach Should I Use?

```
┌─────────────────────────────────────────────────┐
│ Are you using Kubernetes?                      │
└───────────────┬─────────────────────────────────┘
                │
       ┌────────┴────────┐
       │                 │
      YES               NO
       │                 │
       ↓                 ↓
┌──────────────┐  ┌──────────────┐
│ Using        │  │ CI/CD        │
│ Gatekeeper?  │  │ Pipeline?    │
└──────┬───────┘  └──────┬───────┘
       │                 │
  ┌────┴────┐       ┌────┴────┐
 YES       NO      YES       NO
  │         │       │         │
  ↓         ↓       ↓         ↓
External  Ratify  GitHub   Universal
Provider  Plugin  Action   CLI Tool
```

#### Use OPA Gatekeeper Provider if:
- ✅ Already using Gatekeeper
- ✅ Want Rego-based policies
- ✅ Need flexible policy logic

#### Use Ratify Plugin if:
- ✅ Using Ratify (or planning to)
- ✅ Want standard plugin interface
- ✅ Need multi-verifier support (Notation + Cosign + ATProto)

#### Use atcr-verify CLI if:
- ✅ CI/CD pipelines
- ✅ Local development
- ✅ Non-Kubernetes environments
- ✅ Want simple integration

#### Use Containerd Plugin if:
- ✅ Need runtime enforcement
- ✅ Want pull-time verification
- ✅ Using Containerd 2.0+

#### Use Hold-as-CA if:
- ⚠️ Enterprise X.509 PKI compliance required
- ⚠️ Cannot deploy plugins
- ⚠️ Accept centralization trade-off

---

## Best Practices

### 1. Start Simple

Begin with CLI tool integration in CI/CD:
```bash
# Add to .github/workflows/deploy.yml
- run: atcr-verify $IMAGE --policy .github/trust-policy.yaml
```

### 2. Define Trust Policies

Create trust policies early:
```yaml
# trust-policy.yaml
policies:
  - name: production
    scope: "atcr.io/*/prod-*"
    require:
      signature: true
      trustedDIDs: [did:plc:devops-team]
    action: enforce
```

### 3. Progressive Rollout

1. **Week 1:** Add verification to CI/CD (audit mode)
2. **Week 2:** Enforce in CI/CD
3. **Week 3:** Add Kubernetes admission control (audit mode)
4. **Week 4:** Enforce in Kubernetes

### 4. Monitor and Alert

Track verification metrics:
- Verification success/failure rates
- Policy violations
- Signature coverage (% of images signed)

### 5. Plan for Key Rotation

- Document DID key rotation procedures
- Test key rotation in non-production
- Monitor for unexpected key changes

---

## Common Patterns

### Pattern 1: Multi-Layer Defense

```
1. CI/CD verification (atcr-verify)
   ↓ (blocks unsigned images from being pushed)
2. Kubernetes admission (Gatekeeper/Ratify)
   ↓ (blocks unsigned images from running)
3. Runtime verification (Containerd plugin)
   ↓ (blocks unsigned images from being pulled)
```

### Pattern 2: Trust Policy Inheritance

```yaml
# Global policy
trustedDIDs:
  - did:plc:security-team  # Always trusted

# Environment-specific policies
staging:
  trustedDIDs:
    - did:plc:developers  # Additional trust for staging

production:
  trustedDIDs: []  # Only global trust (security-team)
```

### Pattern 3: Offline Verification

```bash
# Build environment (online)
atcr-verify export $IMAGE -o bundle.json

# Air-gapped environment (offline)
atcr-verify $IMAGE --offline --bundle bundle.json
```

---

## Migration Guide

### From Docker Content Trust (DCT)

DCT is deprecated. Migrate to ATCR signatures:

**Old (DCT):**
```bash
export DOCKER_CONTENT_TRUST=1
docker push myimage:latest
```

**New (ATCR):**
```bash
# Signatures created automatically on push
docker push atcr.io/myorg/myimage:latest

# Verify in CI/CD
atcr-verify atcr.io/myorg/myimage:latest
```

### From Cosign

Cosign and ATCR signatures can coexist:

**Dual signing:**
```bash
# Push to ATCR (ATProto signature automatic)
docker push atcr.io/myorg/myimage:latest

# Also sign with Cosign (if needed)
cosign sign atcr.io/myorg/myimage:latest
```

**Verification:**
```bash
# Verify ATProto signature
atcr-verify atcr.io/myorg/myimage:latest

# Or verify Cosign signature
cosign verify atcr.io/myorg/myimage:latest --key cosign.pub
```

---

## Troubleshooting

### Signatures Not Found

**Symptom:** `atcr-verify` reports "no signature found"

**Diagnosis:**
```bash
# Check if Referrers API works
curl "https://atcr.io/v2/OWNER/REPO/referrers/DIGEST"

# Check if signature artifact exists
oras discover atcr.io/OWNER/REPO:TAG
```

**Solutions:**
1. Verify Referrers API is implemented
2. Re-push image to generate signature
3. Check AppView logs for signature creation errors

### DID Resolution Fails

**Symptom:** Cannot resolve DID to public key

**Diagnosis:**
```bash
# Test DID resolution
curl https://plc.directory/did:plc:XXXXXX

# Check DID document has verificationMethod
curl https://plc.directory/did:plc:XXXXXX | jq .verificationMethod
```

**Solutions:**
1. Check internet connectivity
2. Verify DID is valid
3. Ensure DID document contains public key

### Policy Violations

**Symptom:** Verification fails with "trust policy violation"

**Diagnosis:**
```bash
# Verify with verbose output
atcr-verify IMAGE --policy policy.yaml --verbose
```

**Solutions:**
1. Add DID to trustedDIDs list
2. Check signature age vs. maxAge
3. Verify policy scope matches image

---

## See Also

- [ATProto Signatures](./ATPROTO_SIGNATURES.md) - Technical foundation
- [atcr-verify CLI](./ATCR_VERIFY_CLI.md) - CLI tool documentation
- [Signature Integration](./SIGNATURE_INTEGRATION.md) - Tool-specific guides
- [Hold-as-CA](./HOLD_AS_CA.md) - X.509 certificate approach (optional)
- [Examples](../examples/verification/) - Working code examples