# Graph Root Attestation

## Overview

Graph root attestation is a mechanism for creating cryptographically signed, content-addressed proofs of graph state. It enables offline verification that replayed graphs match the original attested state by computing a Merkle root from sorted node/edge IDs and input digests, then wrapping it in a DSSE envelope with an in-toto statement.

## Purpose

Graph root attestations solve the problem of proving graph authenticity without reconstructing the entire proof chain. They enable:

- **Offline Verification**: Download an attestation, recompute the root from stored nodes/edges, compare
- **Audit Snapshots**: Point-in-time proof of graph state for compliance
- **Evidence Linking**: Reference attested roots (not transient IDs) in evidence chains
- **Transparency**: Optional Rekor publication for public auditability

## Architecture

### Components

```
┌─────────────────────────────────────────────────────────────────┐
│                    GraphRootAttestor                           │
├─────────────────────────────────────────────────────────────────┤
│  AttestAsync(request) → GraphRootAttestationResult             │
│  VerifyAsync(envelope, nodes, edges) → VerificationResult      │
└─────────────────┬───────────────────────────────────────────────┘
                  │
                  ▼
┌─────────────────────────────────────────────────────────────────┐
│                 IMerkleRootComputer                             │
├─────────────────────────────────────────────────────────────────┤
│  ComputeRoot(leaves) → byte[]                                   │
│  Algorithm → "sha256"                                           │
└─────────────────────────────────────────────────────────────────┘
                  │
                  ▼
┌─────────────────────────────────────────────────────────────────┐
│              EnvelopeSignatureService                           │
├─────────────────────────────────────────────────────────────────┤
│  Sign(payload, key) → EnvelopeSignature                         │
└─────────────────────────────────────────────────────────────────┘
```

### Data Flow

1. **Request** → Sorted node/edge IDs + input digests
2. **Merkle Tree** → Compute SHA-256 root from leaves
3. **In-Toto Statement** → Build attestation with predicate
4. **Canonicalize** → JCS (RFC 8785) with version marker
5. **Sign** → DSSE envelope with Ed25519/ECDSA
6. **Store/Publish** → CAS storage + optional Rekor

## Graph Types

The `GraphType` enum identifies what kind of graph is being attested:

| GraphType | Description |
|-----------|-------------|
| `Unknown` | Unspecified graph type |
| `CallGraph` | Function/method call relationships |
| `DependencyGraph` | Package/library dependencies (SBOM) |
| `SbomGraph` | SBOM component graph |
| `EvidenceGraph` | Linked evidence records |
| `PolicyGraph` | Policy decision trees |
| `ProofSpine` | Proof chain spine segments |
| `ReachabilityGraph` | Code reachability analysis |
| `VexLinkageGraph` | VEX statement linkages |
| `Custom` | Application-specific graph |

## Models

### GraphRootAttestationRequest

Input to the attestation service:

```csharp
public sealed record GraphRootAttestationRequest
{
    public required GraphType GraphType { get; init; }
    public required IReadOnlyList<string> NodeIds { get; init; }
    public required IReadOnlyList<string> EdgeIds { get; init; }
    public required string PolicyDigest { get; init; }
    public required string FeedsDigest { get; init; }
    public required string ToolchainDigest { get; init; }
    public required string ParamsDigest { get; init; }
    public required string ArtifactDigest { get; init; }
    public IReadOnlyList<string> EvidenceIds { get; init; } = [];
    public bool PublishToRekor { get; init; } = false;
    public string? SigningKeyId { get; init; }
}
```

### GraphRootAttestation (In-Toto Statement)

The attestation follows the in-toto Statement/v1 format:

```json
{
  "_type": "https://in-toto.io/Statement/v1",
  "subject": [
    {
      "name": "sha256:abc123...",
      "digest": { "sha256": "abc123..." }
    },
    {
      "name": "sha256:artifact...",
      "digest": { "sha256": "artifact..." }
    }
  ],
  "predicateType": "https://stella-ops.org/attestation/graph-root/v1",
  "predicate": {
    "graphType": "DependencyGraph",
    "rootHash": "sha256:abc123...",
    "rootAlgorithm": "sha256",
    "nodeCount": 1247,
    "edgeCount": 3891,
    "nodeIds": ["node-a", "node-b", ...],
    "edgeIds": ["edge-1", "edge-2", ...],
    "inputs": {
      "policyDigest": "sha256:policy...",
      "feedsDigest": "sha256:feeds...",
      "toolchainDigest": "sha256:tools...",
      "paramsDigest": "sha256:params..."
    },
    "evidenceIds": ["ev-1", "ev-2"],
    "canonVersion": "stella:canon:v1",
    "computedAt": "2025-12-26T10:30:00Z",
    "computedBy": "stellaops/attestor/graph-root",
    "computedByVersion": "1.0.0"
  }
}
```

## Merkle Root Computation

The root is computed from leaves in this deterministic order:

1. **Sorted node IDs** (lexicographic, ordinal)
2. **Sorted edge IDs** (lexicographic, ordinal)
3. **Policy digest**
4. **Feeds digest**
5. **Toolchain digest**
6. **Params digest**

Each leaf is SHA-256 hashed, then combined pairwise until a single root remains.

```
                    ROOT
                   /    \
              H(L12)    H(R12)
             /    \     /    \
         H(n1)  H(n2) H(e1) H(policy)
```

## Usage

### Creating an Attestation

```csharp
var services = new ServiceCollection();
services.AddGraphRootAttestation(sp => keyId => GetSigningKey(keyId));
var provider = services.BuildServiceProvider();

var attestor = provider.GetRequiredService<IGraphRootAttestor>();

var request = new GraphRootAttestationRequest
{
    GraphType = GraphType.DependencyGraph,
    NodeIds = graph.Nodes.Select(n => n.Id).ToList(),
    EdgeIds = graph.Edges.Select(e => e.Id).ToList(),
    PolicyDigest = "sha256:abc...",
    FeedsDigest = "sha256:def...",
    ToolchainDigest = "sha256:ghi...",
    ParamsDigest = "sha256:jkl...",
    ArtifactDigest = imageDigest
};

var result = await attestor.AttestAsync(request);
Console.WriteLine($"Root: {result.RootHash}");
Console.WriteLine($"Nodes: {result.NodeCount}");
Console.WriteLine($"Edges: {result.EdgeCount}");
```

### Verifying an Attestation

```csharp
var envelope = LoadEnvelope("attestation.dsse.json");
var nodes = LoadNodes("nodes.ndjson");
var edges = LoadEdges("edges.ndjson");

var result = await attestor.VerifyAsync(envelope, nodes, edges);

if (result.IsValid)
{
    Console.WriteLine($"✓ Verified: {result.ComputedRoot}");
    Console.WriteLine($"  Nodes: {result.NodeCount}");
    Console.WriteLine($"  Edges: {result.EdgeCount}");
}
else
{
    Console.WriteLine($"✗ Failed: {result.FailureReason}");
    Console.WriteLine($"  Expected: {result.ExpectedRoot}");
    Console.WriteLine($"  Computed: {result.ComputedRoot}");
}
```

## Offline Verification Workflow

1. **Obtain attestation**: Download DSSE envelope from storage or transparency log
2. **Verify signature**: Check envelope signature against trusted public keys
3. **Extract predicate**: Parse `GraphRootPredicate` from the payload
4. **Fetch graph data**: Download nodes and edges by ID from CAS
5. **Recompute root**: Apply Merkle tree algorithm to node/edge IDs + input digests
6. **Compare**: Computed root must match `predicate.RootHash`

## Determinism Guarantees

Graph root attestations are fully deterministic:

- **Sorting**: All IDs sorted lexicographically (ordinal comparison)
- **Canonicalization**: RFC 8785 JCS with `stella:canon:v1` version marker
- **Hashing**: SHA-256 only
- **Timestamps**: UTC ISO-8601 (not included in root computation)

Same inputs always produce the same root hash, enabling replay verification.

## Security Considerations

- **Signature verification**: Always verify DSSE envelope signatures before trusting attestations
- **Key management**: Use short-lived signing keys; rotate regularly
- **Transparency**: Publish to Rekor for tamper-evident audit trail
- **Input validation**: Validate all digests are properly formatted before attestation

## Related Documentation

- [DSSE Envelopes](./dsse-envelopes.md) - Envelope format and signing
- [Proof Chain](./proof-chain.md) - Overall proof chain architecture
- [Canonical JSON](../../modules/platform/canonical-json.md) - Canonicalization scheme