# Change-Trace JSON Schema Contract

> **Version:** 1.0.0
> **Status:** Draft
> **Last Updated:** 2026-01-12

---

## Overview

This document defines the JSON schema for Change-Trace artifacts (`trace.cdxchange.json`). All implementations must conform to this schema for interoperability.

---

## Schema Identifier

```
Schema URI: stella.change-trace/1.0
File Extension: .cdxchange.json
MIME Type: application/vnd.stella.change-trace+json
```

---

## Root Object

```json
{
  "$schema": "https://stella-ops.org/schemas/change-trace/1.0/schema.json",
  "schema": "stella.change-trace/1.0",
  "subject": { ... },
  "deltas": [ ... ],
  "summary": { ... },
  "analyzedAt": "2026-01-12T14:30:00.000Z",
  "algorithmVersion": "1.0"
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `schema` | string | Yes | Schema version identifier. Must be `stella.change-trace/1.0` |
| `subject` | object | Yes | Subject of the comparison |
| `deltas` | array | Yes | Array of package deltas (may be empty) |
| `summary` | object | Yes | Aggregated summary metrics |
| `analyzedAt` | string | Yes | ISO 8601 timestamp (UTC) |
| `algorithmVersion` | string | No | Algorithm version used (default: "1.0") |

---

## Subject Object

```json
{
  "imageRef": "docker.io/library/nginx:1.25.3",
  "fromDigest": "sha256:abc123...",
  "toDigest": "sha256:def456...",
  "fromScanId": "scan-12345",
  "toScanId": "scan-67890"
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `imageRef` | string | Yes | Container image reference |
| `fromDigest` | string | Yes | SHA-256 digest of source image |
| `toDigest` | string | Yes | SHA-256 digest of target image |
| `fromScanId` | string | No | Source scan identifier |
| `toScanId` | string | No | Target scan identifier |

---

## PackageDelta Object

```json
{
  "purl": "pkg:deb/debian/libssl3@3.0.9-1",
  "fromVersion": "3.0.9-1",
  "toVersion": "3.0.9-1+deb12u3",
  "changeType": "patched",
  "symbols": [ ... ],
  "bytes": [ ... ],
  "trustDelta": { ... }
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `purl` | string | Yes | Package URL (RFC 3986 compliant) |
| `fromVersion` | string | No | Source version (null if added) |
| `toVersion` | string | No | Target version (null if removed) |
| `changeType` | enum | Yes | Type of change |
| `symbols` | array | No | Symbol-level deltas (if available) |
| `bytes` | array | No | Byte-level deltas (if enabled) |
| `trustDelta` | object | Yes | Trust impact calculation |

### ChangeType Enum

| Value | Description |
|-------|-------------|
| `unchanged` | No change detected |
| `added` | Package added in target |
| `removed` | Package removed in target |
| `upgraded` | Version increased |
| `downgraded` | Version decreased |
| `patched` | Security patch applied (same base version) |
| `rebuilt` | Same version, different build |

---

## SymbolDelta Object

```json
{
  "symbolName": "ssl3_get_record",
  "changeType": "patched",
  "sizeDelta": -24,
  "cfgBlockDelta": 2,
  "confidence": 0.97,
  "matchMethod": "CFGHash+ChunkMatch",
  "explanation": "Function patched: 2 basic blocks changed"
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `symbolName` | string | Yes | Symbol/function name |
| `changeType` | enum | Yes | Type of symbol change |
| `sizeDelta` | integer | No | Size change in bytes |
| `cfgBlockDelta` | integer | No | CFG basic block count change |
| `confidence` | number | No | Match confidence (0.0-1.0) |
| `matchMethod` | string | No | Method used for matching |
| `explanation` | string | No | Human-readable explanation |

### SymbolChangeType Enum

| Value | Description |
|-------|-------------|
| `unchanged` | No change detected |
| `added` | Symbol added in target |
| `removed` | Symbol removed in target |
| `modified` | Symbol modified (general) |
| `patched` | Security patch detected |

---

## ByteDelta Object

```json
{
  "offset": 4096,
  "size": 2048,
  "fromHash": "sha256:abc123...",
  "toHash": "sha256:def456...",
  "section": ".text",
  "context": "Function: ssl3_get_record"
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `offset` | integer | Yes | Byte offset in binary |
| `size` | integer | Yes | Size of changed region |
| `fromHash` | string | Yes | SHA-256 hash of source bytes |
| `toHash` | string | Yes | SHA-256 hash of target bytes |
| `section` | string | No | Binary section name |
| `context` | string | No | Context hint (e.g., function name) |

**Privacy Note:** Raw byte content is never included; only hashes are stored.

---

## TrustDelta Object

```json
{
  "score": -0.27,
  "beforeScore": 0.65,
  "afterScore": 0.92,
  "reachabilityImpact": "reduced",
  "exploitabilityImpact": "down",
  "proofSteps": [
    "CVE-2026-12345 affects ssl3_get_record",
    "Version changed: 3.0.9-1 -> 3.0.9-1+deb12u3",
    "Patch verified via CFG match: 97% confidence",
    "Reachable call paths: 3 -> 0 after patch",
    "DSSE attestation present",
    "Verdict: risk_down (-0.27)"
  ]
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `score` | number | Yes | Trust delta value (-1.0 to +1.0) |
| `beforeScore` | number | Yes | Trust score before change |
| `afterScore` | number | Yes | Trust score after change |
| `reachabilityImpact` | enum | Yes | Impact on code reachability |
| `exploitabilityImpact` | enum | Yes | Impact on exploitability |
| `proofSteps` | array | Yes | Human-readable proof steps |

### ReachabilityImpact Enum

| Value | Description |
|-------|-------------|
| `unchanged` | No change in reachability |
| `introduced` | Path now reachable |
| `eliminated` | Path no longer reachable |
| `reduced` | Fewer reachable paths |
| `increased` | More reachable paths |

### ExploitabilityImpact Enum

| Value | Description |
|-------|-------------|
| `unchanged` | No change in exploitability |
| `introduced` | New exploitability concern |
| `eliminated` | Exploitability eliminated |
| `up` | Increased exploitability |
| `down` | Decreased exploitability |

---

## Summary Object

```json
{
  "packagesChanged": 5,
  "packagesAdded": 1,
  "packagesRemoved": 0,
  "symbolsChanged": 23,
  "bytesChanged": 8192,
  "trustDelta": -0.15,
  "overallVerdict": "risk_down"
}
```

### Properties

| Property | Type | Required | Description |
|----------|------|----------|-------------|
| `packagesChanged` | integer | Yes | Total packages with changes |
| `packagesAdded` | integer | Yes | Packages added |
| `packagesRemoved` | integer | Yes | Packages removed |
| `symbolsChanged` | integer | Yes | Total symbols with changes |
| `bytesChanged` | integer | Yes | Total bytes changed |
| `trustDelta` | number | Yes | Aggregate trust delta |
| `overallVerdict` | string | Yes | Overall verdict |

### Verdict Values

| Value | Trust Delta Range | Description |
|-------|-------------------|-------------|
| `risk_down` | < -0.3 | Risk decreased significantly |
| `neutral` | -0.3 to +0.3 | No significant risk change |
| `risk_up` | > +0.3 | Risk increased significantly |
| `inconclusive` | N/A | Unable to determine |

---

## Full Example

```json
{
  "$schema": "https://stella-ops.org/schemas/change-trace/1.0/schema.json",
  "schema": "stella.change-trace/1.0",
  "subject": {
    "imageRef": "docker.io/library/nginx:1.25.3",
    "fromDigest": "sha256:abc123def456...",
    "toDigest": "sha256:789ghi012jkl...",
    "fromScanId": "scan-20260112-001",
    "toScanId": "scan-20260112-002"
  },
  "deltas": [
    {
      "purl": "pkg:deb/debian/libssl3@3.0.9-1",
      "fromVersion": "3.0.9-1",
      "toVersion": "3.0.9-1+deb12u3",
      "changeType": "patched",
      "symbols": [
        {
          "symbolName": "ssl3_get_record",
          "changeType": "patched",
          "sizeDelta": -24,
          "cfgBlockDelta": 2,
          "confidence": 0.97,
          "matchMethod": "CFGHash+ChunkMatch",
          "explanation": "Function patched: 2 basic blocks changed"
        }
      ],
      "bytes": [
        {
          "offset": 4096,
          "size": 2048,
          "fromHash": "sha256:abc...",
          "toHash": "sha256:def...",
          "section": ".text"
        }
      ],
      "trustDelta": {
        "score": -0.27,
        "beforeScore": 0.65,
        "afterScore": 0.92,
        "reachabilityImpact": "reduced",
        "exploitabilityImpact": "down",
        "proofSteps": [
          "CVE-2026-12345 affects ssl3_get_record",
          "Version changed: 3.0.9-1 -> 3.0.9-1+deb12u3",
          "Patch verified via CFG match: 97% confidence",
          "Reachable call paths: 3 -> 0 after patch",
          "DSSE attestation present",
          "Verdict: risk_down (-0.27)"
        ]
      }
    }
  ],
  "summary": {
    "packagesChanged": 1,
    "packagesAdded": 0,
    "packagesRemoved": 0,
    "symbolsChanged": 1,
    "bytesChanged": 2048,
    "trustDelta": -0.27,
    "overallVerdict": "risk_down"
  },
  "analyzedAt": "2026-01-12T14:30:00.000Z",
  "algorithmVersion": "1.0"
}
```

---

## Serialization Rules

### Determinism Requirements

1. **Key ordering**: Object keys must be sorted alphabetically
2. **Array ordering**:
   - `deltas`: Sorted by `purl` (lexicographic)
   - `symbols`: Sorted by `symbolName` (lexicographic)
   - `bytes`: Sorted by `offset` (numeric ascending)
   - `proofSteps`: Preserve generation order
3. **Number formatting**: No trailing zeros, no exponent notation
4. **String escaping**: Minimal escaping per RFC 8785
5. **Timestamps**: ISO 8601 with milliseconds, UTC (`Z` suffix)

### RFC 8785 Compliance

All JSON output must conform to [RFC 8785](https://datatracker.ietf.org/doc/html/rfc8785) (JSON Canonicalization Scheme) for digest computation.

---

## Validation

### JSON Schema

The formal JSON Schema is available at:
```
https://stella-ops.org/schemas/change-trace/1.0/schema.json
```

### Validation Command

```bash
stella change-trace verify trace.cdxchange.json
```

### Required Fields

The following fields are required for a valid trace:
- `schema`
- `subject.imageRef`
- `subject.fromDigest`
- `subject.toDigest`
- `deltas` (may be empty array)
- `summary.*` (all summary fields)
- `analyzedAt`

---

## CycloneDX Integration

### Embedded Mode

When embedded in CycloneDX, the change trace is attached as a `component-evidence` extension:

```json
{
  "bomFormat": "CycloneDX",
  "specVersion": "1.7",
  "components": [
    {
      "purl": "pkg:deb/debian/libssl3@3.0.9-1+deb12u3",
      "evidence": {
        "extensions": [
          {
            "extensionType": "stella-change-trace",
            "extension": {
              "schema": "stella.change-trace/1.0",
              "changeType": "patched",
              "trustDelta": { ... },
              "symbols": [ ... ]
            }
          }
        ]
      }
    }
  ]
}
```

### Standalone Mode

Export as separate file: `<basename>.cdxchange.json`

---

## Version History

| Version | Date | Changes |
|---------|------|---------|
| 1.0.0 | 2026-01-12 | Initial release |

---

## References

- [Architecture Document](../modules/scanner/design/change-trace-architecture.md)
- [Trust-Delta Contract](./change-trace-trust-delta.md)
- [RFC 8785 - JSON Canonicalization Scheme](https://datatracker.ietf.org/doc/html/rfc8785)
- [Package URL Specification](https://github.com/package-url/purl-spec)

---

*Document Version: 1.0.0*
*Last Updated: 2026-01-12*