Sidecar Format

The sidecar file contains both layers of cryptographic proof in a single JSON document that travels alongside the media file.

Overview

A sidecar file (e.g., photo.sidecar.json) accompanies each media file (e.g., photo.jpg). This separation keeps the original media untouched while providing all verification data.

Why JSON?

The proof format is JSON for three reasons:

• Human-readable — Developers can inspect proofs without special tools
• Flexible storage — Store as a file, in a database, on a blockchain, or transmit via API
• Easy integration — Every platform has JSON libraries

Structure

{
  "version": "1.0",
  "capture_trust": {
    "jwt": "eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6Ii4uLiJ9..."
  },
  "media_integrity": {
    "content_hash": "a1b2c3d4e5f6...",
    "signature": "MEUCIQC...",
    "public_key": "BHx5y...",
    "capture_id": "550e8400-e29b-41d4-a716-446655440000",
    "captured_at": "2025-01-15T10:30:00Z"
  }
}

Fields

Root

Field	Type	Description
version	string	Schema version (currently "1.0")
capture_trust	object	Server-issued trust token
media_integrity	object	Device-generated integrity proof

capture_trust

Field	Type	Description
jwt	string	ES256-signed JWT from the SignedShot API

media_integrity

Field	Type	Description
content_hash	string	SHA-256 hash of the media file (hex, 64 characters)
signature	string	ECDSA signature over the signed message (base64)
public_key	string	Device's public key (base64, uncompressed EC point, 65 bytes)
capture_id	string	UUID of the capture session (must match JWT)
captured_at	string	ISO8601 UTC timestamp of capture

JWT Payload

The capture_trust.jwt is a standard JWT. When decoded, it contains:

{
  "iss": "https://api.signedshot.io",
  "aud": "signedshot",
  "sub": "capture-service",
  "iat": 1705312200,
  "capture_id": "550e8400-e29b-41d4-a716-446655440000",
  "publisher_id": "9a5b1062-a8fe-4871-bdc1-fe54e96cbf1c",
  "device_id": "ea5c9bfe-6bbc-4ee2-b82d-0bcfcc185ef1",
  "attestation": {
    "method": "app_check",
    "app_id": "io.signedshot.capture"
  }
}

JWT Fields

Field	Type	Description
iss	string	Issuer (API URL)
aud	string	Audience
sub	string	Subject (always "capture-service")
iat	number	Issued at (Unix timestamp)
capture_id	string	Unique capture session ID
publisher_id	string	Publisher UUID
device_id	string	Device UUID
attestation	object	Attestation details

attestation Object

Field	Type	Description
method	string	"sandbox", "app_check", or "app_attest"
app_id	string	App bundle ID (only present when attested)

Signature Format

Media Integrity Signature

The media_integrity.signature signs a message constructed from:

{content_hash}:{capture_id}:{captured_at}

For example:

a1b2c3d4e5f6...:550e8400-e29b-41d4-a716-446655440000:2025-01-15T10:30:00Z

The signature is:

• Algorithm: ECDSA with P-256 curve
• Generated by the device's Secure Enclave
• Encoded as base64

JWT Signature

The JWT uses:

• Algorithm: ES256 (ECDSA with P-256)
• Key ID (kid) in header for JWKS lookup
• Verification via /.well-known/jwks.json

File Naming Convention

Media File	Sidecar File
photo.jpg	photo.sidecar.json
video.mp4	video.sidecar.json
IMG_1234.HEIC	IMG_1234.sidecar.json

Example: Complete Sidecar

{
  "version": "1.0",
  "capture_trust": {
    "jwt": "eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6ImtleS0xIn0.eyJpc3MiOiJodHRwczovL2FwaS5zaWduZWRzaG90LmlvIiwiYXVkIjoic2lnbmVkc2hvdCIsInN1YiI6ImNhcHR1cmUtc2VydmljZSIsImlhdCI6MTcwNTMxMjIwMCwiY2FwdHVyZV9pZCI6IjU1MGU4NDAwLWUyOWItNDFkNC1hNzE2LTQ0NjY1NTQ0MDAwMCIsInB1Ymxpc2hlcl9pZCI6IjlhNWIxMDYyLWE4ZmUtNDg3MS1iZGMxLWZlNTRlOTZjYmYxYyIsImRldmljZV9pZCI6ImVhNWM5YmZlLTZiYmMtNGVlMi1iODJkLTBiY2ZjYzE4NWVmMSIsImF0dGVzdGF0aW9uIjp7Im1ldGhvZCI6ImFwcF9jaGVjayIsImFwcF9pZCI6ImlvLnNpZ25lZHNob3QuY2FwdHVyZSJ9fQ.signature"
  },
  "media_integrity": {
    "content_hash": "9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08",
    "signature": "MEUCIQDKZokqnCjrRtw+3S0P2mjJH+E8zRqgaG6R4bG6V7oONwIgF3lQsGV1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3M=",
    "public_key": "BHx5yK3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3K1K3M=",
    "capture_id": "550e8400-e29b-41d4-a716-446655440000",
    "captured_at": "2025-01-15T10:30:00Z"
  }
}

Verification Steps

To verify a sidecar:

1. Parse the sidecar JSON
2. Verify Capture Trust (JWT)
   • Fetch JWKS from issuer's /.well-known/jwks.json
   • Find key by kid in JWT header
   • Verify ES256 signature
3. Verify Media Integrity
   • Compute SHA-256 of media file
   • Compare with content_hash
   • Reconstruct signed message: {hash}:{capture_id}:{captured_at}
   • Verify ECDSA signature using public_key
4. Cross-validate
   • Confirm capture_id matches in both JWT and media_integrity

Next Steps

• Two-Layer Trust Model — Understand why both layers are needed
• Python Validation — Verify sidecars programmatically