Data Structures

This page provides detailed technical schemas for the core data structures used in the ATL Protocol.

1. Checkpoint (Binary Wire Format)

Before signing, a Checkpoint must be serialized into a strict 98-byte binary blob to ensure byte-for-byte consistency across different implementations.


Offset  Size   Field         Encoding
------  ----   -----         --------
0       18     Magic Bytes   "ATL-Protocol-v1-CP" (ASCII)
18      32     Origin ID     SHA256 of Instance UUID (binary)
50      8      Tree Size     u64 little-endian
58      8      Timestamp     u64 little-endian (Unix nanoseconds)
66      32     Root Hash     SHA256 root of Merkle tree (binary)
------  ----
Total:  98 bytes

2. Evidence Receipt (JSON)

The Evidence Receipt is the primary unit of proof. It is a JSON file (standard extension .atl) that contains the entry data and the cryptographic proof of its inclusion.


{
  "spec_version": "1.0.0",
  "entry": {
    "id": "550e8400-e29b-41d4-a716-446655440000",
    "payload_hash": "sha256:e3b0c442...",
    "metadata": {
      "filename": "contract.pdf",
      "type": "legal-agreement"
    }
  },
  "proof": {
    "tree_size": 1450,
    "root_hash": "sha256:7f83b165...",
    "leaf_index": 142,
    "inclusion_path": [
      "sha256:a1b2...",
      "sha256:c3d4..."
    ],
    "checkpoint": {
      "origin": "sha256:9988...",
      "tree_size": 1450,
      "root_hash": "sha256:7f83...",
      "timestamp": 1704067200000000000,
      "key_id": "sha256:4433...",
      "signature": "base64:SGVsbG8..."
    },
    "consistency_proof": {
      "from_tree_size": 1200,
      "path": [
        "sha256:e5f6...",
        "sha256:a7b8..."
      ]
    }
  },
  "anchors": [
    {
      "type": "rfc3161",
      "timestamp": "2026-01-04T12:00:00Z",
      "token_der": "base64:MIAGCS..."
    },
    {
      "type": "bitcoin",
      "data": "base64:AE9wZW5..."
    }
  ]
}

3. Merkle Leaf Construction

ATL leaves are constructed with a domain separation prefix to prevent second-preimage attacks.


LeafHash = SHA-256(
  0x00 ||             // Prefix (1 byte)
  PayloadHash ||      // 32 bytes
  MetadataHash        // 32 bytes
)

Where MetadataHash is the SHA-256 of the JCS-canonicalized (RFC 8785) metadata JSON object.